val message:Message= Message()
val message1:Message=Message.obtain()//它利用了Message中消息池（sPool）

Handler()
Handler(callback：Handler.Callback)

override fun onDestroy() {
        super.onDestroy()
        handler.removeCallbacksAndMessages(null)
    }

class  MyHandler(activity: HandlerActivity):Handler(Looper.getMainLooper()){
        private val myWeakReference:WeakReference<HandlerActivity> = WeakReference(activity)
        override fun handleMessage(msg: Message) {//處理消息的回調(diào)方法
            myWeakReference.get()?.run {
            ...
        }
      }
 }

sendMessage（Message msg）
    ->sendMessageDelayed(msg,0)
sendEmptyMessage(int what)
    ->sendEmptyMessageDelayed(what,0)
sendEmptyMessageDelayed(what,0)//發(fā)送不帶數(shù)據(jù)的消息
     ->sendMessageDelayed(msg,delayMillis)
sendMessageDelayed(Message msg,long delayMillis)
     ->sendMessageAtTime(msg,SystemClock.uptimeMillis()+delayMillis)
sendMessageAtTime(Message msg,long uptimeMillis)
     ->enaueueMessage(queue,msg,uptimeMillis)//將消息添加到消息隊(duì)列中
enaueueMessage(MessageQueue queue,Message msg,long uptimeMillis)
     ->queue.enqueueMessage(msg,uptimeMillis)//調(diào)用消息隊(duì)列保存消息對(duì)象
removeMessage(int what)//移除消息
     ->mQueue.removeMessage(this,what,null)//調(diào)用消息隊(duì)列移除它內(nèi)部的指定what消息
handleMessage(Message msg)//處理消息的回調(diào)方法

boolean enqueueMessage(Message msg, long when) {//將Messages插入消息隊(duì)列
    ...
   msg.when = when;//指定消息應(yīng)該被處理的時(shí)間
   ...
   for (;;) {//將當(dāng)前消息對(duì)象保存到消息隊(duì)列中的一個(gè)合適的位置
            prev = p;
            p = p.next;
            if (p == null || when < p.when) {
                break;
            }
            if (needWake && p.isAsynchronous()) {
                 needWake = false;
             }
    }//最終的結(jié)果是:消息隊(duì)列是按when來排序的
    ...
    nativeWake(mPtr);//通過本地方法實(shí)現(xiàn)對(duì)處理等待狀態(tài)的底層線程
    ...
}

Message next() {//取出一個(gè)合適的Message對(duì)象，可能不會(huì)立刻返回
    ...
    nativePollOnce(ptr, nextPollTimeoutMillis);//本地方法，會(huì)導(dǎo)致可能處理等待狀態(tài)，但不會(huì)阻塞主線程
    ...
    Message msg = mMessages;//取出消息隊(duì)列中的第一個(gè)消息
    ...
    return msg;//返回
    ...
}

thread {
            Looper.prepare()
            val  handler:Handler=MyHandler(this)
            Looper.loop()
        }

 public static void loop() {
    final Looper me = myLooper();//得到looper對(duì)象
    ...
    for (;;) {//無限循環(huán)
         ...
         Message msg = me.mQueue.next(); // 從消息隊(duì)列中取出消息
         ...
         msg.target.dispatchMessage(msg);//調(diào)用Handler去分發(fā)并處理消息
         ...
         msg.recycleUnchecked();//回收利用Message
         ...
    }
}

class HandlerActivity : AppCompatActivity() {
    lateinit var handler: Handler
    lateinit var number:TextView
    lateinit var increase:Button
    lateinit var decrease:Button
    lateinit var pause:Button
    //static+弱引用
    class  MyHandler(activity: HandlerActivity):Handler(Looper.getMainLooper()){
        private val myWeakReference:WeakReference<HandlerActivity> = WeakReference(activity)
        override fun handleMessage(msg: Message) {//處理消息的回調(diào)方法
            myWeakReference.get()?.run {
                var numberint:Int=Integer.parseInt(number.text.toString())
                when(msg.what){
                    1->{
                        if(numberint==20){
                            Toast.makeText(this,"已經(jīng)達(dá)到最大值",Toast.LENGTH_SHORT).show()
                             return
                        }
                        numberint++
                        number.text= numberint.toString()
                        handler.sendEmptyMessageDelayed(1,1000)
                    }
                    2->{
                        if(numberint==1){
                            Toast.makeText(this,"已經(jīng)達(dá)到最小值",Toast.LENGTH_SHORT).show()
                            return
                        }
                        numberint--
                        number.text=numberint.toString()
                        handler.sendEmptyMessageDelayed(2,1000)
                    }
                    else -> {}
                }
            }
        }
    }
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_handler)
        number=findViewById(R.id.number)
        increase=findViewById(R.id.increase)
        decrease=findViewById(R.id.decrease)
        pause=findViewById(R.id.pause)
        handler=MyHandler(this)
        increase.setOnClickListener {
            increase.isEnabled=false
            decrease.isEnabled=true
            pause.isEnabled=true
            handler.removeMessages(2)
            handler.sendEmptyMessage(1)
        }
        decrease.setOnClickListener {
            increase.isEnabled=true
            decrease.isEnabled=false
            pause.isEnabled=true
            handler.removeMessages(1)
            handler.sendEmptyMessage(2)
        }
        pause.setOnClickListener {
            increase.isEnabled=true
            decrease.isEnabled=true
            pause.isEnabled=false
            handler.removeMessages(1)
            handler.removeMessages(2)
        }
    }
    override fun onDestroy() {
        super.onDestroy()
        handler.removeCallbacksAndMessages(null)
    }
}