异步消息机制源码分析

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

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。