java - Android Handler.handleMessage() 不会被调用

Question

我试图理解这个版本的 Message.obtain() http://developer.android.com/reference/android/os/Message.html#obtain%28android.os.Handler,%20java.lang.Runnable%29

肯定会调用传递给 Message.obtain() 的 Runnable，但未调用定义的 Handler.handleMessage()(在 msg.sendToTarget() 或什至 mHandler.sendMessage(msg) 上)

Handler mHandler;

class MyThread implements Runnable {

  @Override
  public void run() {
    Message msg = Message.obtain(mHandler, new Runnable() {
        @Override
        public void run() {
            System.out.println("This is printed for sure"); // This is printed for sure
        }
    });

    msg.obj = "My message!";

    msg.sendToTarget();
  }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  mHandler = new Handler() {
      @Override
      public void handleMessage(Message msg) {
          System.out.println(msg.obj); // This is never printed
      }
  };

  Thread t = new Thread(new MyThread());
  t.start();
}

Answer 1

让我们来看看 Message 类中的这个方法:

public Runnable getCallback()

Retrieve callback object that will execute when this message is handled. This object must implement Runnable. This is called by the target Handler that is receiving this Message to dispatch it. If not set, the message will be dispatched to the receiving Handler's handleMessage(Message).

以及您为获取消息

而调用的内容

public static Message obtain (Handler h, Runnable callback)

所以因为你设置的回调 handleMessage(Message) 没有被调用 :-)