Swipe to Check In/Out in Open Event Organizer App

Open Event Organizer App didn’t provide any option for the Event Organizer to view the list of Attendees present under an Order and check them in/out the event. Therefore, we designed a system such that the Organizer can just swipe the attendee present under an order to check them in or out. In this blog post, I will discuss how we implemented this functionality in Open Event Organizer App without using any third party libraries.

Specifications

We will create a separate class SwipeController.java which extends ItemTouchHelper.SimpleCallback and provide the swiping functionalities to our plain old recyclerview. We will call the super constructor with ItemTouchHelper.LEFT and ItemTouchHelper.RIGHT as arguments to provide left as well as right movements in each recyclerview list item. The bitmaps and paint object initialized here will be used later in onDraw.

public SwipeController(OrderDetailViewModel orderDetailViewModel, OrderAttendeesAdapter orderAttendeesAdapter, Context context) {
super(0, ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT);
this.orderDetailViewModel = orderDetailViewModel;
this.orderAttendeesAdapter = orderAttendeesAdapter;

closeIcon = BitmapFactory.decodeResource(context.getResources(), R.drawable.close);
doneIcon = BitmapFactory.decodeResource(context.getResources(), R.drawable.done);

paintGreen.setColor(context.getResources().getColor(R.color.light_green_500));
paintRed.setColor(context.getResources().getColor(R.color.red_500));
}

Next, we will override getMovementFlags method. This method decides the allowed movement directions for each recyclerview item. The deciding logic is that, if an attendee is checked in then the allowed movement is left to check out and if an attendee is checked out then the allowed movement is right to check in. If neither of the above case, then both movements are allowed.

@Override
public int getMovementFlags(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder) {
int dragFlags = 0;

If (orderDetailViewModel.getCheckedInStatus(

viewHolder.getAdapterPosition()) == null)
makeMovementFlags(dragFlags, ItemTouchHelper.LEFT | ItemTouchHelper.RIGHT);

if (orderDetailViewModel.getCheckedInStatus(

viewHolder.getAdapterPosition())) {
return makeMovementFlags(dragFlags, ItemTouchHelper.LEFT);
} else {
return makeMovementFlags(dragFlags, ItemTouchHelper.RIGHT);
}
}

The onChildDraw method involves the code doing actual drawing. The variables used in code are discussed below.

  1. ActionState – Checks the state of the recycler view item. We proceed with the below logic if the item is being swiped.
  2. dX – The distance by which the item is swiped. Positive for left and negative for right.
  3. Background – Background of the viewholder. Rectangular in shape and dimensions changed with change in dX.
  4. IconDest – Calculates the position where the icons (close icon or done icon) is placed in canvas
  5. Canvas – Java Canvas on which the drawing is done. We set the background and draw the bitmaps on their location in canvas.
@Override
public void onChildDraw(Canvas canvas, RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder,
float dX, float dY, int actionState, boolean isCurrentlyActive) {
if (actionState == ItemTouchHelper.ACTION_STATE_SWIPE) {
View itemView = viewHolder.itemView;
float height = (float) itemView.getBottom() – (float) itemView.getTop();
float width = height / 3;
RectF background;
Paint paint;
Bitmap icon;
RectF iconDest;

if (dX > 0) {
background = new RectF((float) itemView.getLeft(), (float) itemView.getTop(), dX,
(float) itemView.getBottom());
paint = paintGreen;
icon = doneIcon;
iconDest = new RectF((float) itemView.getLeft() + width,
(float) itemView.getTop() + width, (float) itemView.getLeft() + 2 * width,
(float) itemView.getBottom() – width);
} else {
background = new RectF((float) itemView.getRight() + dX, (float) itemView.getTop(),
(float) itemView.getRight(), (float) itemView.getBottom());
paint = paintRed;
icon = closeIcon;
iconDest = new RectF((float) itemView.getRight() – 2 * width,
(float) itemView.getTop() + width, (float) itemView.getRight() – width,
(float) itemView.getBottom() – width);
}

canvas.drawRect(background, paint);
canvas.drawBitmap(icon, null, iconDest, paint);
}
super.onChildDraw(canvas, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive);
}

Now after the item is swiped out or in, we need to restore its original state again. For this we override the onSwiped method and call notifyItemChanged(). Also, the changes in UI (showing green side strip for checked in and red side strip for checked out) are done by. We call the toggleCheckin() method in ViewModel to toggle the checking status of the attendee in server and local database.

@Override
public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
int position = viewHolder.getAdapterPosition();

orderDetailViewModel.toggleCheckIn(position);
orderAttendeesAdapter.notifyItemChanged(position);
}

Last but not the least, we will override the onMove method to return false. Since we are not supporting drag and drop features therefore this method will never be called.

@Override
public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
return false;
}

Resources

  1. Codebase for Open Event Organizer App https://github.com/fossasia/open-event-orga-app
  2. Official documentation for ItemTouchHelper.SimpleCallback https://developer.android.com/reference/android/support/v7/widget/helper/ItemTouchHelper.SimpleCallback

Share Events in the Open Event Organizer Android App

In the Open Event Organizer Android App, after creating an event the organizer was unable to share it. We handled this challenge and came up with options to share Event with other social media apps. Along with that user can send Email to users containing event description with just a click. All this through a UI that our user will love interacting with. Let’s see how we implemented this.

Specifications

We designed a UI given below which offer four functionalities to the user in a single screen.

  1. The Event Name and Date are shown in the CardView.
  2. User can copy Event External URL by clicking on Copy URL option.
  3. User can send Email containing information about the Event like ( Name, Description, Starting and Ending Date-Time for the Event) via Email by clicking on Email option.
  4. User can share the same information described in point three via other social media/chatting apps etc by clicking on many more option.

The is the Event Model class ia a POJO containing the associated attributes.

We will use Retrofit to fetch Event object from server through a GET request in EventApi class.

@GET(“events/{id}?include=tickets”)
Observable<Event> getEvent(@Path(“id”) long id);

Then we will use the getEvent method of EventRepositoryImpl class to make the request for us using EventApi class and then pass on the Response Event object to the ViewModel by wrapping it in RxJava Observable. We are accepting a boolean field named reload which decdes whether we need to reuse the existing Event object from Local Database  or fetch a new object from server.

@Override
public Observable<Event> getEvent(long eventId, boolean reload) {
Observable<Event> diskObservable = Observable.defer(() ->
repository
.getItems(Event.class, Event_Table.id.eq(eventId))
.filter(Event::isComplete)
.take(1)
);

Observable<Event> networkObservable = Observable.defer(() ->
eventApi
.getEvent(eventId)
.doOnNext(this::saveEvent));

return repository.observableOf(Event.class)
.reload(reload)
.withDiskObservable(diskObservable)
.withNetworkObservable(networkObservable)
.build();
}

In ShareEventVewModel, we are calling the getEvent from EventRepositoryImpl class, construct a LiveData object from it so that UI could observe changes on it. Methods getShareableInformation, getShareableUrl, getShareableSubject provide the shareable information to the UI which is further shared with other apps.

public class ShareEventViewModel extends ViewModel {

protected LiveData<Event> getEvent(long eventId, boolean reload) {
if (eventLiveData.getValue() != null && !reload)
return eventLiveData;

compositeDisposable.add(eventRepository.getEvent(eventId, reload)
.doOnSubscribe(disposable -> progress.setValue(true))
.doFinally(() -> progress.setValue(false))
.subscribe(event -> {
this.event = event;
eventLiveData.setValue(event);
},
throwable -> error.setValue(ErrorUtils.getMessage(throwable))));

return eventLiveData;
}

public String getShareableInformation() {
return Utils.getShareableInformation(event);
}

}

In ShareEventFragment class does the work of binding the UI to the model using data binding. It observes the LiveData objects supplied by presenter and reflect the changes in UI to the LiveData object.

public class ShareEventFragment extends BaseFragment implements ShareEventView {

public void shareEvent() {
Intent shareIntent = new Intent();
shareIntent.setAction(Intent.ACTION_SEND);
shareIntent.putExtra(Intent.EXTRA_TEXT, shareEventViewModel.getShareableInformation());
shareIntent.setType(“text/plain”);
startActivity(Intent.createChooser(shareIntent, getResources().getText(R.string.send_to)));
}

public void shareByEmail() {
Intent shareIntent = new Intent();
shareIntent.setAction(Intent.ACTION_SENDTO);
shareIntent.setType(“message/rfc822”);
shareIntent.setData(Uri.parse(“mailto:”));
shareIntent.putExtra(Intent.EXTRA_SUBJECT, shareEventViewModel.getEmailSubject());
shareIntent.putExtra(Intent.EXTRA_TEXT, shareEventViewModel.getShareableInformation());
try {
startActivity(Intent.createChooser(shareIntent, getResources().getText(R.string.send_to)));
} catch (android.content.ActivityNotFoundException ex) {
ViewUtils.showSnackbar(binding.getRoot(), “There are no email clients installed”);
}
}

public void copyUrlToClipboard() {
ClipboardManager clipboard = (ClipboardManager) getActivity().getSystemService(Context.CLIPBOARD_SERVICE);
String eventUrl = shareEventViewModel.getShareableUrl();
if (eventUrl == null) {
ViewUtils.showSnackbar(binding.getRoot(), “Event does not have a Public URL”);
} else {
ClipData clip = ClipData.newPlainText(“Event URL”, shareEventViewModel.getShareableUrl());
clipboard.setPrimaryClip(clip);
ViewUtils.showSnackbar(binding.getRoot(), “Event URL Copied to Clipboard”);
}
}
}

The layout file contains the Event object bind to the UI using Two way Data Binding. Here is an extract from the layout file. For viewing entire file, please refer here.

References

  1. Official documentation of Retrofit 2.x http://square.github.io/retrofit/
  2. Official documentation for RxJava 2.x https://github.com/ReactiveX/RxJava
  3. Official documentation for ViewModel https://developer.android.com/topic/libraries/architecture/viewmodel
  4. Codebase for Open Event Organizer App https://github.com/fossasia/open-event-orga-app

Change Password Feature for Open Event Android Organizer App

In Open Event Organizer Android App, the users were able to successfully login and sign up but in case they wanted to change their login password they could not. So, we added a feature to allow users to change their existing password. This blog explains the technical details to implement this feature following MVVM architecture and using highly efficient libraries like Retrofit, RxJava, Raziz Labs DbFlow, Data Binding.

Specifications

We will implement a page where users can enter their old password and new password along with a confirm password field. Their will be a login button to send the password change request to server. Server then return a response and we will provide feedback regarding the request. We are following MVP architecture so there will be a Model class, Fragment class, Presenter class and Network Layer to make network requests.

Let’s start with creating ChangePassword model class. There are three fields to store old password, new password and new confirmed password. Several Lombok annotations like @Data, @AllArgsConstructor, @NoArgsConstructor are used to avoid boilerplate code for getters, setters and constructors. @JsonNaming annotation is used to translate the Java Object names to KebabCase when they are serialized.

@Data
@AllArgsConstructor
@NoArgsConstructor
@JsonNaming(PropertyNamingStrategy.KebabCaseStrategy.class)
public class ChangePassword {

public String oldPassword;
public String newPassword;

@JsonIgnore
public String confirmNewPassword;
}

The layout file is binded to model using Data Binding. There will be three TextInputEditText fields for user input. An option to toggle password visibility and a login button.

The Fragment class binds layout file to the Fragment and handle UI stuff. Presenter is called to make Login request when login button is pressed.

public class ChangePasswordFragment extends BaseFragment<ChangePasswordPresenter> implements ChangePasswordView {

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
binding = DataBindingUtil.inflate(inflater, R.layout.change_password_fragment, container, false);
validator = new Validator(binding);

AppCompatActivity activity = ((AppCompatActivity) getActivity());
activity.setSupportActionBar(binding.toolbar);

ActionBar actionBar = activity.getSupportActionBar();
if (actionBar != null) {
actionBar.setHomeButtonEnabled(true);
actionBar.setDisplayHomeAsUpEnabled(true);
}

return binding.getRoot();
}

@Override
public void onStart() {
super.onStart();
getPresenter().attach(this);
binding.setOrganizerPassword(getPresenter().getChangePasswordObject());
getPresenter().start();

binding.btnChangePassword.setOnClickListener(view -> {
if (!validator.validate())
return;

String url = binding.url.baseUrl.getText().toString().trim();
getPresenter().setBaseUrl(url, binding.url.overrideUrl.isChecked());
getPresenter().changePasswordRequest(binding.oldPassword.getText().toString(),
binding.newPassword.getText().toString(),
binding.confirmNewPassword.getText().toString());

});
}

When the Login button is pressed, changePasswordRequest() method is called which makes an asynchronous call to ChangePasswordModel in order to perform the task of sending and receiving data from network in a different thread than the UI thread. Along with making requests, this method also verifies the password typed in confirm password field and send the the error as well as success message to the fragment.

public class ChangePasswordPresenter extends AbstractBasePresenter<ChangePasswordView> {

public void changePasswordRequest(String oldPassword, String newPassword, String confirmPassword) {
if (!newPassword.equals(confirmPassword)) {
getView().showError(“Passwords Do Not Match”);
return;
}

organizerPasswordObject.setOldPassword(oldPassword);
organizerPasswordObject.setNewPassword(newPassword);
organizerPasswordObject.setConfirmNewPassword(confirmPassword);

changePasswordModel.changePassword(organizerPasswordObject)
.compose(disposeCompletable(getDisposable()))
.compose(progressiveErroneousCompletable(getView()))
.subscribe(() -> getView().onSuccess(“Password Changed Successfully”), Logger::logError);
}
}

We are using Retrofit to make POST Request to server using the REST API. @Body annotation denotes the object request body which here contains a Map<String, ChangePassword> object. The Response from server is captured in Observable<ChangePasswordResponse> which is an RxJava Observable.

@POST(“auth/change-password”)
Observable<ChangePasswordResponse> changePassword(@Body Map<String, ChangePassword> changePassword);

This is the declaration for the method in Network Layer where the actual network request is made. It takes as input the changePassword object from Presenter which is already binded with data. Then it uses RxJava to asynchronously call the Api class and pass in the Map<String, ChangePassword> object. The result is then processed and Completable object is returned to the presenter. The Presenter processes the Completable object and shows user feedback in the form of a message in SnackBar.

References

  1. Official documentation for RxJava by ReactiveX https://github.com/ReactiveX/RxJava
  2. Official documentation for Retrofit by Square Inc https://github.com/square/retrofit
  3. Codebase for Open Event Organizer App on Github https://github.com/fossasia/open-event-orga-app
  4. Open Event Server deployment at heroku https://open-event-api-dev.herokuapp.com/

Create Session in Open Event Android Organizer Application

Open Event Android Organizer Application offered variety of features to Organizers from all over the world to help them host their Events globally but it didn’t had the functionality to create Sessions in the app itself and associate it to Tracks. This feature addition was crucial since it enables Organizer to create Sessions which every common person enquires about before attending and event. In this Blog Post we will see how we added this functionality in the app.

Open Event Android Organizer Application is a client for Open Event Server which provides the REST API.

Problem

There can be various Sessions associated with Tracks for an Event. Open Event API had the endpoint to implement Creating Session but the Orga app didn’t, so we worked on creating a Session in the app.

The UI for creating a Session is shown above. User can fill in the necessary details and click on the green Floating Action Button to create a Session.

How to implement functionality?

We will follow MVP Architecture and use Retrofit 2.x, RxJava, Dagger, Jackson, Data Binding and other industry standard libraries to implement this functionality.

Firstly, let’s create Session model class specifying the attributes and relationships to set up in database using RazizLabs DbFlow library. The POJO will be serialized into JSON by Jackson library to be passed on as a part of RequestBody to server.

Now we will create SessionApi class that will contain the request details to be passed to Retrofit. @POST denotes a POST request and @Body denotes the requestBody of the request which is a Session object.

public interface SessionApi {
@POST(“sessions”)
Observable<Session> postSession(@Body Session session);
}

This is the CreateSessionFragment class that contains the code binding model to the view. The Fragment class implements the CreateSessionView class overriding the method declarations present there. The @Inject annotation of Dagger is used to load singleton presenter instance lazily to improve app’s performance.

Event-Id and Track-Id’s are retrieved from Bundle from Fragment Transaction. These are then passed on to presenter when Create Session button is pressed. There are other methods to show binding progressbar, snackbar and other UI components to show progress of the background request to server and database.

public class CreateSessionFragment extends BaseFragment<CreateSessionPresenter> implements CreateSessionView {

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup

container, @Nullable Bundle savedInstanceState) {
binding = DataBindingUtil.inflate(inflater,

R.layout.session_create_layout, container, false);
validator = new Validator(binding.form);

binding.sessionCreate.setOnClickListener(view -> {
if (validator.validate()) {
getPresenter().createSession(trackId, eventId);
}
});

return binding.getRoot();
}

@Override
public void onStart() {
super.onStart();
getPresenter().attach(this);
binding.setSession(getPresenter().getSession());
}
}

In the Presenter createSession method is called when create button is pressed in UI. The method attaches track-id and event-id to Session object. This is necessary for Relationship constraints on Session Model. Then after binding all the data to Session object, we pass it on to SessionRepository. The success response is provided to user by passing success response in getView().onSuccess() method.

public class CreateSessionPresenter extends

AbstractBasePresenter<CreateSessionView> {

public Session getSession() {
return session;
}

public void createSession(long trackId, long eventId) {

Track track = new Track();
Event event = new Event();

track.setId(trackId);
event.setId(eventId);
session.setTrack(track);
session.setEvent(event);

sessionRepository
.createSession(session)
.compose(dispose(getDisposable()))
.compose(progressiveErroneous(getView()))
.subscribe(createdSession ->

getView().onSuccess(“Session Created”), Logger::logError);
}
}

The SessionRepository uses RxJava to make asynchronous Retrofit Call to Server. We throw a Network Error to user if the device does not have Internet Connectivity.

The session object accepted as a parameter in createSession method is passed on to sessionApi. It will return Observable<Session> Response which we will process in doOnNext() method. Then the Session object along with required foreign key relationships with Track and Event is saved in database for offline use.

@Override
public Observable<Session> createSession(Session session) {
if (!repository.isConnected()) {
return Observable.error(new Throwable(Constants.NO_NETWORK));
}return sessionApi
.postSession(session)
.doOnNext(created -> {
created.setTrack(session.getTrack());
created.setEvent(session.getEvent());
repository
.save(Session.class, created)
.subscribe();
})
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}

The above code snippets are from Open Event Orga Application. For exploring the entire codebase please refer here. For details about the REST API used by the app please visit here.

References

  1. Official RxJava Project on Github by ReactiveX https://github.com/ReactiveX/RxJava.
  2. Official Retrofit Project on Github by Square Inc https://github.com/square/retrofit.
  3. Official Open Event Organizer App on Github by FOSSASIA https://github.com/fossasia/open-event-orga-app.
  4. Documentation for REST API of Open Event Server on Heroku by FOSSASIA https://open-event-api-dev.herokuapp.com/.

Create Call for Speakers with Open Event Organizer Android App

In The Open Event Organizer Android app, we were providing variety of features to the user but the functionality to create call for speakers for an Event was missing. This feature was required to attract speakers from all over the world to participate in the event and make it a success. Theis blog explains how we added this feature to the project us following MVVM Architecture and using libraries like Retrofit, RxJava, Db Flow etc.

Objective

The goal will be to provide an option to the organizer to create Call for Speakers for an Event (if it doesn’t exist already). We will provide the organizer a Floating Action Button in Speakers Call detail layout which will open a fragment which contain all the relevant fields provided by Open Event Server. The organizer can fill up all the fields as per requirement and create the call for speakers.

Specifications

Let’s move on to the implementation details.

First, we will create the model which will contain all the fields offered by server. We are using Lombok library to reduce boilerplate code using annotations. For example, @Data annotation is used to generate getters and setters. @NoArgsConstructor to generate no arguments constructor as the name suggests. Along with that we are using @JonNaming annotation provided by Jackson library to serialize the model using KebabCaseStrategy. Also, the Table annotation provided by Raziz DbFlow library to make a table in database for this model.

@Data
@Type(“speakers-call”)
@NoArgsConstructor
@JsonNaming(PropertyNamingStrategy.KebabCaseStrategy.class)
@Table(database = OrgaDatabase.class, allFields = true)
public class SpeakersCall {@Id(LongIdHandler.class)
@PrimaryKey
public Long id;@Relationship(“event”)
@ForeignKey(onDelete = ForeignKeyAction.CASCADE)
public Event event;

public String announcement;
public String hash;
public String privacy;
public String startsAt;
public String endsAt;
}

To handle this problem we will be using Retrofit 2.3.0 to make Network Requests, which is a REST client for Android and Java by Square Inc. It makes it relatively easy to retrieve and upload JSON (or other structured data) via a REST based Web Service. Also we will be using other awesome libraries like RxJava 2.1.10 (by ReactiveX) to handle tasks asynchronously, Jackson, Jasminb-Json-Api in an MVVM architecture.

We will make a POST request to the server . So we specify the declaration in SpeakersCallApi.java

@POST(“speakers-calls”)
Observable<SpeakersCall> postSpeakersCall(@Body SpeakersCall speakersCall);

We will use the method createSpeakersCall(SpeakersCall speakersCall) in SpeakersCallRepositoryImpl.java to interact with the SpeakersCallApi and make the network request for us. The response will be passed on to the SpeakersCallViewModel method which calls it.

@Override
public Observable<SpeakersCall> createSpeakersCall(SpeakersCall speakersCall) {
if (!repository.isConnected()) {
return Observable.error(new Throwable(Constants.NO_NETWORK));
}return speakersCallApi
.postSpeakersCall(speakersCall)
.doOnNext(created -> {
created.setEvent(speakersCall.getEvent());
repository
.save(SpeakersCall.class, created)
.subscribe();
})
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread());
}

Next, we will see the code for SpeakersCallViewModel which is using MVVM ViewModel to handle the processing logic for SpeakesCallFragment. Here we are passing SpeakersCallRepository instance in constructor. Methods like getProgress, getError and getSuccess pass on a LiveData Object from ViewModel to Fragment and the latter observes the changes on the object and show these in UI. CreateSpeakersCall method is used to call speakersCallRepository to send a POST request to server asynchronously using Retrofit and RxJava. Further, initialize method is used to set initial values of time and date to SpeakersCall object.

public class CreateSpeakersCallViewModel extends ViewModel {
public void createSpeakersCall(long eventId) {
if (!verify())
return;Event event = new Event();event.setId(eventId);
speakersCallLive.getValue().setEvent(event);

compositeDisposable.add(speakersCallRepository.createSpeakersCall(speakersCallLive.getValue())
.doOnSubscribe(disposable -> progress.setValue(true))
.doFinally(() -> progress.setValue(false))
.subscribe(var -> success.setValue(“Speakers Call Created Successfully”),
throwable -> error.setValue(ErrorUtils.getMessage(throwable))));
}
}

We will create a Fragment class to bind the UI elements to the Model class so that they can be processed by ViewModel. The creation form contains less fields therefore BaseBottomSheetFragment is used. In the onStart method, we are binding several LiveData objects like SpeakersCall, Progress, Error, Success to the UI and then observe changes in them. Using LiveData we don’t have to write extra code to handle screen rotation and to update UI when the binded object is modified.

public class CreateSpeakersCallFragment extends BaseBottomSheetFragment implements CreateSpeakersCallView {
@Override
public void onStart() {
super.onStart();
createSpeakersCallViewModel.getSpeakersCall().observe(this, this::showSpeakersCall);
createSpeakersCallViewModel.getProgress().observe(this, this::showProgress);
createSpeakersCallViewModel.getError().observe(this, this::showError);
createSpeakersCallViewModel.getSuccess().observe(this, this::onSuccess);
createSpeakersCallViewModel.initialize();
}
}

We will design the speaker create form using Two Way Data Binding to bind SpeakersCall object to the UI and use TextInputEditText to take user Input. Along with that we have used Time and Date pickers to make it easier to select Date and Time. For viewing the complete code, please refer here.

Thus, we are able to build a responsive, reactive UI using Android Architectural Components.

References

  1. Official documentation of Retrofit 2.x http://square.github.io/retrofit/
  2. Official documentation for RxJava 2.x https://github.com/ReactiveX/RxJava
  3. Official documentation for ViewModel https://developer.android.com/topic/libraries/architecture/viewmodel
  4. Codebase for Open Event Orga App https://github.com/fossasia/open-event-orga-app

Retrofit2 Rxjava2 Error Response Handling in Open Event Organizer App

In the Open Event Organizer Android app the challenge is to provide user, a readable error description for the input requests. The Organizer App was showing a coded message which was understandable only to a programmer making it unfriendly to the common user. The blog describes how we tackled this problem and implemented a mechanism to provide user friendly error messages for user requests (if any).

Let’s consider the scenario when an organizer want to create an Event or maybe a Ticket so what he has to do is fill out a form containing some user input fields and click on submit button to send the data to the server which in turn sends a response for the request. If the information is valid the server sends a HTTP 201 Response and user gets feedback that the Event/Ticket is created. But if the information is not valid the user gets feedback that there is HTTP 422 error in your request. There is no readable description of error provided to the user.

To handle this problem we will be using Retrofit 2.3.0 to make Network Requests, which is a REST client for Android and Java by Square Inc. It makes it relatively easy to retrieve and upload JSON (or other structured data) via a REST based Web Service. Further, we will be using other awesome libraries like RxJava 2.1.10 (by ReactiveX) to handle tasks asynchronously, Jackson, Jasminb-Json-Api in an MVP architecture. Let’s move on to the code.

Retrofit to the rescue

Now we will see how we can extract the details about the error response and provide user a better feedback rather than just throwing the error code.

Firstly let’s make the Request to our REST API Server using Retrofit2.

@POST(“faqs”)
Observable<Faq> postFaq(@Body Faq faq);

Here we are making a POST request and expecting a Observable<Faq> Response from the server.The  @Body annotation indicates the Request Body is an Faq object.

Please note that Observable used here is ReactiveX Observable so don’t confuse it with java.util Observable.

public class Faq {
@Id(LongIdHandler.class)
public Long id;  @Relationship(“event”)
@ForeignKey(stubbedRelationship = true,

onDelete = ForeignKeyAction.CASCADE)
public Event event;

public String question;
public String answer;
}

Let’s say the API declares both question and answer as mandatory fields  for creation of an Faq. We supply the following input to the app.

question = “Do I need to buy a Ticket to get In ?”;
answer = null

We used RxJava to make an asynchronous request to server. In case of error we will get a Retrofit throwable and we will pass that on to ErrorUtils.java which will do all the processing and return a readable error message.

faqRepository
.createFaq(faq)
.compose(dispose(getDisposable()))
.compose(progressive(getView()))  .doOnError(throwable ->

getView().showError(ErrorUtils.getMessage(throwable)))
.subscribe(createdFaq -> {
getView().onSuccess(“Faq Created”);
getView().dismiss();
}, Logger::logError);

Now we will extract the ResponseBody from the Retrofit throwable.

ResponseBody responseBody = ((HttpException) throwable)

.response().errorBody();
return getErrorDetails(responseBody);

The ResponseBody is a JSON containing all the information about the error.

{
“errors”: [
{
“status”: “422”,
“source”: {
“pointer”: “/data/attributes/answer”
},
“detail”: “Missing data for required field.”,
“title”: “Validation error”
}
],
“jsonapi”: {
“version”: “1.0”
}
}

In order to provide better feedback to user regarding the error we have to parse the response JSON and extract the pointed field (which in this case is – “answer”) and then combine it with the value corresponding to the “detail” key. Following is the relevant section from ErrorUtils.java (for viewing the complete ErrorUtils.java please refer here).

public static String getErrorDetails(ResponseBody responseBody) {
try {
JSONObject jsonObject = new JSONObject(responseBody.string());
JSONObject jsonArray = new    JSONObject(jsonObject.getJSONArray(“errors”).get(0).toString());
JSONObject errorSource =

new JSONObject(jsonArray.get(“source”).toString());

try {
String pointedField =

getPointedField(errorSource.getString(“pointer”));
if (pointedField == null)
return jsonArray.get(“detail”).toString();
else
return jsonArray.get(“detail”).toString()

.replace(“.”, “”) + “: ” + pointedField;
} catch (Exception e) {
return jsonArray.get(“detail”).toString();
}

} catch (Exception e) {
return null;
}
}

public static String getPointedField(String pointerString) {
if (pointerString == null || Utils.isEmpty(pointerString))
return null;
else {
String[] path = pointerString.split(“/”);
if (path.length > 3)
return path[path.length – 1];
else
return null;
}
}

The final error message returned by ErrorUtils in case of HTTP 422 –

Missing data for required field: answer

Similar approach can be followed for extracting the error “title” or “status” .

References

  1. Official documentation of Retrofit 2.x http://square.github.io/retrofit/
  2. Official documentation for RxJava 2.x https://github.com/ReactiveX/RxJava
  3. Codebase for Open Event Organizer App on Github https://github.com/fossasia/open-event-orga-app

Posting Tweet from Loklak Wok Android

Loklak Wok Android is a peer harvester that posts collected tweets to the Loklak Server. Not only it is a peer harvester, but also provides users to post their tweets from the app. Images and location of the user can also be attached in the tweet. This blog explains

Adding Dependencies to the project

In app/build.gradle:

apply plugin: 'com.android.application'
apply plugin: 'me.tatarka.retrolambda'

android {
   ...
   packagingOptions {
       exclude 'META-INF/rxjava.properties'
   }
}

dependencies {
   ...
   compile 'com.google.code.gson:gson:2.8.1'

   compile 'com.squareup.retrofit2:retrofit:2.3.0'
   compile 'com.squareup.retrofit2:converter-gson:2.3.0'
   compile 'com.squareup.retrofit2:adapter-rxjava2:2.3.0'

   compile 'io.reactivex.rxjava2:rxjava:2.0.5'
   compile 'io.reactivex.rxjava2:rxandroid:2.0.1'
}

 

In build.gradle project level:

dependencies {
   classpath 'com.android.tools.build:gradle:2.3.3'
   classpath 'me.tatarka:gradle-retrolambda:3.2.0'
}

 

Implementation

User first authorize the application, so that they are able to post tweet from the app. For posting tweet statuses/update API endpoint of twitter is used and for attaching images with tweet media/upload API endpoint is used.

As, photos and location can be attached in a tweet, for Android Marshmallow and above we need to ask runtime permissions for camera, gallery and location. The related permissions are mentioned in Manifest file first

<uses-permission android:name="android.permission.CAMERA" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
// for location
<uses-feature android:name="android.hardware.location.gps"/>
<uses-feature android:name="android.hardware.location.network"/>

 

If, the device is using an OS below Android Marshmallow, there will be no runtime permissions, the user will be asked permissions at the time of installing the app.

Now, runtime permissions are asked, if the user had already granted the permission the related activity (camera, gallery or location) is started.

For camera permissions, onClickCameraButton is called

@OnClick(R.id.camera)
public void onClickCameraButton() {
   int permission = ContextCompat.checkSelfPermission(
           getActivity(), Manifest.permission.CAMERA);
   if (isAndroidMarshmallowAndAbove && permission != PackageManager.PERMISSION_GRANTED) {
       String[] permissions = {
               Manifest.permission.CAMERA,
               Manifest.permission.WRITE_EXTERNAL_STORAGE,
               Manifest.permission.READ_EXTERNAL_STORAGE
       };
       requestPermissions(permissions, CAMERA_PERMISSION);
   } else {
       startCameraActivity();
   }
}

 

To start the camera activity if the permission is already granted, startCameraActivity method is called

private void startCameraActivity() {
   Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
   File dir = getActivity().getExternalFilesDir(Environment.DIRECTORY_PICTURES);
   mCapturedPhotoFile = new File(dir, createFileName());
   Uri capturedPhotoUri = getImageFileUri(mCapturedPhotoFile);
   intent.putExtra(MediaStore.EXTRA_OUTPUT, capturedPhotoUri);
   startActivityForResult(intent, REQUEST_CAPTURE_PHOTO);
}

 

If the user decides to save the photo clicked from camera activity, the photo should be saved by creating a file and its uri is required to display the saved photo. The filename is created using createFileName method

private String createFileName() {
   String timeStamp = new SimpleDateFormat("ddMMyyyy_HHmmss").format(new Date());
   return "JPEG_" + timeStamp + ".jpg";
}

 

and uri is obtained using getImageFileUri

private Uri getImageFileUri(File file) {
   if (Build.VERSION.SDK_INT < Build.VERSION_CODES.N) {
       return Uri.fromFile(file);
   } else {
       return FileProvider.getUriForFile(getActivity(), "org.loklak.android.provider", file);
   }
}

 

Similarly, for the gallery, onClickGalleryButton method is implemented to ask runtime permissions and launch gallery activity if the permission is already granted.

@OnClick(R.id.gallery)
public void onClickGalleryButton() {
   int permission = ContextCompat.checkSelfPermission(
           getActivity(), Manifest.permission.WRITE_EXTERNAL_STORAGE);
   if (isAndroidMarshmallowAndAbove && permission != PackageManager.PERMISSION_GRANTED) {
       String[] permissions = {
               Manifest.permission.WRITE_EXTERNAL_STORAGE,
               Manifest.permission.READ_EXTERNAL_STORAGE
       };
       requestPermissions(permissions, GALLERY_PERMISSION);
   } else {
       startGalleryActivity();
   }
}

 

For starting the gallery activity, startGalleryActivity is used

private void startGalleryActivity() {
   Intent intent = new Intent();
   intent.setType("image/*");
   intent.setAction(Intent.ACTION_GET_CONTENT);
   intent.putExtra(Intent.EXTRA_ALLOW_MULTIPLE, true);
   startActivityForResult(
           Intent.createChooser(intent, "Select images"), REQUEST_GALLERY_MEDIA_SELECTION);
}

 

And finally for location onClickAddLocationButton is implemented

@OnClick(R.id.location)
public void onClickAddLocationButton() {
   int permission = ContextCompat.checkSelfPermission(
           getActivity(), Manifest.permission.ACCESS_FINE_LOCATION);
   if (isAndroidMarshmallowAndAbove && permission != PackageManager.PERMISSION_GRANTED) {
       String[] permissions = {Manifest.permission.ACCESS_FINE_LOCATION};
       requestPermissions(permissions, LOCATION_PERMISSION);
   } else {
       getLatitudeLongitude();
   }
}

 

If, the permission is already granted getLatitudeLongitude is called. Using LocationManager last known location is tried to obtain, if there is no last known location, current location is requested using a LocationListener.

private void getLatitudeLongitude() {
   mLocationManager =
           (LocationManager) getActivity().getSystemService(Context.LOCATION_SERVICE);

   // last known location from network provider
   Location location = mLocationManager.getLastKnownLocation(LocationManager.NETWORK_PROVIDER);
   if (location == null) { // last known location from gps
       location = mLocationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);
   }

   if (location != null) { // last known loaction available
       mLatitude = location.getLatitude();
       mLongitude = location.getLongitude();
       setLocation();
   } else { // last known location not available
       mLocationListener = new TweetLocationListener();
       // current location requested
       mLocationManager.requestLocationUpdates("gps", 1000, 1000, mLocationListener);
   }
}

 

TweetLocationListener implements a LocationListener that provides the current location. If GPS is disabled, settings is launched so that user can enable GPS. This is implemented in onProviderDisabled callback of the listener.

private class TweetLocationListener implements LocationListener {

   @Override
   public void onLocationChanged(Location location) {
       mLatitude = location.getLatitude();
       mLongitude = location.getLongitude();
       setLocation();
   }

   @Override
   public void onStatusChanged(String s, int i, Bundle bundle) {

   }

   @Override
   public void onProviderEnabled(String s) {

   }

   @Override
   public void onProviderDisabled(String s) {
       Intent intent = new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS);
       startActivity(intent);
   }
}

 

If the user is asked for permissions, onRequestPermissionResult callback is invoked, if the permission is granted then the respective activities are opened or latitude and longitude are obtained.

@Override
public void onRequestPermissionsResult(
       int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
   boolean isResultGranted = grantResults[0] == PackageManager.PERMISSION_GRANTED;
   switch (requestCode) {
       case CAMERA_PERMISSION:
           if (grantResults.length > 0 && isResultGranted) {
               startCameraActivity();
           }
           break;
       case GALLERY_PERMISSION:
           if (grantResults.length > 0 && isResultGranted) {
               startGalleryActivity();
           }
           break;
       case LOCATION_PERMISSION:
           if (grantResults.length > 0 && isResultGranted) {
               getLatitudeLongitude();
           }
   }
}

 

Since, the camera and gallery activities are started to obtain a result i.e. photo(s). So, onActivityResult callback is called

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
   switch (requestCode) {
       case REQUEST_CAPTURE_PHOTO:
           if (resultCode == Activity.RESULT_OK) {
               onSuccessfulCameraActivityResult();
           }
           break;
       case REQUEST_GALLERY_MEDIA_SELECTION:
           if (resultCode == Activity.RESULT_OK) {
               onSuccessfulGalleryActivityResult(data);
           }
           break;
       default:
           super.onActivityResult(requestCode, resultCode, data);
   }
}

 

If the result of Camera activity is success i.e. the image is saved by the user. The saved image is displayed in a RecyclerView in TweetPostingFragment. This is implemented in onSuccessfulCameraActivityResult mehtod

private void onSuccessfulCameraActivityResult() {
   tweetMultimediaContainer.setVisibility(View.VISIBLE);
   Bitmap bitmap = BitmapFactory.decodeFile(mCapturedPhotoFile.getAbsolutePath());
   mTweetMediaAdapter.clearAdapter();
   mTweetMediaAdapter.addBitmap(bitmap);
}

 

For a gallery activity, if a single image is selected then the uri of image can be obtained using getData method of an Intent. If multiple images are selected, the uri of images are stored in ClipData. After uris of images are obtained, it is checked if more than 4 images are selected as Twitter allows at most 4 images in a tweet. If more than 4 images are selected than the uris of extra images are removed. Using the uris of the images, the file is obtained and then from file Bitmap is obtained which is displayed in RecyclerView. This is implemented in onSuccessfulGalleryActivityResult

private void onSuccessfulGalleryActivityResult(Intent intent) {
   tweetMultimediaContainer.setVisibility(View.VISIBLE);
   Context context = getActivity();

   // get uris of selected images
   ClipData clipData = intent.getClipData();
   List<Uri> uris = new ArrayList<>();
   if (clipData != null) {
       for (int i = 0; i < clipData.getItemCount(); i++) {
           ClipData.Item item = clipData.getItemAt(i);
           uris.add(item.getUri());
       }
   } else {
       uris.add(intent.getData());
   }

   // remove of more than 4 images
   int numberOfSelectedImages = uris.size();
   if (numberOfSelectedImages > 4) {
       while (numberOfSelectedImages-- > 4) {
           uris.remove(numberOfSelectedImages);
       }
       Utility.displayToast(mToast, context, moreImagesMessage);
   }

   // get bitmap from uris of images
   List<Bitmap> bitmaps = new ArrayList<>();
   for (Uri uri : uris) {
       String filePath = FileUtils.getPath(context, uri);
       Bitmap bitmap = BitmapFactory.decodeFile(filePath);
       bitmaps.add(bitmap);
   }

   // display images in RecyclerView
   mTweetMediaAdapter.setBitmapList(bitmaps);
}

 

Now, to post images with tweet, first the ID of the image needs to be obtained using media/upload API endpoint, a multipart post request and then the obtained ID(s) is passed as the value of “media_ids” in statuses/update API endpoint. Since, there can be more than one image, a single observable is created for each image. The bitmap is converted to raw bytes for the multipart post request. As the process includes a network request and converting bitmap to bytes – a heavy resource consuming task which shouldn’t be on the main thread -, so an observable is created for the same as a result of which the tasks are performed concurrently i.e. in a separate thread.

private Observable<String> getImageId(Bitmap bitmap) {
   return Observable
           .defer(() -> {
               // convert bitmap to bytes
               ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
               bitmap.compress(Bitmap.CompressFormat.JPEG, 100, byteArrayOutputStream);
               byte[] bytes = byteArrayOutputStream.toByteArray();
               RequestBody mediaBinary = RequestBody.create(MultipartBody.FORM, bytes);
               return Observable.just(mediaBinary);
           })
           .flatMap(mediaBinary -> mTwitterMediaApi.getMediaId(mediaBinary, null))
           .flatMap(mediaUpload -> Observable.just(mediaUpload.getMediaIdString()))
           .subscribeOn(Schedulers.newThread());
}

 

The tweet is posted when the “Tweet” button is clicked by invoking onClickTweetPostButton mehtod

@OnClick(R.id.tweet_post_button)
public void onClickTweetPostButton() {
   String status = tweetPostEditText.getText().toString();

   List<Bitmap> bitmaps = mTweetMediaAdapter.getBitmapList();
   List<Observable<String>> mediaIdObservables = new ArrayList<>();
   for (Bitmap bitmap : bitmaps) { // observables for images is created
       mediaIdObservables.add(getImageId(bitmap));
   }

   if (mediaIdObservables.size() > 0) {
       // Post tweet with image
       postImageAndTextTweet(mediaIdObservables, status);
   } else if (status.length() > 0) {
       // Post text only tweet
       postTextOnlyTweet(status);
   } else {
       Utility.displayToast(mToast, getActivity(), tweetEmptyMessage);
   }
}

 

Tweet containing images are posted by calling postImageAndTextTweet, once the tweet data is obtained, the data is cross posted to loklak server. The image IDs are obtained concurrently by using the zip operator.

private void postImageAndTextTweet(List<Observable<String>> imageIdObservables, String status) {
   mProgressDialog.show();
   ConnectableObservable<StatusUpdate> observable = Observable.zip(
           imageIdObservables,
           mediaIdArray -> {
               String mediaIds = "";
               for (Object mediaId : mediaIdArray) {
                   mediaIds = mediaIds + String.valueOf(mediaId) + ",";
               }
               return mediaIds.substring(0, mediaIds.length() - 1);
           })
           .flatMap(imageIds -> mTwitterApi.postTweet(status, imageIds, mLatitude, mLongitude))
           .subscribeOn(Schedulers.io())
           .publish();

   Disposable postingDisposable = observable
           .subscribeOn(Schedulers.io())
           .observeOn(AndroidSchedulers.mainThread())
           .subscribe(this::onSuccessfulTweetPosting, this::onErrorTweetPosting);
   mCompositeDisposable.add(postingDisposable);

   // cross posting to loklak server   
   Disposable crossPostingDisposable = observable
           .flatMap(this::pushTweetToLoklak)
           .subscribeOn(Schedulers.io())
           .observeOn(AndroidSchedulers.mainThread())
           .subscribe(
                   push -> {},
                   t -> Log.e(LOG_TAG, "Cross posting failed: " + t.toString())
           );
   mCompositeDisposable.add(crossPostingDisposable);

   Disposable publishDisposable = observable.connect();
   mCompositeDisposable.add(publishDisposable);
}

 

In case of only text tweets, the text is obtained from editText and mediaIds are passed as null. And once the tweet data is obtained it is cross posted to loklak_server. This is executed by calling postTextOnlyTweet

private void postTextOnlyTweet(String status) {
   mProgressDialog.show();
   ConnectableObservable<StatusUpdate> observable =
           mTwitterApi.postTweet(status, null, mLatitude, mLongitude)
           .subscribeOn(Schedulers.io())
           .publish();

   Disposable postingDisposable = observable
           .subscribeOn(Schedulers.io())
           .observeOn(AndroidSchedulers.mainThread())
           .subscribe(this::onSuccessfulTweetPosting, this::onErrorTweetPosting);
   mCompositeDisposable.add(postingDisposable);


   // cross posting to loklak server
   Disposable crossPostingDisposable = observable
           .flatMap(this::pushTweetToLoklak)
           .subscribeOn(Schedulers.io())
           .observeOn(AndroidSchedulers.mainThread())
           .subscribe(
                   push -> Log.e(LOG_TAG, push.getStatus()),
                   t -> Log.e(LOG_TAG, "Cross posting failed: " + t.toString())
           );
   mCompositeDisposable.add(crossPostingDisposable);

   Disposable publishDisposable = observable.connect();
   mCompositeDisposable.add(publishDisposable);
}

 

Resources

Data Access Layer in Open Event Organizer Android App

Open Event Organizer is an Android App for Organizers and Entry Managers. Its core feature is scanning a QR Code to validate Attendee Check In. Other features of the App are to display an overview of sales and tickets management. The App maintains a local database and syncs it with the Open Event API Server. The Data Access Layer in the App is designed such that the data is fetched from the server or taken from the local database according to the user’s need. For example, simply showing the event sales overview to the user will fetch the data from the locally saved database. But when the user wants to see the latest data then the App need to fetch the data from the server to show it to the user and also update the locally saved data for future reference. I will be talking about the data access layer in the Open Event Organizer App in this blog.

The App uses RxJava to perform all the background tasks. So all the data access methods in the app return the Observables which is then subscribed in the presenter to get the data items. So according to the data request, the App has to create the Observable which will either load the data from the locally saved database or fetch the data from the API server. For this, the App has AbstractObservableBuilder class. This class gets to decide which Observable to return on a data request.

Relevant Code:

final class AbstractObservableBuilder<T> {
   ...
   ...
   @NonNull
   private Callable<Observable<T>> getReloadCallable() {
       return () -> {
           if (reload)
               return Observable.empty();
           else
               return diskObservable
                   .doOnNext(item -> Timber.d("Loaded %s From Disk on Thread %s",
                       item.getClass(), Thread.currentThread().getName()));
       };
   }

   @NonNull
   private Observable<T> getConnectionObservable() {
       if (utilModel.isConnected())
           return networkObservable
               .doOnNext(item -> Timber.d("Loaded %s From Network on Thread %s",
                   item.getClass(), Thread.currentThread().getName()));
       else
           return Observable.error(new Throwable(Constants.NO_NETWORK));
   }

   @NonNull
   private <V> ObservableTransformer<V, V> applySchedulers() {
       return observable -> observable
           .subscribeOn(Schedulers.io())
           .observeOn(AndroidSchedulers.mainThread());
   }

   @NonNull
   public Observable<T> build() {
       if (diskObservable == null || networkObservable == null)
           throw new IllegalStateException("Network or Disk observable not provided");

       return Observable
               .defer(getReloadCallable())
               .switchIfEmpty(getConnectionObservable())
               .compose(applySchedulers());
   }
}

 

The class is used to build the Abstract Observable which contains both types of Observables, making data request to the API server and the locally saved database. Take a look at the method build. Method getReloadCallable provides an observable which will be the default one to be subscribed which is a disk observable which means data is fetched from the locally saved database. The method checks parameter reload which if true suggests to make the data request to the API server or else to the locally saved database. If the reload is false which means data can be fetched from the locally saved database, getReloadCallable returns the disk observable and the data will be fetched from the locally saved database. If the reload is true which means data request must be made to the API server, then the method returns an empty observable.

The method getConnectionObservable returns a network observable which makes the data request to the API server. In the method build, switchIfEmpty operator is applied on the default observable which is empty if reload is true, and the network observable is passed to it. So when reload is true, network observable is subscribed and when it is false disk observable is subscribed. For example of usage of this class to make a events data request is:

public Observable<Event> getEvents(boolean reload) {
   Observable<Event> diskObservable = Observable.defer(() ->
       databaseRepository.getAllItems(Event.class)
   );

   Observable<Event> networkObservable = Observable.defer(() ->
       eventService.getEvents(JWTUtils.getIdentity(getAuthorization()))
           ...
           ...

   return new AbstractObservableBuilder<Event>(utilModel)
       .reload(reload)
       .withDiskObservable(diskObservable)
       .withNetworkObservable(networkObservable)
       .build();
}

 

So according to the boolean parameter reload, a correct observable is subscribed to complete the data request.

Links:
1. Documentation about the Operators in ReactiveX
2. Information about the Data Access Layer on Wikipedia

Automatic handling of view/data interactions in Open Event Orga App

During the development of Open Event Orga Application (Github Repo), we have strived to minimize duplicate code wherever possible and make the wrappers and containers around data and views intelligent and generic. When it comes to loading the data into views, there are several common interactions and behaviours that need to be replicated in each controller (or presenter in case of MVP architecture as used in our project). These interactions involve common ceremony around data loading and setting patterns and should be considered as boilerplate code. Let’s look at some of the common interactions on views:

Loading Data

While loading data, there are 3 scenarios to be considered:

  • Data loading succeeded – Pass the data to view
  • Data loading failed – Show appropriate error message
  • Show progress bar on starting of the data loading and hide when completed

If instead of loading a single object, we load a list of them, then the view may be emptiable, meaning you’ll have to show the empty view if there are no items.

Additionally, there may be a success message too, and if we are refreshing the data, there will be a refresh complete message as well.

These use cases present in each of the presenter cause a lot of duplication and can be easily handled by using Transformers from RxJava to compose common scenarios on views. Let’s see how we achieved it.

Generify the Views

The first step in reducing repetition in code is to use Generic classes. And as the views used in Presenters can be any class such as Activity or Fragment, we need to create some interfaces which will be implemented by these classes so that the functionality can be implementation agnostic. We broke these scenarios into common uses and created disjoint interfaces such that there is little to no dependency between each one of these contracts. This ensures that they can be extended to more contracts in future and can be used in any View without the need to break them down further. When designing contracts, we should always try to achieve fundamental blocks of building an API rather than making a big complete contract to be filled by classes. The latter pattern makes it hard for this contract to be generally used in all classes as people will refrain from implementing all its methods for a small functionality and just write their own function for it. If there is a need for a class to make use of a huge contract, we can still break it into components and require their composition using Java Generics, which we have done in our Transformers.

First, let’s see our contracts. Remember that the names of these Contracts are opinionated and up to the developer. There is no rule in naming interfaces, although adjectives are preferred as they clearly denote that it is an interface describing a particular behavior and not a concrete class:

Emptiable

A view which contains a list of items and thus can be empty

public interface Emptiable<T> {
   void showResults(List<T> items);
   void showEmptyView(boolean show);
}

Erroneous

A view that can show an error message on failure of loading data

public interface Erroneous {
   void showError(String error);
}

ItemResult

A view that contains a single object as data

public interface ItemResult<T> {
   void showResult(T item);
}

Progressive

A view that can show and hide a progress bar while loading data

public interface Progressive {
   void showProgress(boolean show);
}

Note that even though Progressive view can only be the one which is either ItemResult or Emptiable as they are the ones containing any data, but we have decoupled it, making it possible for a view to load data without progress or show progress for any other implementation other than loading data.

Refreshable

A view that can be refreshed and show the refresh complete message

public interface Refreshable {
   void onRefreshComplete();
}

There should also be a method for refresh failure, but the app is under development and will be added soon

Successful

A view that can show a success message

public interface Successful {
   void onSuccess(String message);
}

Implementation

Now, we will implement the Observable Transformers for these contracts

Erroneous

public static <T, V extends Erroneous> ObservableTransformer<T, T> erroneous(V view) {
   return observable ->  observable
             .doOnError(throwable -> view.showError(throwable.getMessage()));
}

We simply call showError on a view implementing Erroneous on the call of doOnError of the Observable

Progressive

private static <T, V extends Progressive> ObservableTransformer<T, T> progressive(V view) {
   return observable -> observable
           .doOnSubscribe(disposable -> view.showProgress(true))
           .doFinally(() -> view.showProgress(false));
}

Here we show the progress when the observable is subscribed and finally, we hide it whether it succeeded or failed

ItemResult

public static <T, V extends ItemResult<T>> ObservableTransformer<T, T> result(V view) {
   return observable -> observable.doOnNext(view::showResult);
}

We call showResult on call of onNext

 

Refreshable

private static <T, V extends Refreshable> ObservableTransformer<T, T> refreshable(V view, boolean forceReload) {
   return observable ->
       observable.doFinally(() -> {
           if (forceReload) view.onRefreshComplete();
       });
}

As we only refresh a view if it is a forceReload, so we check it before calling onRefreshComplete

 

Emptiable

public static <T, V extends Emptiable<T>> SingleTransformer<List<T>, List<T>> emptiable(V view, List<T> items) {
   return observable -> observable
       .doOnSubscribe(disposable -> view.showEmptyView(false))
       .doOnSuccess(list -> {
           items.clear();
           items.addAll(list);
           view.showResults(items);
       })
       .doFinally(() -> view.showEmptyView(items.isEmpty()));
}

Here we hide the empty view on start of the loading of data and finally we show it if the items are empty. Also, since we keep only one copy of a final list variable which is also used in view along with the presenter, we clear and add all items in that variable and call showResults on the view

Bonus: You can also merge the functions for composite usage as mentioned above like this

public static <T, V extends Progressive & Erroneous> ObservableTransformer<T, T> progressiveErroneous(V view) {
   return observable -> observable
       .compose(progressive(view))
       .compose(erroneous(view));
}

public static <T, V extends Progressive & Erroneous & ItemResult<T>> ObservableTransformer<T, T> progressiveErroneousResult(V view) {
   return observable -> observable
       .compose(progressiveErroneous(view))
       .compose(result(view));
}

Usage

Finally we use the above transformers

eventsDataRepository
   .getEvents(forceReload)
   .compose(dispose(getDisposable()))
   .compose(progressiveErroneousRefresh(getView(), forceReload))
   .toSortedList()
   .compose(emptiable(getView(), events))
   .subscribe(Logger::logSuccess, Logger::logError);

To give you an idea of what we have accomplished here, this is how we did the same before adding transformers

eventsView.showProgressBar(true);
eventsView.showEmptyView(false);

getDisposable().add(eventsDataRepository
   .getEvents(forceReload)
   .toSortedList()
   .subscribeOn(Schedulers.computation())
   .subscribe(events -> {
       if(eventsView == null)
           return;
       eventsView.showEvents(events);
       isListEmpty = events.size() == 0;
       hideProgress(forceReload);
   }, throwable -> {
       if(eventsView == null)
           return;

       eventsView.showEventError(throwable.getMessage());
       hideProgress(forceReload);
   }));

Sure looks ugly as compared to the current solution.

Note that if you don’t provide the error handler in subscribe method of the observable, it will throw an onErrorNotImplemented exception even if you have added a doOnError side effect

Here are some resources related to RxJava Transformers:

Posting Scraped Tweets to Loklak server from Loklak Wok Android

Loklak Wok Android is a peer harvester that posts collected  messages to the Loklak Server. The suggestions to search tweets are fetched using suggest API endpoint. Using the suggestion queries, tweets are scraped. The scraped tweets are shown in a RecyclerView and simultaneously they are posted to loklak server using push API endpoint. Let’s see how this is implemented.

Adding Dependencies to the project

This feature heavily uses Retrofit2, Reactive extensions(RxJava2, RxAndroid and Retrofit RxJava adapter) and RetroLambda (for Java lambda support in Android).

In app/build.gradle:

apply plugin: 'com.android.application'
apply plugin: 'me.tatarka.retrolambda'

android {
   ...
   packagingOptions {
       exclude 'META-INF/rxjava.properties'
   }
}

dependencies {
   ...
   compile 'com.google.code.gson:gson:2.8.1'

   compile 'com.squareup.retrofit2:retrofit:2.3.0'
   compile 'com.squareup.retrofit2:converter-gson:2.3.0'
   compile 'com.squareup.retrofit2:adapter-rxjava2:2.3.0'

   compile 'io.reactivex.rxjava2:rxjava:2.0.5'
   compile 'io.reactivex.rxjava2:rxandroid:2.0.1'
}

 

In build.gradle project level:

dependencies {
   classpath 'com.android.tools.build:gradle:2.3.3'
   classpath 'me.tatarka:gradle-retrolambda:3.2.0'
}

 

Implementation

The suggest and push API endpoint is defined in LoklakApi interface

public interface LoklakApi {

   @GET("/api/suggest.json")
   Observable<SuggestData> getSuggestions(@Query("q") String query, @Query("count") int count);

   @POST("/api/push.json")
   @FormUrlEncoded
   Observable<Push> pushTweetsToLoklak(@Field("data") String data);
}

 

The POJOs (Plain Old Java Objects) for suggestions and posting tweets are obtained using jsonschema2pojo, Gson uses POJOs to convert JSON to Java objects.

The REST client is created by Retrofit2 and is implemented in RestClient class. The Gson converter and RxJava adapter for retrofit is added in the retrofit builder. create method is called to generate the API methods(retrofit implements LoklakApi Interface).

public class RestClient {

   private RestClient() {
   }

   private static void createRestClient() {
       sRetrofit = new Retrofit.Builder()
               .baseUrl(BASE_URL)
               // gson converter
               .addConverterFactory(GsonConverterFactory.create(gson))
               // retrofit adapter for rxjava
               .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
               .build();
   }

   private static Retrofit getRetrofitInstance() {
       if (sRetrofit == null) {
           createRestClient();
       }
       return sRetrofit;
   }

   public static <T> T createApi(Class<T> apiInterface) {
       // create method to generate API methods
       return getRetrofitInstance().create(apiInterface);
   }

}

 

The suggestions are fetched by calling getSuggestions after LoklakApi interface is implemented. getSuggestions returns an Observable of type SuggestData, which contains the suggestions in a List. For scraping tweets only a single query needs to be passed to LiquidCore, so flatmap is used to transform the observabe and then fromIterable operator is used to emit single queries as string to LiquidCore which then scrapes tweets, as implemented in fetchSuggestions

private Observable<String> fetchSuggestions() {
   LoklakApi loklakApi = RestClient.createApi(LoklakApi.class);
   Observable<SuggestData> observable = loklakApi.getSuggestions("", 2);
   return observable.flatMap(suggestData -> {
       List<Query> queryList = suggestData.getQueries();
       List<String> queries = new ArrayList<>();
       for (Query query : queryList) {
           queries.add(query.getQuery());
       }
       return Observable.fromIterable(queries);
   });
}

 

As LiquidCore uses callbacks to create a connection between NodeJS instance and Android, to maintain a flow of observables a custom observable is created using create operator which encapsulates the callbacks inside it. For a detail understanding of how LiquidCore event handling works, please go through the example. The way it is implemented in getScrapedTweets:

private Observable<ScrapedData> getScrapedTweets(final String query) {
   final String LC_TWITTER_URI = "android.resource://org.loklak.android.wok/raw/twitter";
   URI uri = URI.create(LC_TWITTER_URI);

   return Observable.create(emitter -> { // custom observable creation
       EventListener startEventListener = (service, event, payload) -> {
               service.emit(LC_QUERY_EVENT, query);
           service.emit(LC_FETCH_TWEETS_EVENT);
       };

       EventListener getTweetsEventListener = (service, event, payload) -> {
           ScrapedData scrapedData = mGson.fromJson(payload.toString(), ScrapedData.class);
           emitter.onNext(scrapedData); // data emitted using ObservableEmitter
       };

       MicroService.ServiceStartListener serviceStartListener = (service -> {
           service.addEventListener(LC_START_EVENT, startEventListener);
           service.addEventListener(LC_GET_TWEETS_EVENT, getTweetsEventListener);
       });

       MicroService microService = new MicroService(getActivity(), uri, serviceStartListener);
       microService.start();
   });
}

 

Now that we are getting suggestions and using them to get scraped tweets, this needs to be done periodically, so that tweets are pushed continuously to the loklak server. For this interval operator is used. A List is maintained which contains the suggestion queries based on which tweets are to be scraped. Once the scraping is done, the suggestion query is removed from the list when they are displayed in RecyclerView. And if the list is empty, then only a new set of suggestions are fetched.

Observable.interval(4, TimeUnit.SECONDS)
       .flatMap(this::getSuggestionsPeriodically)
       .flatMap(query -> {
           mSuggestionQuerries.add(query); // query added to list
           return getScrapedTweets(query);
       });

 

Method reference is used to maintain the modularity, so the logic of periodically fetching suggestions is implemented in getSuggestionsPeriodically

private Observable<String> getSuggestionsPeriodically(Long time) {
   if (mSuggestionQuerries.isEmpty()) { // checks if list is empty
       mInnerCounter = 0;
       return fetchSuggestions(); // new suggestions
   } else { // wait for a previous request to complete
       mInnerCounter++;
       if (mInnerCounter > 3) { // if some strange error occurs
           mSuggestionQuerries.clear();
       }
       return Observable.never(); // no observable is passed to subsriber, subscriber waits
   }
}

 

Now, it’s time to display the fetched tweets and then push the tweets to loklak server. When periodic fetching of suggestions was implemented we used interval operator and then flatMap to transform observables i.e. chaining network requests.

Till this point the observable we were creating were Cold Observable.Cold observables only emit values when a subscription is made. As we need to display scraped tweets and then push it, i.e. one source of observables and two (multiple) subscribers. By intuition the observable should be subscribed two times, for example:

Observable observable = Observable.interval(4, TimeUnit.SECONDS)
       .flatMap(this::getSuggestionsPeriodically)
       .flatMap(query -> {
           mSuggestionQuerries.add(query);
           return getScrapedTweets(query);
       });

// first time subscription
observable
       .subscribeOn(Schedulers.io())
       .observeOn(AndroidSchedulers.mainThread())
       .subscribe(
	// display in RecyclerVIew
       );


// second time subscription
observable
       .flatMap(// trnasformations to push data to server)
       .subscribeOn(Schedulers.io())
       .observeOn(AndroidSchedulers.mainThread())
       .subscribe(
	// display in number of tweets pushed
       );

 

But the source observable is cold observable i.e. it emits objects when it is subscribed to, due to which there will be two different network calls, one for first subscription and one for second subscription. So, both the subscriptions will have different data, which is not what is desired. The expected result is that there should be a single network call, and the data obtained from that call should be displayed and pushed to loklak server.

For this, hot Observables are used. Hot observables start emitting objects the moment they are created, irrespective of whether they are subscribed or not.

A cold observable can be converted to a hot observable by using publish operator and it starts emitting objects when connect operator is used. This is implemented in displayAndPostScrapedData:

ConnectableObservable<ScrapedData> observable = Observable.interval(4, TimeUnit.SECONDS)
           .flatMap(this::getSuggestionsPeriodically)
           .flatMap(query -> {
               mSuggestionQuerries.add(query);
               return getScrapedTweets(query);
           })
           .retry(2)
           .publish();

   // first time subscription to display scraped data
   Disposable viewDisposable = observable
           .subscribeOn(Schedulers.io())
           .observeOn(AndroidSchedulers.mainThread())
           .subscribe(
                   this::displayScrapedData,
                   this::setNetworkErrorView
           );
   mCompositeDisposable.add(viewDisposable);
  
   // second time subscription for pushing data to loklak
   Disposable pushDisposable = observable
           .flatMap(this::pushScrapedData) // scraped data transformed for pushing
           .subscribeOn(Schedulers.io())
           .observeOn(AndroidSchedulers.mainThread())
           .subscribe(
                   push -> {
                       mHarvestedTweets += push.getRecords();
                       harvestedTweetsCountTextView.setText(String.valueOf(mHarvestedTweets));
                   },
                   throwable -> {}
           );
   mCompositeDisposable.add(pushDisposable);

   Disposable publishDisposable = observable.connect(); // hot observable starts emitting
   mCompositeDisposable.add(publishDisposable);
}

 

The two subscriptions are made before connect operator is invoked because the hot observable emits objects due to successful network calls and network calls can’t be done on MainThread (UI Thread). So, doing the subscription before, channels the network calls to a background thread.

The scraped data is converted to JSON from objects using Gson, the JSON is converted to string and then using push API endpoint it is posted to loklak server. This is implemented in pushScrapedData method, which is used in second subscription by using method referencing.

private Observalbe<Push> pushScrapedData(ScrapedData scrapedData) throws Exception{    
    LoklakApi loklakApi = RestClient.createApi(LoklakApi.class);
    List<Status> statuses = scrapedData.getStatuses();
    String data = mGson.toJson(statuses);
    JSONArray jsonArray = new JSONArray(data);
    JSONObject jsonObject = new JSONObject();
    jsonObject.put("statuses", jsonArray);
    return loklakApi.pushTweetsToLoklak(jsonObject.toString());
}

 

Method reference for displayScrapedData and setNetworkErrorView methods are used to display the scraped data and handle unsuccessful network requests.

Only 80 tweets are preserved in RecyclerView. If number of tweets exceeds 80, then old tweets are removed.

private void displayScrapedData(ScrapedData scrapedData) {
   String query = scrapedData.getQuery();
   List<Status> statuses = scrapedData.getStatuses();
   mSuggestionQuerries.remove(query);
   if (mHarvestedTweetAdapter.getItemCount() > 80) {
       mHarvestedTweetAdapter.clearAdapter(); // old tweets removed
   }
   mHarvestedTweetAdapter.addHarvestedTweets(statuses);
   int count = mHarvestedTweetAdapter.getItemCount() - 1;
   recyclerView.scrollToPosition(count);
}

 

In case of a network error, the visibility of RecyclerView and TextView (which shows number of tweets pushed) is changed to gone and a message is displayed that there is network error.

private void setNetworkErrorView(Throwable throwable) {
   Log.e(LOG_TAG, throwable.toString());
   // recyclerView and TextView showing count of harvested tweets are hidden
   ButterKnife.apply(networkViews, GONE);
   // network error message displayed
   networkErrorTextView.setVisibility(View.VISIBLE);
}

References

Resources