Handler原理简述

只做简单概述,具体详细细节可查看相关源码

1.handler发送消息根据时间先后顺序插入到消息队列

  public final boolean sendMessageDelayed(@NonNull Message msg, long delayMillis) {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        //根据消息具体的执行时间点排序
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

2.Looper.loop()不断从消息队列中取消息

 for (;;) {
            //不断从消息队列中取消息 queue.next()会阻塞
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }
            ...
            ...
            //取消消息后回调 handler分发消息 msg.target是发送消息的handler
            msg.target.dispatchMessage(msg);
            ...
            ...
        }
    }

3.MessageQueue.next() 取消息，重点分析

  Message next() {
        //mPtr 相当于一把锁
        final long ptr = mPtr;
        if (ptr == 0) {
            return null;
        }

        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        //距离最近的消息需要等待的时间
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
            //等待 nextPollTimeoutMillis
            nativePollOnce(ptr, nextPollTimeoutMillis);

            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                
                //重点 如果消息头是一个屏障消息(标志：msg.target == null)
                
                if (msg != null && msg.target == null) {
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    //查找下一个 msg.isAsynchronous() = true 的消息,如果没有 msg =null
                    //将会等待进入休眠状态,不会处理msg.isAsynchronous() = false的消息
                    //所以当消息头为屏障消息是，只会处理异步消息，之后的同步消息和屏障消息都不处理
                    //如果没有异步消息将会进入休眠状态
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());
                }
                //msg != null 说明头部不是屏障头,或者是屏障消息头且找到了异步消息
                if (msg != null) {
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) { 
                            //如果是消息头是屏障消息，队列移除找到的异步消息，消息头不变，还是屏障消息
                            prevMsg.next = msg.next;
                        } else {
                            //如果是消息头是不是屏障消息，队列移除找到的消息，消息头后移
                            mMessages = msg.next;
                           
                        }
                        msg.next = null;
                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                        msg.markInUse();
                        return msg;
                    }
                } else {
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }

               ····
               ····
        }
    }

4.简单总结一下

1.消息队列中可存放三种消息，屏障消息(msg.target == null)，异步消息(msg.isAsynchronous()= true)，同步消息(msg.isAsynchronous()= false)

2.如果消息头不是屏障消息，则异步消息和同步消息没有区别，消息队列头会后移

3.如果消息头是屏障消息，则只能通过异步消息，不能通过同步消息，消息队列头不会后移,只能手动移除

发送三种消息

1.发送普通消息,当然handler可指定默认消息类型

    //构造方法 async
    public Handler(boolean async) {
        this(null, async);
    }
    
    private boolean enqueueMessage(@NonNull MessageQueue queue, @NonNull Message msg,
            long uptimeMillis) {
        msg.target = this;
        msg.workSourceUid = ThreadLocalWorkSource.getUid();

        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }
    
    //如果handler mAsynchronous = ture ,设置message的isAsynchronous= false无效，
    //此handler发送的消息都是异步消息
    val obtainMessage = handler.obtainMessage().apply {
            what = 1
            isAsynchronous = false
            obj = "普通消息”
        }
  handler.sendMessageDelayed(obtainMessage, time)

2.发送异步消息

    val obtainMessage = handler.obtainMessage().apply {
            what = 2
            isAsynchronous = true
            obj = "异步消息”
        }
    handler.sendMessageDelayed(obtainMessage, time)

3.发送屏障消息,同步异步消息只有在遇到屏障头消息时才有效，查看源码

    /*
     * @hide
     */
    @TestApi
    public int postSyncBarrier() {
        return postSyncBarrier(SystemClock.uptimeMillis());
    }
    // 注意：when是计算后的时间
    //可以看出屏障消息的特点是msg.target == null，且msg.isAsynchronous() = false
    private int postSyncBarrier(long when) {
        synchronized (this) {
            final int token = mNextBarrierToken++;
            final Message msg = Message.obtain();
            msg.markInUse();
            msg.when = when;
            msg.arg1 = token;

            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 = msg;
            }
            return token;
        }
    }

4.发送屏障消息,反射调用

    //获取 发送屏障消息postSyncBarrier函数
    private val postSyncBarrier = MessageQueue::class.java.getDeclaredMethod("postSyncBarrier", Long::class.java).apply {
        isAccessible = true
    }

    //获取 撤销屏障消息 removeSyncBarrier函数
    private val removeSyncBarrier = MessageQueue::class.java.getDeclaredMethod("removeSyncBarrier", Int::class.java).apply {
        isAccessible = true
    }

使用Demo

    //初始化handler 
  private val handler = object : Handler(Looper.getMainLooper()) {
        override fun handleMessage(msg: Message) {
            super.handleMessage(msg)
            Log.d(TAG, "handleMessage:-${msg.what}--${msg.isAsynchronous}-- ${msg.obj}--${msg.`when`}")
            
            when (msg.what) {
                0 -> {
                    //收到消息后异常屏障，注意只能用异步消息移除屏障
                    removeSyncBarrier.invoke(queue, msg.g1)
                }
            }
        }
    }

    //获取当前队列
    private val queue = handler.looper.queue

    //获取队列的postSyncBarrier函数
    private val postSyncBarrier = MessageQueue::class.java.getDeclaredMethod("postSyncBarrier", Long::class.java).apply {
        isAccessible = true
    }

    //获取队列的removeSyncBarrier函数
    private val removeSyncBarrier = MessageQueue::class.java.getDeclaredMethod("removeSyncBarrier", Int::class.java).apply {
        isAccessible = true
    }
  


    private fun senMsg(handler: Handler, w: Int, async: Boolean, str: String, time: Long, arg: Int = 0) {
        val obtainMessage = handler.obtainMessage().apply {
            what = w
            arg1 = arg
            isAsynchronous = async
            obj = str
        }
        handler.sendMessageDelayed(obtainMessage, time)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        senMsg(handler, 1, false, "同步消息", 1000)
        //发送屏障消息 返回屏障标识token
        val token = postSyncBarrier.invoke(queue, SystemClock.uptimeMillis() + 2000)
        senMsg(handler, 2, false, "同步消息", 3000)
        senMsg(handler, 3, false, "同步消息", 4000)
        senMsg(handler, 4, true, "isAsynchronous 异步消息 ", 5000)
        senMsg(handler, 5, false, "同步消息", 6000)
        senMsg(handler, 6, true, "isAsynchronous 异步消息", 7000)
        senMsg(handler, 7, false, "同步消息", 8000)
        senMsg(handler, 8, true, "isAsynchronous 异步消息", 9000)
        //发送0  移除屏障token
        senMsg(handler, 0, true, "isAsynchronous 移除屏障", 9000, token as Int)
        senMsg(handler, 9, false, "同步消息", 10 * 1000)
        senMsg(handler, 10, false, "同步消息", 11 * 1000)
        senMsg(handler, 11, true, "isAsynchronous 异步消息", 12 * 1000)
        senMsg(handler, 12, false, "同步消息", 13 * 1000)
        senMsg(handler, 13, false, "同步消息", 14 * 1000)
        senMsg(handler, 11, true, "isAsynchronous 异步消息", 15 * 1000)
    }

测试结果

    
 //1 消息后是一个屏障消息
 19:54:37.854 : handleMessage:-1--false-- 同步消息--11075427 
 //只处理异步消息
 19:54:41.857 : handleMessage:-4--true-- isAsynchronous 异步消息 --11079427
 19:54:43.855 : handleMessage:-6--true-- isAsynchronous 异步消息--11081427 
 19:54:45.855 : handleMessage:-8--true-- isAsynchronous 异步消息--11083427
 //收到0后 会移除屏障消息
 19:54:45.855 : handleMessage:-0--true-- isAsynchronous 移除屏障--11083427 
 //继续按照队列的顺序正常执行
 19:54:45.855 : handleMessage:-2--false-- 同步消息--11077427               
 19:54:45.856 : handleMessage:-3--false-- 同步消息--11078427               
 19:54:45.856 : handleMessage:-5--false-- 同步消息--11080427               
 19:54:45.856 : handleMessage:-7--false-- 同步消息--11082427               
 19:54:46.854 : handleMessage:-9--false-- 同步消息--11084427               
 19:54:47.854 : handleMessage:-10--false-- 同步消息--11085427              
 19:54:48.854 : handleMessage:-11--true-- isAsynchronous 异步消息--11086427
 19:54:49.854 : handleMessage:-12--false-- 同步消息--11087427              
 19:54:50.855 : handleMessage:-13--false-- 同步消息--11088428              
 19:54:51.855 : handleMessage:-11--true-- isAsynchronous 异步消息--11089428

扩展:Looper.myQueue().addIdleHandler(mPurgeIdler);

添加空闲任务 MessageQueue#addIdleHandler(@NonNull IdleHandler handler)

  //添加空闲任务
 public void addIdleHandler(@NonNull IdleHandler handler) {
        if (handler == null) {
            throw new NullPointerException("Can't add a null IdleHandler");
        }
        synchronized (this) {
            mIdleHandlers.add(handler);
        }
    }

消息队列没有消息时会执行mIdleHandlers任务

 Message next() {
       //...
      //...
     //mMessages队列没有消息时
     for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler

                boolean keep = false;
                try {
                    keep = idler.queueIdle(); //执行任务
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }
               //任务是否持续执行
                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
       }
      //...
      //...
 }