I want to use a progress bar with RxJava like you can with AsyncTask

Question

I want to replace my AsyncTask with RxJava in android. My current AsyncTask goes like this:

public class ProgressBarAsyncTask extends AsyncTask<Void,Void,Void> { 

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        ringProgressDialog = ProgressDialog.show(context,"MyProgressBarTitle","Working please wait",true, false);                  
    }

    @Override
    protected Void doInBackground(Void... void) { 
       //do work
       myTask();
       return null;
    }

    @Override
    protected void onPostExecute(Void void) {
        super.onPostExecute();
        ringProgressDialog.dismiss();
    }
}

Here's my RxJava replacement:

    public static Observable<Void> getObservable(final Context context,final String... params) {

       return Observable.defer(new Func0<Observable<Void>>() {
          @Override
          public Observable<Void> call() {
             return Observable.just(myTask());
          }
       });

   }

    public static Subscriber<Void> getSubscriber() {
    Subscriber<Void> subscriber = new Subscriber<Void>() {
        @Override
        public void onCompleted() {
           ringProgressDialog.dismiss();

        }

        @Override
        public void onError(Throwable e) {
            Log.d(TAG,e.toString());

        }

        @Override
        public void onNext(Void aVoid) {
             manipulateData();

        }
    };
    return subscriber;
}

My Activity:

public class MainActivity extends Activity { 
   private ProgressDialog ringProgressDialog;

   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      GetNumberObservable.Observable()
                       .subscribeOn(Schedulers.io())
                       .observeOn(AndroidSchedulers.mainThread()))
                       .subscribe(getSubscriber());
   }
}

How do I mimic the onPreExecute() method in the AsyncTask where I kick off the progressDialog?

Answer 1

• In RxJava you have the do operators, that creates Observable lifecycle events listeners, in your case you want to do something (update the UI) before the task started, which mean you want the doOnSubscribe event. (side note it is true with 'cold' Observables that started thier work when subscrbied to - like your case) Just beware to call .observeOn(AndroidSchedulers.mainThread())) before the doOnSubscribe in order to get notified on the mainThread, as you're updating the UI.

• Instead of using both defer and just

  return Observable.defer(new Func0<Observable<Void>>() {
       @Override
       public Observable<Void> call() {
          return Observable.just(myTask());
       }
    });  
  

  you can use fromCallable:

      Observable.fromCallable(new Callable<Object>() {
         @Override
         public Object call() throws Exception {
             return myTask();
         }
     })