Android的消息机制

最新推荐文章于 2025-05-01 08:54:53 发布

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最新推荐文章于 2025-05-01 08:54:53 发布

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文章标签： android 线程

本文链接：https://blog.youkuaiyun.com/qq1194222919/article/details/70196242

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最近工作不是很忙，闲来没事去看了Handler消息的源码，在这里记录一下，别过几天又忘记了。
通过Handler可以轻松的将一个任务切换到Handler所在的线程，最常用的就是更新UI。Handler运行需要底层的MessageQueue和Looper的支撑。Handler的创建会采用当前线程的Looper来构造消息循环系统。（在主线程中创建Handler对象的时候不用创建Looper对象，在ActivityThread会被初始化，所以在主线程可以默认使用Handler）

先说说Looper这个类。
它在消息机制中扮演着消息循环的角色，它会不停的从MessageQueue查看是否有新消息，如果有就会立即去处理，否则一直阻塞在那里。在一个线程创建Looper，通过Looper.prepare()来创建，通过Looper,loop()来开启消息循环：

public static void prepare() {
        prepare(true);
    }

    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));//把looper对象放在ThreadLocal里面
    }
    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);//创建MessageQueue
        mThread = Thread.currentThread();//获取当前线程
    }

可以通过prepare()在当前的线程创建一个Looper对象，在创建一个Looper对象的时候也会创建一个MessageQueue（）对象。还有个创建Looper对象的方法是 prepareMainLooper(),这个方法主要是给主线程ActivityThread创建Looper对象用的，还提供一个getMainLooper()对象，这个方法是通过它可以在任何地方获取到主线程的Looper对象。最后说说ThreadLocal这个对象，创建一个ThreadLocal对象，在不同线程中用ThreadLocal.set()方法储存任何东西，在不同线程通过ThreadLocal.get()获取的类容只能是在当前线程通过ThreadLocal.set()方法存储的值。所以创建Looper对象的时候回把创建的对象放到ThreadLocal中，在后只有在当前线程才能通过get（）方法获取到Looper对象，ThreadLocal的工作原理用下面代码说明：

final ThreadLocal<Boolean> mThreadLocal = new ThreadLocal<>();
        mThreadLocal.set(true);
        new Thread(new Runnable() {
            @Override
            public void run() {
                Log.d("Thread1", "run: "+Thread.currentThread()+":"+mThreadLocal.get());
            }
        },"Thread1").start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                mThreadLocal.set(true);
                Log.d("Thread2", "run: "+Thread.currentThread()+":"+mThreadLocal.get());
            }
        },"Thread2").start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                mThreadLocal.set(false);
                Log.d("Thread3", "run: "+Thread.currentThread()+":"+mThreadLocal.get());
            }
        },"Thread3").start();

        Log.d("main", "run: "+Thread.currentThread()+":"+mThreadLocal.get());
输出结果：
04-16 15:02:26.143 18435-18465/com.example.androidtext D/Thread1: run: Thread[Thread1,5,main]:null
04-16 15:02:26.143 18435-18466/com.example.androidtext D/Thread2: run: Thread[Thread2,5,main]:true
04-16 15:02:26.143 18435-18467/com.example.androidtext D/Thread3: run: Thread[Thread3,5,main]:false
04-16 15:02:26.143 18435-18435/com.example.androidtext D/main: run: Thread[main,5,main]:true

可以看出ThreadLocal的get（）和set（）方法所操作的对象都是当前的线程。

说说MessageQueue这个类。
MessageQueue这个类比较简单主要就有两个方法，一个是enqueueMessage(Message msg, long when)方法，向单链表中插入数据，next()向单链表中获取数据并删除，这个方法在Looper.loop()的方法中调用。loop()是一个死循环。
最后说活Handler这个类
handler主要包含了消息的发送和接收，通过post和send的一系列方法实现的

public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
     public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }
     private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

当调用sendMessage(Message msg)方法后，最后会调用MessageQueue的enqueueMessage（）方法插入数据。在sendMessageAtTime方法中，mQueue是初始化Handler中通过Looper.myLooper()获取当前Handler的Looper对象，通过Looper.myLooper().mQueue获取MessageQueue对象。

 public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        final MessageQueue queue = me.mQueue;

        // Make sure the identity of this thread is that of the local process,
        // and keep track of what that identity token actually is.
        Binder.clearCallingIdentity();
        final long ident = Binder.clearCallingIdentity();

        for (;;) {
            Message msg = queue.next(); // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
                return;
            }

            // This must be in a local variable, in case a UI event sets the logger
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            if (logging != null) {
                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
            }

            // Make sure that during the course of dispatching the
            // identity of the thread wasn't corrupted.
            final long newIdent = Binder.clearCallingIdentity();
            if (ident != newIdent) {
                Log.wtf(TAG, "Thread identity changed from 0x"
                        + Long.toHexString(ident) + " to 0x"
                        + Long.toHexString(newIdent) + " while dispatching to "
                        + msg.target.getClass().getName() + " "
                        + msg.callback + " what=" + msg.what);
            }

            msg.recycleUnchecked();
        }
    }

Looper.loop()方法会不停的循环，调用MessageQueue的next()方法获取插入的数据， msg.target.dispatchMessage(msg)中的target就是绑定Looper的Handler对象，调用Handler的dispatchMessage的这个方法。

public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }
     private static void handleCallback(Message message) {
        message.callback.run();
    }
     public void handleMessage(Message msg) {
    }