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

Adding Transition Effect Using RxJS And CSS In Voice Search UI Of Susper

 

Susper has been given a voice search feature through which it provides a user with a better experience of search. We introduced to enhance the speech-recognition user interface by adding transition effects. The transition effect was required to display appropriate messages according to voice being detected or not. The following messages were:

  • When a user should start a voice search, it should display ‘Speak Now’ message for 1-2 seconds and then show up with message ‘Listening…’ to acknowledge user that now it is ready to recognize the voice which will be spoken.
  • If a user should do not speak anything, it should display ‘Please check audio levels or your microphone working’ message in 3-4 seconds and should exit the voice search interface.

The idea of speech UI was taken from the market leader and it was implemented in a similar way. On the homepage, it looks like this:

On the results page, it looks like this:

For creating transitions like, ‘Listening…’ and ‘Please check audio levels and microphone’ messages, we used CSS, RxJS Observables and timer() function.

Let’s start with RxJS Observables and timer() function.

RxJS Observables and timer()

timer() is used to emit numbers in sequence in every specified duration or after a given duration. It acts as an observable. For example:

let countdown = Observable.timer(2000);
The above code will emit value of countdown in 2000 milliseconds. Similarly, let’s see another example:
let countdown = Observable.timer(2000, 6000);
The above code will emit value of countdown in 2000 milliseconds and subsequent values in every 6000 milliseconds.
export class SpeechToTextComponent implements OnInit {
  message: any = ‘Speak Now’;
  timer: any;
  subscription: any;
  ticks: any;
  miccolor: any = #f44;
}
ngOnInit() {
  this.timer = Observable.timer(1500, 2000);
  this.subscription = this.timer.subscribe(t => {
  this.ticks = t;// it will throw listening message after 1.5   sec
  if (t === 1) {
    this.message = Listening;
  }// subsequent events will be performed in 2 secs interval
  // as it has been defined in timer()
  if (t === 4) {
    this.message = Please check your microphone audio levels.;
    this.miccolor = #C2C2C2;
}// if no voice is given, it will throw audio level message
// and unsubscribe to the event to exit back on homepage
  if (t === 6) {
    this.subscription.unsubscribe();
    this.store.dispatch(new speechactions.SearchAction(false));
  }
 });
}
The above code will throw following messages at a particular time. For creating the text-animation effect, most developers go for plain javascript. The text-animation effects can also be achieved by using pure CSS.

Text animation using CSS

@webkitkeyframes typing {from {width:0;}}
.spch {
  fontweight: normal;
  lineheight: 1.2;
  pointerevents: none;
  position: none;
  textalign: left;
  –webkitfontsmoothing: antialiased;
  transition: opacity .1s easein, marginleft .5s easein,                  top  0s linear 0.218s;
  –webkitanimation: typing 2s steps(21,end), blinkcaret .5s                       stepend infinite alternate;
  whitespace: nowrap;
  overflow: hidden;
  animationdelay: 3.5s;
}
@keyframes specifies animation code. Here width: 0; tells that animation begins from 0% width and ends to 100% width of the message. Also, animation-delay: 3.5s has been adjusted w.r.t timer to display messages with animation at the same time.
This is how it works now:

The source code for the implementation can be found in this pull request: https://github.com/fossasia/susper.com/pull/663

Resources: