从 doInBackground() 中调用 AsyncTask.cancel(true) 后，Android 不会调用 onCancelled()，而是调用 onPostExecute()。但是as per the documentation :

Calling this method will result in onCancelled(Object) being invoked on the UI thread after doInBackground(Object[]) returns. Calling this method guarantees that onPostExecute(Object) is never invoked.

这是 Android 中的错误吗？

更多观察:

1. 从任一线程调用 cancel(false) 都按照 文档。
2. 从 UI 任务调用 cancel(true) 不会调用 onPostExecute()，也不会抛出 InterruptedException 在下面的 logcat 跟踪中看到。
3. 从任何线程调用 cancel(false/true) 有时甚至在 doInBackground() 返回之前调用 onCancelled()。这显然违反了文档，which states :

Calling this method will result in onCancelled(Object) being invoked on the UI thread after doInBackground(Object[]) returns.

代码:(在 Android 2.2 设备上测试)

protected Void doInBackground(Void... params) {
    Log.d(TAG, "started doInBackground()");
    while (!isCancelled()) {
        boolean ret = cancel(true);
        Log.d(TAG, "cancel() returned: " + ret);
    }
    Log.d(TAG, "returning from doInBackground()");
    return null;
}

Logcat 输出

04-15 21:38:55.519: D/MyTask(27597): started doInBackground()
04-15 21:38:55.589: W/AsyncTask(27597): java.lang.InterruptedException
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1254)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:219)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.FutureTask.get(FutureTask.java:82)
04-15 21:38:55.589: W/AsyncTask(27597):     at android.os.AsyncTask$3.done(AsyncTask.java:196)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.FutureTask$Sync.innerCancel(FutureTask.java:293)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.FutureTask.cancel(FutureTask.java:75)
04-15 21:38:55.589: W/AsyncTask(27597):     at android.os.AsyncTask.cancel(AsyncTask.java:325)
04-15 21:38:55.589: W/AsyncTask(27597):     at com.example.test.TestActivity$MyTask.doInBackground(TestActivity.java:31)
04-15 21:38:55.589: W/AsyncTask(27597):     at com.example.test.TestActivity$MyTask.doInBackground(TestActivity.java:1)
04-15 21:38:55.589: W/AsyncTask(27597):     at android.os.AsyncTask$2.call(AsyncTask.java:185)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.FutureTask.run(FutureTask.java:137)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1068)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:561)
04-15 21:38:55.589: W/AsyncTask(27597):     at java.lang.Thread.run(Thread.java:1096)
04-15 21:38:55.589: D/MyTask(27597): cancel() returned: true
04-15 21:38:55.589: D/MyTask(27597): returning from doInBackground()
04-15 21:38:55.659: D/MyTask(27597): onPostExecute()

最佳答案

1. 出现异常是因为您调用了 cancel(true)，它向运行 doInBackground() 的线程发送中断 - 但是，在这种情况下，您是从 doInBackground() 内部调用 cancel(true)，从而导致线程立即向自身发送中断。

2. 您的代码在 Android 2 上运行，但您引用的是 Android 4 的文档。问题是 cancel() 的行为在 Android 2 和 Android 4 之间发生了变化。

   Android 2.3.7 onPostExecute :

   Runs on the UI thread after doInBackground. The specified result is the value returned by doInBackground or null if the task was cancelled or an exception occured.

   Android 4.0.1 onPostExecute :

   Runs on the UI thread after doInBackground. The specified result is the value returned by doInBackground. This method won't be invoked if the task was cancelled.

关于android - AsyncTask 取消自身仍然调用 onPostExecute()，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/10163763/