java - 等待多个 AsyncTask 完成

Question

我通过将操作拆分为确切数量的可用内核来并行化我的操作，然后启动相同数量的 AsyncTask，执行相同的操作，但对数据的不同部分执行。

我正在使用 executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, ...) 以并行执行它们。

我想知道每个线程何时完成其工作，以便合并所有结果并执行进一步的操作。

我该怎么办？

Answer 1

作为 onPostExecute 的一部分，您还可以简单地减少共享对象中的计数器。由于 onPostExecute 在同一个线程(主线程)上运行，您不必担心同步问题。

更新 1

共享对象看起来像这样:

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"));
        }
    }
}

更新 2

根据这个答案的评论，OP 正在寻找一种解决方案，他可以在其中避免构建新类。这可以通过在派生的 AsyncTask 之间共享一个 AtomicInteger 来完成:

// 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"));    
        }
    }
}