Serializing Java objects for REST API Requests in Open Event Organizer App

Open Event Organizer App is a client side application which uses REST API for network requests. The server supports sending and receiving of data only in JSONAPI spec, so, we needed to serialize java models into JSON objects and deserialize JSON data into java models following JSONAPI spec. To achieve this we followed the following steps.


We will be using jasminb/jsonapi-converter which handles request/response parsing of models following JSONAPI Spec and Retrofit plugin of jackson converter to serializing JSON to Java Models and vice versa.

Let’s create a java model. We are using some annotations provided by Lombok library to avoid writing boilerplate code. @JsonNaming annotation is used to apply KebabCaseStrategy while serializing fields

@Table(database = OrgaDatabase.class, allFields = true)
public class Order {

public Long id;

public float amount;
public String completedAt;
public String identifier;
public String paidVia;
public String paymentMode;
public String status;

@ForeignKey(stubbedRelationship = true, onDelete = ForeignKeyAction.CASCADE)
public Event event;

public Order() { }

In the NetworkModule class, there is a method providesMappedClasses() containing a list of classes that needs to be serialized/deserialized. We need to add the above model in the list. Then, this list is provided to Singleton instance of JSONAPIConvertorFactory through Dagger. JSONAPIConvertorFactory uses the Retrofit ObjectMapper and maps the classes that are handled by this instance.

Class[] providesMappedClasses() {
return new Class[]{Event.class, Attendee.class, Ticket.class, Order.class};

Further, various serialization properties can be used while building Singleton ObjectMapper instance. Adding any properties here ensures that these are applied to all the mapped classes by JSONAPIConvertorFactory. For eg, we are using the serialization property to throw an exception and fail whenever empty beans are encountered.

ObjectMapper providesObjectMapper() {
return new ObjectMapper()
// Handle constant breaking changes in API by not including null fields
// TODO: Remove when API stabilizes and/or need to include null values is there


  1. Github Repository for jsonapi-converter
  2. Github repository for Jackson Retrofit Plugin
  3. Official Website for Project Lombok

Github Repository for Open-Event-Orga-App

Adding Preference Settings using Preference Fragment Compat

It is very much likely that one needs to add preferences to their app which span the entire application and therefore can be accessed anywhere in the app without storing anything in database or making global variables. For an instance, in Open Event Organizer App we added the preferences to store the privacy policy, cookie policy etc. The user can access these items in Settings Preference which in device settings. In this blog post we will see how to add preference settings to the app by storing the data in shared preferences.


The benefit of storing the data in shared preference and not in local storage is that the access time for the data is drastically reduced and the data persists even when the app is closed. We will use this library which is built on top of official preference-v7 library.

Firstly, we will make a preference resource layout file and add the preference for privacy policy and cookie policy in the preference screen.

<PreferenceScreen xmlns:android=””>

android:title=”@string/privacy_policy” />

android:title=”@string/cookie_policy” />


Make a separate preference fragment class named LegalPreferenceFragment which extends PreferenceFragmentCompat. Then we will override onCreatePreferenceFix() method.

Inside this, we will create an instance of Preference Manager and set shared preference name for it and set the preference using the layout file. This enables us to use findPreference() method to retrieve the layout preferences by their key. After, retrieving the preference we will set onClick listener to launch activity with an intent to open browser for the url passed in data bundle.

public void onCreatePreferencesFix(@Nullable Bundle bundle, String rootKey) {
PreferenceManager manager = getPreferenceManager();

setPreferencesFromResource(R.xml.legal_preferences, rootKey);

findPreference(getString(R.string.privacy_policy_key)).setOnPreferenceClickListener(preference -> {
BrowserUtils.launchUrl(getContext(), PRIVACY_POLICY_URL);
return true;
findPreference(getString(R.string.cookie_policy_key)).setOnPreferenceClickListener(preference -> {
BrowserUtils.launchUrl(getContext(), COOKIE_POLICY_URL);
return true;


  1. Preference Fragment Compat library by Takisoft
  2. Android Preference Documentation

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.


We will create a separate class 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);


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.

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.
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.

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


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.

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


  1. Codebase for Open Event Organizer App
  2. Official documentation for ItemTouchHelper.SimpleCallback

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.


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.

public class ChangePassword {

public String oldPassword;
public String newPassword;

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 {

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());

ActionBar actionBar = activity.getSupportActionBar();
if (actionBar != null) {

return binding.getRoot();

public void onStart() {

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

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


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”);


.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.

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.


  1. Official documentation for RxJava by ReactiveX
  2. Official documentation for Retrofit by Square Inc
  3. Codebase for Open Event Organizer App on Github
  4. Open Event Server deployment at heroku

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.


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 {
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 {

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();

public void onStart() {

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();


.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.

public Observable<Session> createSession(Session session) {
if (!repository.isConnected()) {
return Observable.error(new Throwable(Constants.NO_NETWORK));
}return sessionApi
.doOnNext(created -> {
.save(Session.class, created)

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.


  1. Official RxJava Project on Github by ReactiveX
  2. Official Retrofit Project on Github by Square Inc
  3. Official Open Event Organizer App on Github by FOSSASIA
  4. Documentation for REST API of Open Event Server on Heroku by FOSSASIA

Adding Events’ Payment Preferences to Eventyay Organizer Android App

The Open Event Organizer Android App allows creating events from the app itself. But organizers had to enter the payment information every time. To solve this problem, the PR#1058 was opened which saves the Organizers’ payment preferences in Event Settings.

The Open Event project offers 5 types of payment options:

1. Paypal
2. Stripe

3. Cash payment
4. Bank Transfer
5. Cheques

Each of the above need the payment specific details to be saved. And stuffing all of them into a single Event Settings screen isn’t a good option. Therefore the following navigation was desired:

Event Settings -> Payment Preferences -> All options with their preferences

Android Developer guide references a simple method to achieve the above, which is by using nested preference screens. But unfortunately, there’s a bug in the support library and it cannot be implemented with  PreferenceFragmentCompat

So we had to apply a hack to the UI. We set an OnPreferenceClickListener as follows:

public class EventSettingsFragment extends PreferenceFragmentCompat {
   public void onCreatePreferencesFix(@Nullable Bundle bundle, String rootKey) {

       findPreference(“payment_preferences”).setOnPreferenceClickListener(preference -> {
           FragmentTransaction transaction = getFragmentManager().beginTransaction();
               .replace(, PaymentPrefsFragment.newInstance())
           return true;

Once the preference item “Payment Preferences” is clicked, we initiate a fragment transaction opening the Payment Preferences screen, and add it to the fragment back stack.

For each payment option, we have two things to consider:

  1. Is that payment option supported by the organizer?
  2. If yes, we need to store the necessary details in order to direct the payment to the organizer.

We are also keeping track of whether the organizer wants to keep using the same payment preferences for future events as well. This way we save the organizer’s effort of entering payment details for each new event.

<?xml version=“1.0” encoding=“utf-8”?>
<PreferenceScreen xmlns:android=“”>

       android:defaultValue=“false” />

       android:title=“Bank Transfer”>

           android:title=“@string/bank_details” />


Now the only thing remaining is to set payment preferences once the Event Creation form is opened. Hence the following method is called in  CreateEventPresenter  sets the payment details for each option that the organizer has already saved the information for. It does this by using constants named like PREF… all declared in the  file.

using a custom Preference class which abstracts away some boilerplate code for us.

   public void setPaymentPreferences(Preferences preferences) {

       if (preferences.getBoolean(PREF_USE_PAYMENT_PREFS, false)) {
               preferences.getBoolean(PREF_ACCEPT_BANK_TRANSFER, false)
               preferences.getString(PREF_BANK_DETAILS, null)

This is how the result looks like:


Implement Tests for Feedback List in Open Event Orga App

In the Open Event Orga App test have been written for all the presenters and viewmodel classes to ensure that the implemented functionalities work well.

In the following blog post I have discussed one particular test which I implemented which is the FeedbackList Presenter Test.


  1. Instantiation of the variables.
public MockitoRule mockitoRule = MockitoJUnit.rule();
public FeedbackListView feedbackListView;
public FeedbackRepository feedbackRepository;

We should first know the meaning of the Annotations being used:

@Rule : It tells mockito to create the mocks based on the @Mock annotation. This annotation always needs to be used.

@Mock: It tells Mockito to mock the FeedbackListView interface and FeedbackRepository class.

Here we are mocking 3 classes namely: MockitoRule, FeedbackListView, FeedbackRepository.

Before moving forward we first need to understand the meaning of Mock. A mock object is a dummy implementation for an interface or a class in which you define the output of certain method calls. Mock objects are configured to perform a certain behavior during a test. They typically record the interaction with the system and tests can validate that.


private static final List<Feedback> FEEDBACKS = Arrays.asList(

The list of feedbacks is populated with demo values which can be used for testing purpose later.

2) The @Before annotation is applied before the set up. Before any tests are created, the setUp( ) is executed. A feedbackListPresenter object is created and the required parameters are passed. The RxJava Plugin’s setIoSchedulerHandler, setComputationSchedulerHandler and setInitmainThreadSchedulerHandler use the Scheduler.Trampoline( ) .  It lets the internal call Observable call to end before asserting the result.

setIOSchedulerHandler( ) -> It basically is a type of Scheduler which handles the Input and Output of the RxJava code.

setComputationSchedulerHandler( ) -> It is another Scheduler which handles the computations which are carried out during call to RxJava methods.

setInitMainThreadSchedulerHandler( ) -> It is called to notify the Scheduler that the IO operations would be carried out on the main thread.

public void setUp() {
  feedbackListPresenter = new FeedbackListPresenter(feedbackRepository);
  feedbackListPresenter.attach(ID, feedbackListView);

  RxJavaPlugins.setIoSchedulerHandler(scheduler -> Schedulers.trampoline());
  RxJavaPlugins.setComputationSchedulerHandler(scheduler -> Schedulers.trampoline());
  RxAndroidPlugins.setInitMainThreadSchedulerHandler(schedulerCallable -> Schedulers.trampoline());

Some of the tests are discussed below:

→  The following test is written to ensure that the feedback list gets updated automatically after a feedback is received.

public void shouldLoadFeedbackListAutomatically() {
  when(feedbackRepository.getFeedbacks(anyLong(), anyBoolean())).thenReturn(Observable.fromIterable(FEEDBACKS));


  verify(feedbackRepository).getFeedbacks(ID, false);

As can be seen above , I have used the when and return functionality of Mockito. It is basically used to check the return type of the object. So when the required parameters are passed in the getFeedback( ) , then the return type of what is expected is mentioned in the thenReturn( ).

verify ensures that the getFeedback( ) is called on the feedbackfeedbackRepository mock only.

→ The following test is written to ensure that there is an error message on loading data after swipe refresh is made. Firstly the list of feedbacks is fetched from the feedbackRepository with the help of getFeedbacks( ) where the parameters event id and the boolean variable true are passed. Then the thenReturn( ) has the statement Observable.error(Logger.TEST_ERROR) which is actually written to specify the expected result we want i.e in this case we are expecting the TEST_ERROR statement as a response and hence it is written before.

At the end it is verified using the statement verify statement where the feedbackListView is passed and the error is captured.


public void shouldShowErrorMessageOnSwipeRefreshError() {
  when(feedbackRepository.getFeedbacks(ID, true)).thenReturn(Observable.error(Logger.TEST_ERROR));



3) After the tests have been applied, the RxJava plugins are reset.

public void tearDown() {



→ Mockito tutorial :

→ Testing in RxJava:

Using Two-Way Data Binding in Open Event Organizer Android App:

Data Binding is the simple approach which relieves developers of repeated findViewById() calls. It is something that every developer must use if not using ButterKnife.

The Open Event Organizer Android App provides options to fill in an extensive set of details while creating an event, or any other entities. The problem at hand is that many of these options are common to many of these entities. For instance, currently the element date-time-picker and text fields are common to elements of different forms, as each one of them requires date-time checkboxes.

We need to be able to <include> a separate smaller and reusable layout file and bind event data to make the code shorter. This would help decreasing unnecessary code base and improving code readability.

We will see how using 2 way data binding and <include> tags in the small PR #929 reduced the code of 112 lines to just 9 lines:

Step 1: Configuration:

The very first step is to configure your project to enable data bindings in your
build.gradle (Module:app) file.

dataBinding should be included as follows:

android {
   // Rest of gradle file…
   dataBinding {
   enabled true
   }    // Rest of gradle file…

Step 2: Import and variable tags:

Data Binding uses the tag <data> to signify the data which will be referred to in lambda expressions inside the XML.

We also need to import any class, whose methods we need to use. This can be done using the <import> tag.

Finally, the <variable> tag is used to define any variables that will be referenced in the XML.


  <import type=”android.view.View” />
      type=”String” />

Step 3: Binding the declared variables:

Data binding recognizes methods of the type set<variable>, where <variable> is event in our case.

We need to use  executePendingBindings();  so that any pending bindings are done and the UI of our app responds correctly as soon as the view data is updated.

public void showResult(Event event) {

Step 4: Using the declared variables:

Making use of the declared variables is a very simple task and is as simple as a java statement. You can do almost everything that’s possible in the java file, the only constraint being that the used variables are declared in the xml and binded appropriately.

Most of the data binding expressions use data binding to condense the expression to its smallest possible form.

  android:visibility=“@{ picker.checked ? View.VISIBLE : View.GONE }”>

2 Way Data Binding

In case of the Organizer App, we are using 2 way data binding.

Data Binding allows us to do much more than just set text in TextView or create listener in Button. If we want to use EditText and automatically update text variable in java code, we need to use observable fields and two way binding.

Thus, most variables like date, event that we are binding, are Observable fields.

* Sometimes there’s a use case of using a variable declared in another layout file.

For example, in:

  app:value=“@={ date }” />

The variable date isn’t binded in the java file but the xml files which include the layout time_picker.xml

Using the <include> tag:

The include tag is very simple to use, and we can simply bind the date and label element. The event_create_form.xml binds the variable using the bind attribute like this:

  bind:date=“@={ event.startsAt }”
  bind:label=“@{ @string/starts_at }”/>

The most common error you will face:

Often, when there’s something wrong with the XML, the most common error you will face is:

“Cannot resolve Data Binding class…”

This error is because Android Studio couldn’t generate the Data Binding class for your XML file because of some error. Presently, it doesn’t give much details about what’s wrong, so you’ll have to look for the errors yourselves.

The most common mistake newbie developers make is forgetting to bind the variables appropriately.


Android Developer Guide:

Handling Click Events using Custom Binding Adapters

The Open Event Organiser Android App is the Event management app for organizers using the Open Event Platform. It is currently released in the Alpha phase on the Google Play Store here and is being actively developed by the community.

The Data Binding Library is one of the most popular libraries among the android developers. We use it extensively in the application in order to greatly simplify the UI binding logic. While trying to show the details of a speaker in the application, we wanted to list his/her social media links using Image buttons.

Upon clicking one of these buttons, the user was supposed to be directed to the link after opening the default web browser. This blog post discusses how we used custom Binding Adapters to handle click events on an Image Button by defining a custom attribute.

Defining the Binding Adapter

We defined a simple Binding Adapter for an Image button meant to handle social media links. We used “imageOnClick” as the custom attribute name for specifying the URL that will be opened once the button is clicked.

public static void bindOnImageButtonClickListener(ImageButton imageButton, String url) {
  imageButton.setOnClickListener(view -> {
    if (url != null) {
      Context context = imageButton.getContext();
      Intent intent = new Intent(Intent.ACTION_VIEW);
      if (intent.resolveActivity(context.getPackageManager()) != null) {
      } else {
        Toast.makeText(context, "No Web browser found", Toast.LENGTH_SHORT).show();


The method can be named anything you want and can be placed anywhere in the project but we would recommend creating a separate class for all the Binding adapters.
The important things to take away from the above method are:

  • The method needs to be public otherwise the Data binding framework won’t be able to find it.
  • We need to pass in the view as the first parameter and the attribute value as the second parameter.

Then we simply set the familiar click listener to handle the click interaction. We use the Context from the view passed in the method as the first parameter. Then we create an Intent and set the passed in URL as the data. We make sure that the user has a browser installed on his/her android phone before we try to open the browser. We show a suitable error message if they don’t.

Using it in Layout

Using the custom attribute in the layout was extremely simple. We specified the url using the attribute “imageOnClick” and the rest was handled by the Binding Adapter and the Data binding framework.  

     app:imageOnClick="@{ speaker.linkedin }"
     android:visibility="@{ (speaker.linkedin != null) ? View.VISIBLE : View.GONE }"


Create an AutocompleteTextView dropdown for the email input in the Open Event Orga Android App

In the first version of the Open Event Organizer App, the event organizer was required to enter his full email each time he logged out of his account and therefore it was hindering the user experience. AutoCompleteTextView with shared preferences is a solution to this problem. This feature provides an editable text view that shows completion suggestions automatically while the user is typing. The list of suggestions is displayed in a drop down menu. The user can choose an item to replace the content of the edit box with. It is extremely useful in enhancing user experience.

The solution we implemented was to create an autocomplete textview for the email input, store the email address of the user on a successful login in the shared preference in a set of strings to prevent duplicacy and display it in the dropdown on subsequent login attempts.


Change your TextInputLayout structure to accommodate the autocompletetextview. Remember to create a separate autocompletetextview object with the specific id of the view.

       android:inputType="textEmailAddress" />


Create Utility methods to get/store the emails in the shared preferences. The set data structure has been used here so that there is no duplicacy while storing the emails in the shared preferences.

public Set<String> getStringSet(String key, Set<String> defaultValue) {
   return sharedPreferences.getStringSet(key, defaultValue);

public void saveStringSet(String key, Set<String> value) {
   SharedPreferences.Editor editor = sharedPreferences.edit();
   editor.putStringSet(key, value);

public void addStringSetElement(String key, String value) {
   Set<String> set = getStringSet(key, new HashSet<>());
   saveStringSet(key, set);


Create helper methods to add an email and retrieve the list of emails from the shared preferences to provide it to the views.

private void saveEmail(String email) {
   utilModel.addStringSetElement(Constants.SHARED_PREFS_SAVED_EMAIL, email);

private Set<String> getEmailList() {
   return utilModel.getStringSet(Constants.SHARED_PREFS_SAVED_EMAIL, null);


Create an autocompleteTextView object in your activity with the help of the R id from the layout and set the adapter with the set of strings retrieved from the shared preferences. You could create a custom adapter for this case too, but as far as the Open Event Orga App was concerned, using the array adapter made sense.

autoCompleteEmail.setAdapter(new ArrayAdapter<>(this, android.R.layout.simple_list_item_1,
   new ArrayList<String>(emails)));



It is important that the user is served with the best possible experience of the application and the autocomplete text view for the email serves just that.