handle

handle:是發送消息，處理消息
looper:是輪詢消息
messageQueen:消息隊列
程序的啓動有個主線程，也就是我們說的ui線程，ActivityThread線程有個main方法,在裏面調用了looper的啓動。
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線程是系統給我們寫了looper的創建，但是我們自己寫的線程是沒有開啓消息循環的，必須要自己調用Looper.prepare(), 然後調用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是如何與線程綁定的呢？
ThreadLocal<Looper>, Looper.prepare方法裏面，我們看到ThreadLocal.set方法將looper傳遞到Threadlocal裏面，這個裏面存在一個類似於map結構的model，將looper與thread作爲<k,value>傳遞進去。

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與messagequeue是如何聯繫的：
handle的構造方法裏面存在

public Handler(Callback callback, boolean async) {
       ....
        mLooper = Looper.myLooper(); //獲取到當前線程裏面的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裏面new出來的，可以直接獲取
        mCallback = callback;
        mAsynchronous = async;
    }

Looper裏面：

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