Handle机制源码总结

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以下是个人总结：
一般我们都是直接Handle handle=new Handle();
但是子线程中直接new Handle会报错,因为Handle的构造方法中的mLooper = Looper.myLooper();这个地方是mLooper是从Looper.myLooper()中获取的。如果我们没用调用Looper. prepare()进行初始化的话，Looper.myLooper()获取到的值就会是空的。而Looper. prepare()是通过sThreadLocal.set(new Looper())的形式新建Looper对象的，ThreadLocal通过获取当前线程将数据存放new Looper()在ThreadLocalMap中;
然后消息队列在Looper的构造方法中被创建。

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

然后准备工作完成，当我们用handle发送消息时，不管你怎么send,最终都会走
sendMessageAtTime，或者sendMessageAtFrontOfQueue；这两个的区别就是消息执行的时间不一样而已。而他们都会调用enqueueMessage这个方法执行消息的入队操作。需要注意的是msg.target = this;这一句。在消息上传入了当前的handle，后面在消息分发的时候就直接分发到哪个handle上了。

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);    }

最后我们还要调用Looper.loop()事件才会进行会发，来看下loop()的源码：

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();        }    }

看重要的代码：

for (;;) {            Message msg = queue.next(); // might block            ………………            msg.target.dispatchMessage(msg);            …………            ｝

一个死循环，queue.next();链表一直next下去，告诉你可能会Block。
然后就是刚才说的要注意的msg.target,这个我们在上面在将消息入队之前进行了赋值，然后现在就是调用它的dispatchMessage方法，也就是handle的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);        }    }

如果Message没有设置callback则直接调用handleMessage，反之刚执行handleCallback;