Wait for multiple AsyncTask to complete

Question

I am parallelizing my operation by splitting it in the exact number of cores available and then, by start the same number of AsyncTask, performing the same operation but on

Answer 1

RX Merge operator is your friend.

Get rid of AsyncTark is slower than RX and you cannot handle Errors

Answer 2

First, add this class to your project

public abstract class MultiTaskHandler {
    private int mTasksLeft;
    private boolean mIsCanceled = false;

    public MultiTaskHandler(int numOfTasks) {
        mTasksLeft = numOfTasks;
    }

    protected abstract void onAllTasksCompleted();

    public void taskComplete()  {
        mTasksLeft--;
        if (mTasksLeft==0 && !mIsCanceled) {
            onAllTasksCompleted();
        }
    }

    public void reset(int numOfTasks) {
        mTasksLeft = numOfTasks;
        mIsCanceled=false;
    }

    public void cancel() {
        mIsCanceled = true;
    }
}

Then:

int totalNumOfTasks = 2; //change this to the number of tasks that you are running
final MultiTaskHandler multiTaskHandler = new MultiTaskHandler(totalNumOfTasks) {
    @Override
    protected void onAllTasksCompleted() {
       //put the code that runs when all the tasks are complete here
    }
};

Then in each task - when completed, add the line: multiTaskHandler.taskComplete();

Example:

(new AsyncTask<Void,Void,Void>() {

    @Override
    protected Void doInBackground(Void... voids) {
        // do something...
        return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        multiTaskHandler.taskComplete();
    }
}).execute();

You can use multiTaskHandler.cancel() if you want to cancel the code that runs when all the tasks have completed. For instance - if you have an error (don't forget to also cancel all the other tasks).

* This solution will not pause the main thread!

Answer 3

Another Option could be to store all your new threads in an Array.

Then you could iterate over the Array and wait with thread[i].join for the thread to finish.

see join() http://developer.android.com/reference/java/lang/Thread.html#Thread(java.lang.Runnable)

When the Iteration is finished all your threads are done and you can work on

Answer 4

You should use a CountDownLatch. Here the documentation with examples: java.util.concurrent.CountDownLatch

Basically you give a reference of CountDownLatch to your threads, and each of them will decrement it when finished:

countDownLatch.countDown();

The main thread will wait on the termination of all threads using:

countDownLatch.await();

Answer 5

You could also simply decrement a counter in a shared object as part of onPostExecute. As onPostExecute runs on the same thread (the main thread), you won't have to worry about synchronization.

UPDATE 1

The shared object could look something like this:

public class WorkCounter {
    private int runningTasks;
    private final Context ctx;

    public WorkCounter(int numberOfTasks, Context ctx) {
        this.runningTasks = numberOfTasks;
        this.ctx = ctx;
    }
    // Only call this in onPostExecute! (or add synchronized to method declaration)
    public void taskFinished() {
        if (--runningTasks == 0) {
            LocalBroadcastManager mgr = LocalBroadcastManager.getInstance(this.ctx);
            mgr.sendBroadcast(new Intent("all_tasks_have_finished"));
        }
    }
}

UPDATE 2

According to the comments for this answer, OP is looking for a solution in which he can avoid building a new class. This can be done by sharing an AtomicInteger among the spawned AsyncTasks:

// TODO Update type params according to your needs.
public class MyAsyncTask extends AsyncTask<Void,Void,Void> {
    // This instance should be created before creating your async tasks.
    // Its start count should be equal to the number of async tasks that you will spawn.
    // It is important that the same AtomicInteger is supplied to all the spawned async tasks such that they share the same work counter.
    private final AtomicInteger workCounter;

    public MyAsyncTask(AtomicInteger workCounter) {
        this.workCounter = workCounter;
    }

    // TODO implement doInBackground

    @Override
    public void onPostExecute(Void result) {
        // Job is done, decrement the work counter.
        int tasksLeft = this.workCounter.decrementAndGet();
        // If the count has reached zero, all async tasks have finished.
        if (tasksLeft == 0) {
            // Make activity aware by sending a broadcast.
            LocalBroadcastManager mgr = LocalBroadcastManager.getInstance(this.ctx);
            mgr.sendBroadcast(new Intent("all_tasks_have_finished"));    
        }
    }
}