异步消息机制源码分析

我们进入Handler的源码我们可以看到

public Handler(Callback callback) {        this(callback, false);    }       public Handler(Callback callback, boolean async) {        if (FIND_POTENTIAL_LEAKS) {            final Class<? extends Handler> klass = getClass();            if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&                    (klass.getModifiers() & Modifier.STATIC) == 0) {                Log.w(TAG, "The following Handler class should be static or leaks might occur: " +                    klass.getCanonicalName());            }        }        mLooper = Looper.myLooper();        if (mLooper == null) {            throw new RuntimeException(                "Can't create handler inside thread that has not called Looper.prepare()");        }        mQueue = mLooper.mQueue;        mCallback = callback;        mAsynchronous = async;    }

Looper.myLooper();拿到当前线程的Looper实例
mLooper.mQueue;拿到他的消息队列
并进行一个判空
如果Looper为空则会报一个异常

在来看看 Looper.myLooper();进入looper源码看到

public static @Nullable Looper myLooper() {        return sThreadLocal.get();    }

返回的就是一个looper

但是主线程他已经帮我们调用了Looper.prepare()

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));    }   private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mThread = Thread.currentThread();    }

可以看到在这里会在本线程冲保存一个Looper实例和他的消息队列

我们在看看handler的发送消息都是通过sendMessage();进入源码查看我们可以看到最终
调用的是

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

msg.target = this;
他调用把消息队列enqueueMessage（）的方法把消息入队

现在我们知道了入队，那整么出对呢？？

就要看看Looper的另一个方法loop（）

 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();        }    }

他主要就是创一个消息循环，不断的从消息队列中拿取消息
Message msg = queue.next();
最后通过msg.target.dispatchMessage(msg);发送消息，
开始说道msg.target其实就是Handler

再来看看dispatchMessage(msg)；

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

最终调用的就是 handleMessage(msg);来处理消息

总结：
1.创建Handler的时候会拿到当前线程的Looper实例，和他的消息队列构成关联，形成一个循环通过queue.enqueueMessage(msg, uptimeMillis);保存消息到消息队列中

2.主线程调用Looper.prepare();在本线程中保存一个Looper实例和他的消息队列。

3.Looper.loop()拿到当前线程的Looper实例和他的消息队列，创建消息循环体。

4.通过Message msg = queue.next(); 不断的从消息队列中取出消息通过 。msg.target.dispatchMessage(msg);发送消息 。

5.最终调用handleMessage(msg);来处理消息形成一个循环。