Android开发学习之路-Handler消息派发机制源码分析

注：这里只是说一下sendmessage的一个过程，post就类似的

如果我们需要发送消息，会调用sendMessage方法

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

这个方法会调用如下的这个方法

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

接下来设定延迟时间，然后继续调用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);
}

这里获得了消息队列，检查队列是否存在，然后返回enqueMessage的方法的执行结果，这个结果是说明消息能否进入队列的一个布尔值

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

这里是对消息进行入队处理，下面就是在MessageQueue中对消息进行入队

boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }

    synchronized (this) {
        if (mQuitting) {
            IllegalStateException e = new IllegalStateException(
                    msg.target + " sending message to a Handler on a dead thread");
            Log.w(TAG, e.getMessage(), e);
            msg.recycle();
            return false;
        }

        msg.markInUse();
        msg.when = when;
        Message p = mMessages;
        boolean needWake;
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            // up the event queue unless there is a barrier at the head of the queue
            // and the message is the earliest asynchronous message in the queue.
            needWake = mBlocked && p.target == null && msg.isAsynchronous();
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;
        }

        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

就是对传递过来的消息进行一些封装然后放到队列中，至此我们的sendMessage处理完毕，返回的结果是进队是否成功的布尔值，那么究竟消息之后是如何被处理的呢？

我们可以看到在Handler构造的时候记录了一个Looper对象，也记录了一个回掉函数

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;
}

这里的myLooper方法返回的是当前线程关联的一个Looper对象

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

当Looper实例化了以后会执行自己的prepare方法然后执行loop方法，loop方法就是不断的读取消息队列中的消息然后执行相应的操作的方法，因为是在其他线程中执行的循环所以不会影响其他线程

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
        Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }

        msg.target.dispatchMessage(msg);

        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();
    }
}

在循环中如果读取到了消息，就会执行dispatchMessage方法，然后分派完消息之后再执行一次recycleUnchecked方法来重用这个Message，我们看到dispatchMessage方法

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

这里看到直接执行了一个handlerMessage方法，这个方法是一个回调方法，我们是必须实现的，否则Handler什么都不会做，为什么呢？还记得刚刚说构造Handler的时候我们记录了一个CallBack的回掉吗？Handler中的这个handlerMessage方法是一个空方法，如果我们重写了这个方法，在回调的时候就会执行我们先写下的代码，也就是接收到消息之后要做什么。

public interface Callback {
    public boolean handleMessage(Message msg);
}

public void handleMessage(Message msg) {
}

这里简单说下整个过程：

当我们实例化一个Handler的子类并重写handleMessage方法之后，这个时候系统已经帮我们做了几个事情

1.实例化了一个消息队列MessageQueue

2.实例化了一个关联的Looper对象，并让Looper不断的读取消息队列

3.把我们重写的handleMessage方法记录为我们需要回调的方法

当我们执行Handler的sendMessage方法的时候，系统会把我们传过去的Message对象添加到消息队列，这个时候如果Looper读取到了消息，就会把消息派发出去，然后回调handleMessage方法，执行我们设定的代码。