Question

When I use Handler.dispatchMessage(msg), the handleMessage(Message msg) will be run on new thread but when I use Handler.sendMessage(msg), the handleMessage(Message msg) will be run on main thread. Who can tell me the difference between them?

Thanks!

Demo:

public class MainActivity extends Activity
{
private String TAG = "MainActivity";
private Handler mHandler = new Handler()
{
    @Override
    public void handleMessage(Message msg)
    {
        Log.i(TAG, "Handler:" + Thread.currentThread().getId() + " & arg1=" + msg.arg1);
        super.handleMessage(msg);
    }

};

@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    Log.i(TAG, "Main:" + Thread.currentThread().getId());
    testMethod();
}

private void testMethod()
{
    Thread thread = new Thread()
    {
        @Override
        public void run()
        {
            Log.i(TAG, "Thread:" + Thread.currentThread().getId());
            Message msg = mHandler.obtainMessage();
            msg.arg1 = 1;
            mHandler.dispatchMessage(msg);
            Message msg2 = mHandler.obtainMessage();
            msg2.arg1 = 2;
            mHandler.sendMessage(msg2);
        }

    };
    thread.start();
}

}

Output:

04-19 11:32:10.452: INFO/MainActivity(774): Main:1
04-19 11:32:10.488: INFO/MainActivity(774): Thread:8
04-19 11:32:10.492: INFO/MainActivity(774): Handler:8 & arg1=1
04-19 11:32:10.635: INFO/MainActivity(774): Handler:1 & arg1=2

Answer 1

mHandler.dispatchMessage(msg) is like directly calling handleMessage(Message msg) and I don’t know when that would be useful. The point of Handlers is the ability to send messages to other threads. That’s what you do with sendMessage.

Edit: as you can see it just calls handleMessage() for you.

/**
 * Handle system messages here.
 */
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        // callback = the Runnable you can post "instead of" Messages.
        msg.callback.run();
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}