避免使用 AsyncTask 泄漏活动

需要注意的是 ：AsyncTask 与此处描述的内存泄漏有很多区别。因此，请谨慎使用此 API，或者如果你不完全了解其含义，请完全避免使用此 API。有很多选择（Thread，EventBus，RxAndroid 等）。

AsyncTask 的一个常见错误是捕获对主机 Activity（或 Fragment）的强引用：

class MyActivity extends Activity {
  private AsyncTask<Void, Void, Void> myTask = new AsyncTask<Void, Void, Void>() {
    // Don't do this! Inner classes implicitly keep a pointer to their
    // parent, which in this case is the Activity!
  }
}

这是一个问题，因为 AsyncTask 可以轻松地超过父 Activity，例如，如果在任务运行时发生配置更改。

正确的方法是让你的任务成为一个 static 类，它不捕获父类，并持有对主机 Activity 的弱引用 ：

class MyActivity extends Activity {
  static class MyTask extends AsyncTask<Void, Void, Void> {
    // Weak references will still allow the Activity to be garbage-collected
    private final WeakReference<MyActivity> weakActivity;

    MyTask(MyActivity myActivity) {
      this.weakActivity = new WeakReference<>(myActivity);
    }

    @Override
    public Void doInBackground(Void... params) {
      // do async stuff here
    }

    @Override
    public void onPostExecute(Void result) {
      // Re-acquire a strong reference to the activity, and verify
      // that it still exists and is active.
      MyActivity activity = weakActivity.get();
      if (activity == null
          || activity.isFinishing()
          || activity.isDestroyed()) {
        // activity is no longer valid, don't do anything!
        return;
      }

      // The activity is still valid, do main-thread stuff here
    }
  }
}