Android : Wait for a background task to finish in main thread

Question

I am currently creating an Android app and encounter an issue and I haven't solved yet.

I use the Retrofit library to send requests to a server, according to the Retrofit library it is done in a background thread.

I call this Retrofit request in the main thread and I want to wait for this background thread to finish in order to work on its output.

I found a similar question on the forum but I don't know how to implement : Android: how to wait AsyncTask to finish in MainThread?

I call the request with Retrofit, when the request is finished, the success method of the Callback object starts, but main thread is still running and it reaches the last line before the background task has finished.

How can I force my main thread to wait for the background task to finish ?

My code :

    // ... (main thread)
    // CALL TO THE SERVER

    RestAdapter restAdapter = new RestAdapter.Builder()
            .setEndpoint(LoginScreen.ENDPOINT).build();
    WebServices getFiles = restAdapter.create(WebServices.class);

    Callback<Response> callback = new Callback<Response>() 
    {
        @Override
        public void success(Response s, Response response) 
        {
            fileAvailable = new String(((TypedByteArray) s.getBody()).getBytes());
            //THIS IS EXECUTED WHEN THE BACKGROUND TASK IS FINISHED
        }

        @Override
        public void failure(RetrofitError retrofitError) 
        {
            Log.e(TAG, retrofitError.toString());
        }
    };
    getFiles.getFiles(id, callback);

    // I want here to work with fileAvailable, but the code
    // reaches this line before the background task is over.

I tried to create a Handler object as in the previous link and call sendEmptyMessage in the success method but it didn't work.

I would be really grateful if somebody could help me.

Thank you in advance,

there is a way you could force your main thread to block but it's not a good idea and android system may kill your app. You could use a countdown latch or wait/notify calls to do this. — Matt Wolfe, Sep 03 '15 at 17:32

Matt Wolfe · Answer 1 · 2015-09-03T17:12:07.033

You don't want to block your main thread, that's the whole point of this library. You need to reconsider how you do this because if you block the main thread the UI will hang (be unresponsive) and the user will be upset. Think about the last time you used an app and everything locked up for several seconds. That is what happens if you block the main thread.

Instead, you should continue your logic in the response handler, or you can call into a function of the outer class from the inner class (callback is the inner class).

RestAdapter restAdapter = new RestAdapter.Builder()
        .setEndpoint(LoginScreen.ENDPOINT).build();
WebServices getFiles = restAdapter.create(WebServices.class);

Callback<Response> callback = new Callback<Response>() 
{
    @Override
    public void success(Response s, Response response) 
    {
        fileAvailable = new String(((TypedByteArray) s.getBody()).getBytes());
        doSomethingOnSuccess();
    }

    @Override
    public void failure(RetrofitError retrofitError) 
    {
        Log.e(TAG, retrofitError.toString());
    }
};

//this runs before success(), so show a spinner or something,
//eg:http://stackoverflow.com/questions/9157504/put-a-progressbar-on-actionbar

}

-- Update (per your comment about having multiple background tasks -- If you've got 2 or more background tasks running in parallel (meaning they don't depend on each others output) but you need all of them to complete before you can do anything with them, it would probably look like this:

public class MyActivity extends Activity {

  private Result result1 = null;
  private Result result2 = null;
  public void onCreate(Bundle savedState) {
    super.onCreate(savedState);   
    Callback<Response> callback1 = new Callback<Response>() {
      @Override
      public void success(Response s, Response response) 
      {
          result1 = new String(((TypedByteArray) s.getBody()).getBytes());
          checkComplete();
      }
      @Override
      public void failure(RetrofitError retrofitError) 
      {
          Log.e(TAG, retrofitError.toString());
      }
    };

    Callback<Response> callback2 = new Callback<Response>() {
      @Override
      public void success(Response s, Response response) 
      {
          result2 = new String(((TypedByteArray) s.getBody()).getBytes());
          checkComplete();
      }
      @Override
      public void failure(RetrofitError retrofitError) 
      {
          Log.e(TAG, retrofitError.toString());
      }
    };
    getFiles.getFiles(id, callback1);
    otherthing.doSomething(id, callback2);
  }

  private void checkComplete() {
    if (result1 != null && result2 != null) {
        doSomethingWithResults();
    }
  }

  private void doSomethingWithResults() {
     //update UI
  }

}

Thanks a lot for your answer, but if I have to wait for 2 of these background tasks to finish before displaying anything on the screen. How can I set something like a flag to be sure that all queries are done ? — marcsarfa, Sep 03 '15 at 16:38
Are the two background tasks dependent on each other? Meaning, do you have to wait for 1 to finish before executing another? If so you just chain them (call one when the other finishes). If they are running at the same time then I'd suggest that in the callback of each you set the result of the data to a class variable and then call into a function that will test to see if both results are complete and if so then you can continue. — Matt Wolfe, Sep 03 '15 at 17:04

score 0 · Answer 2 · answered Sep 03 '15 at 16:21

There's no "last line" for the main thread, it's ongoing. Main thread is not finished after onCreate() is done executing. Just put the code you want to execute on the data into the success handler right where

//THIS IS EXECUTED WHEN THE BACKGROUND TASK IS FINISHED

is.

Android : Wait for a background task to finish in main thread

2 Answers2