// 內(nèi)部持有的變量如下：
    ThreadLocal<Looper>
    MainLooper
    Observer
    MessageQueue
    Thread

class FragmentContentActivity : AppCompatActivity() {
    val FLAG = 1
    lateinit var handler: Handler
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        handler = object : Handler(Looper.getMainLooper()) {
            override fun handleMessage(msg: Message) {
                when (msg.what) {
                    FLAG -> {
                        findViewById<TextView>(R.id.text).text = msg.data["Text"].toString()
                    }
                }
            }
        }
        thread {
            Thread.sleep(2000L)
            handler.sendMessage(Message.obtain().apply {
                what = FLAG
                data = Bundle().apply {
                    this.putString("Text", "ThreadMessage")
                }
            })
        }
    }
}

class FragmentContentActivity : AppCompatActivity() {
    val THREAD_FLAG =2
    lateinit var threadHandler: Handler
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        thread {
            Looper.prepare()
            threadHandler = object :Handler(Looper.myLooper()!!){
                override fun handleMessage(msg: Message) {
                    when(msg.what){
                        THREAD_FLAG -> {
                            Toast.makeText(
                                this@FragmentContentActivity,
                                "${msg.data["Text"]}",
                                Toast.LENGTH_SHORT
                            ).show()
                        }
                    }
                }
            }
            Looper.loop()
        }
    }

    override fun onResume() {
        super.onResume()
        findViewById<TextView>(R.id.text).postDelayed({
           threadHandler.sendMessage(Message.obtain().apply {
               what = THREAD_FLAG
               data = Bundle().apply {
                   putString("Text","UI Message")
               }
           })
        },2000L)
    }
}

// 在調(diào)用prepare()之后一定要調(diào)用loop(),最后結(jié)束消息循環(huán)的時(shí)候調(diào)用quit()
private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));
}
private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

    public static void loop(){
    
    Binder.clearCallingIdentity()
for (;;) {
    Message msg = queue.next(); // might block
 
    long origWorkSource = ThreadLocalWorkSource.setUid(msg.workSourceUid);
    try {
        // 其實(shí) loop()只做了這一個(gè)調(diào)用，其他的都是監(jiān)控當(dāng)前消息循環(huán)時(shí)間是否超時(shí)，應(yīng)該和ANR有關(guān)
        msg.target.dispatchMessage(msg);
        if (observer != null) {
            observer.messageDispatched(token, msg);
        }
        dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
    } catch (Exception exception) {
        if (observer != null) {
            observer.dispatchingThrewException(token, msg, exception);
        }
        throw exception;
    } finally {
        ThreadLocalWorkSource.restore(origWorkSource);
        if (traceTag != 0) {
            Trace.traceEnd(traceTag);
        }
    }
    if (logSlowDelivery) {
        if (slowDeliveryDetected) {
            if ((dispatchStart - msg.when) <= 10) {
                Slog.w(TAG, "Drained");
                slowDeliveryDetected = false;
            }
        } else {
            if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                    msg)) {
                // Once we write a slow delivery log, suppress until the queue drains.
                slowDeliveryDetected = true;
            }
        }
    }
    if (logSlowDispatch) {
        showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
    }
    //消息實(shí)體回收
    msg.recycleUnchecked();

  boolean enqueueMessage(Message msg,long when) //msg 元素插入隊(duì)列
  boolean hasMessages(Handler h,int what,Object object) //查找handler下的msg.what/object相同的Msg
  boolean hasEqualMessages(Handler h,int what,Object obj)//查找 msg.object.equal(obj)的msg
  removeMessages(Handler h,int what,Object obj)/(Handler h,Runnable r,Object obj)
  removeEqualMessages(...) //刪除與參數(shù)msg.object相同或equal的msg
  
  Message next() //獲取隊(duì)列中的頭部元素

Message next() ->
...

// linux機(jī)制下的總線進(jìn)入輪詢(xún)，線程相當(dāng)于掛起狀態(tài)，nextPollTimeOut是掛起多長(zhǎng)時(shí)間
nativePollOnce(ptr, nextPollTimeoutMillis);

synchronized (this) {

   final long now = SystemClock.uptimeMillis();
   Message prevMsg = null;
   Message msg = mMessages;
   //先判斷msg.target是否為null，表示當(dāng)前消息是不是異步消息
   if (msg != null && msg.target == null) {
   // Stalled by a barrier.  Find the next asynchronous message in the queue.
   //同步屏障：取出當(dāng)前隊(duì)列中的異步消息
   do {
       prevMsg = msg;
       msg = msg.next;
       } while (msg != null && !msg.isAsynchronous());
   }
   if (msg != null) {
       if (now < msg.when) {
           // Next message is not ready.  Set a timeout to wake up when it is ready.
           //重新計(jì)算線程進(jìn)入掛起狀態(tài)的時(shí)間
           nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
       } else {
           // Got a message.
               mBlocked = false;
               if (prevMsg != null) {
               prevMsg.next = msg.next;
           } else {
               mMessages = msg.next;
           }
           msg.next = null;
           if (DEBUG) Log.v(TAG, "Returning message: " + msg);
           msg.markInUse();
           return msg;
       }
   } else {
   // No more messages.
       nextPollTimeoutMillis = -1;
}
...

 // 同步屏障的方法，此方法只在 ViewRootImpl類(lèi)中調(diào)用
private int postSyncBarrier(long when) {
    // Enqueue a new sync barrier token.
    // We don't need to wake the queue because the purpose of a barrier is to stall it.
    synchronized (this) {
        final int token = mNextBarrierToken++;
        final Message msg = Message.obtain();
        msg.markInUse();
        msg.when = when;
        msg.arg1 = token;
        //沒(méi)有設(shè)置target
        Message prev = null;
        Message p = mMessages;
        if (when != 0) {
            while (p != null && p.when <= when) {
                prev = p;
                p = p.next;
            }
        }
        if (prev != null) { // invariant: p == prev.next
            msg.next = p;
            prev.next = msg;
        } else {
            msg.next = p;
        //mMessages變?yōu)橥狡琳舷ⅲ琻ext()下一次循環(huán)，首先獲取到的是同步屏障
            mMessages = msg;
        }
        return token;
    }
    // ViewRootImpl
 void scheduleTraversals() {
        if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
    
//設(shè)置同步屏障之后，通過(guò)設(shè)置了Aysnc標(biāo)記位的Handler發(fā)送的Msg都是異步消息，
//MessageQueue也優(yōu)先處理此類(lèi)異步消息，直到移除同步屏障標(biāo)記位，再恢復(fù)到普通消息隊(duì)列。

    //Handler 發(fā)送Message第一種方法，設(shè)置Message的what，data
    //不設(shè)置 runnable：Callback 
    boolean sendMessage(Message msg) -> boolean sendMessageDelayed(Message msg,long delayTime)
    -> boolean sendMessageAtTime(Message msg，SystemClock.uptimeMillis()+delayTime)
    -> mQueue.enqueueMessage(msg,uptime)
    //第二種方法，Message只設(shè)置runnable:Callback
    boolean postAtTime(Runnable r,Object token，long uptime)
    -> sendMessageAtTime(getPostMessage(r,token),uptime)
    --> Message getPostMessage(Runnable r,Object token){
        Message.obtain().callback=r
        ...
        }
   //移除Message和檢驗(yàn)Message
   removeMessages() 
   hasMessages()
   ...
   //Message 回調(diào)執(zhí)行
   void dispatchMessage(Message msg){
       if(msg.callback!=null){
           handleCallback(msg) ->
       }else{
           if(mCallback!=null){
            mCallback.handleMessage(msg)
           }
           handleMessage(msg)
   }
   //可以看到 Message的回調(diào)分為三個(gè)等級(jí)
   //No.1 msg自身的callback
   //No.2 Handler自身的mCallback成員變量，mCallback是final類(lèi)型
   //No.3 Handler的子類(lèi)重載的handleMessage方法

    int what //Handler發(fā)送主體下的Message的消息碼
    int arg1 //低成本的參數(shù)傳遞
    int arg2 
    Object obj //可以為空的token對(duì)象，一般在進(jìn)程通信中用到
    Bundle data //線程通信中常用的參數(shù)容器
    Handler target //發(fā)送主體
    Runnable callback //Message自身回調(diào)
    Messenger replyTo   //進(jìn)程通信，一般在AMS中用到
    ------
    // Message緩存池相關(guān)
    Object sPoolSync = new Object() // 同步標(biāo)記
    Messsage next 
    static Message sPool
    static int sPoolSize

    //可以看出這是一個(gè)非常巧妙的方法
    static Message obtain(){
        synchronized(sPoolsSync){
            if(sPools!=null){
                Message m= sPool;
                sPool = m.next;
                m.next = null;
                sPoolSize--;
                return m;
              }
        }
        return new Message();
    }
    //主體上是一個(gè)帶緩存池鏈表的同步工廠模式，同時(shí)也考慮到較多線程阻塞時(shí)
    //可以直接聲明初始化對(duì)象
    
    //回收Message對(duì)象到緩存池鏈表
    void recycleUnchecked(){
        ...參數(shù)=null
        synchronized(sPoolSync){
            if(sPoolSize < MAX_SIZE){
                next = sPool;
                sPools = this;
                sPoolSize++;
          }
       }
    }