前言

Handler是Android应用比较广泛的对象，了解Handler的作用和如何运用，对理解Android有很大的作用。

Handler是什么

A Handler allows you to send and process Message and Runnable objects associated with a thread's MessageQueue. Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler it is bound to a Looper. It will deliver messages and runnables to that Looper's message queue and execute them on that Looper's thread.
Handler允许您发送和处理与线程MessageQueue关联的消息和可运行对象。每个Handler实例都与一个线程和该线程的消息队列相关联。创建新的Handler时，它将绑定到一个Looper。它将消息和runnables传递到该Looper的消息队列，并在该Looper的线程上执行它们。

Handler的作用

There are two main uses for a Handler: (1) to schedule messages and runnables to be executed at some point in the future; and (2) to enqueue an action to be performed on a different thread than your own.
Handler有两个主要用途：（1）安排消息和runnables在将来某个时候执行；（2）安排在不同线程上执行动作。

Handler的简单运用

下面的代码片段展示Handler最常用的场景，主要作用是派发消息。下面的handler使用MainActivity的Looper，其Thread ID也和MainActivity相同。


public class MainActivity extends AppCompatActivity {
    private final static String TAG = "HandlerTest";

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

        Log.i(TAG, "onCreate: PID = " + Os.getpid() + ", TID = " + Os.gettid());


    }

    Handler mHandler = new Handler() {
        @Override
        public void handleMessage(@NonNull Message msg) {
            super.handleMessage(msg);
            Log.i(TAG, "handleMessage: msg.what = " + msg.what);
            Log.i(TAG, "handleMessage: PID = " + Os.getpid() + ", TID = " + Os.gettid());

            switch (msg.what) {
                case 1:
                    Log.i(TAG, "handleMessage: do something here");
                    break;
                default:
                    break;
            }
        }
    };


    @Override
    protected void onResume() {
        super.onResume();
        mHandler.sendEmptyMessage(1);

    }
}

创建新的Handler Thread

下面的代码创建了新的Handler Thread，注意：下面的Handler处在新的thread中，其Thread ID与MainActivity的Thread ID不同，如果需要在此Handler中操作当前Activity的UI，需要呼叫runOnUiThread(), 在runOnUiThread()中，其Thread ID与MainActivity一样。


public class MainActivity extends AppCompatActivity {
    private final static String TAG = "HandlerTest";

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

        Log.i(TAG, "onCreate: PID = " + Os.getpid() + ", TID = " + Os.gettid());


    }


    Handler backgroundHandler;

    @Override
    protected void onResume() {
        super.onResume();

        HandlerThread backgroundHandlerThread = new HandlerThread("BackgroudHandlerThread");
        backgroundHandlerThread.start();
        backgroundHandler = new Handler(backgroundHandlerThread.getLooper()) {
            @Override
            public void handleMessage(@NonNull Message msg) {
                super.handleMessage(msg);
                Log.i(TAG, "background handleMessage: msg.what = " + msg.what);
                Log.i(TAG, "background handleMessage: PID = " + Os.getpid() + ", TID = " + Os.gettid());

                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Log.i(TAG, "runOnUiThread: PID = " + Os.getpid() + ", TID = " + Os.gettid());
                    }
                });
            }
        };

        backgroundHandler.sendEmptyMessage(2);
    }
}