handle

handle:是发送消息，处理消息
looper:是轮询消息
messageQueen:消息队列
程序的启动有个主线程，也就是我们说的ui线程，ActivityThread线程有个main方法,在里面调用了looper的启动。
ActivityThread:

public static void main(String[] args) {
        ...
        Looper.prepareMainLooper();
        ActivityThread thread = new ActivityThread();
        thread.attach(false);
        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }
        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }
        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

ui线程是系统给我们写了looper的创建，但是我们自己写的线程是没有开启消息循环的，必须要自己调用Looper.prepare(), 然后调用Looper.loop();

   class LooperThread extends Thread {
     public Handler mHandler;
 
        public void run() {
            Looper.prepare();
  
            mHandler = new Handler() {
              public void handleMessage(Message msg) {
                   // process incoming messages here
                }
            };
  
            Looper.loop();
        }
    }

Looper是如何与线程绑定的呢？
ThreadLocal, Looper.prepare方法里面，我们看到ThreadLocal.set方法将looper传递到Threadlocal里面，这个里面存在一个类似于map结构的model，将looper与thread作为传递进去。

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

handle，looper与messagequeue是如何联系的：
handle的构造方法里面存在

public Handler(Callback callback, boolean async) {
       ....
        mLooper = Looper.myLooper(); //获取到当前线程里面的looper。
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue; //messagequeue在looper里面new出来的，可以直接获取
        mCallback = callback;
        mAsynchronous = async;
    }

Looper里面：

 public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }