Android的消息机制（Handler、Looper、MessageQueue）

本文链接： https://blog.csdn.net/u013174702/article/details/54944378

注：本文源码基于Android7.0

先大概概括一下消息机制：

这里有三个角色，Handler、looper、MessageQueue。Handler负责发消息和处理消息，Looper负责从MessageQueue中取出消息给Handler处理，MessageQueue则负责存储Handler发过来的消息。

这个机制主要是将一个任务切换到指定线程中执行，就像我们在子线程中更新UI界面，就可以用Handler。多说一句，特殊情况下子线程中也是可以更新UI的，具体可以看这篇文章，大概意思就是在ViewRootImpl未完全初始化之前更新Android 中子线程真的不能更新 UI 吗？。

我们从Handler的sendMessage方法入手开始分析：

public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

再看sendMessageDelayed方法

public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

再看sendMessageAtTime方法

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);
    }

最后我们看enqueueMessage方法

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

从最后一行我们可以看出，Handler的sendMessage方法最终就是往MessageQueue里插入一条消息。

Looper我们就从Looper.prepare方法来分析

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));
    }

prepare方法是为当前线程创建一个looper，从那个if语句中我们也知道了为什么一个线程中只能有一个Looper。sThreadlocal是Threadlocal类型，是一个线程内部的数据存储类，在该线程存储的数据只能在该线程中获取。

接下来我们看Looper.loop方法

public static void loop() {
	//获取到当前的Looper
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
	//从Looper中得到MessageQueue
        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();

	//不断的从MessageQueue取消息，当没有消息时，next方法会阻塞，直到消息队列被标记为退出
	//这样next方法会返回null
        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.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
		//从这里看到了Handler的消息最终交给它的dispatchMessage方法来处理了
            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();
        }
    }

Android的消息机制（Handler、Looper、MessageQueue）

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