mLooper = Looper.myLooper();
在ActivityThread.main中：
Looper.prepareMainLooper();…Looper.loop();此时Handler和Looper有了关联。
2、obtainMessage()的分析在Handler.obtainMessage()中会调用Message.obtain(this)。Message.obtain()的源码：public static Message obtain(Handler h) {Message m = obtain();m.target = h;return m; }
3、sendToTarget()的分析public void sendToTarget() {target.sendMessage(this);}直接调用target.sendMessage，而target正是当前的Handler。继续跟踪target.sendMessage(this) target中发送消息会辗转来到private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis)在这个方法中有去调用queue.enqueueMessage.
…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.nextprev.next = msg; }// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}…
这里面的MessageQueue就是在Handler保存的那个MessageQueue，也就是说此时，这个Message已经保存到Handler中的那个消息队列中了。而，我们Handler中的MessageQueue哪来的呢？ 来看看这行代码：public Handler(Callback callback, boolean async) {…mLooper = Looper.myLooper();…mQueue = mLooper.mQueue;…}Handler中的MessageQueue正式从Looper中获取的，这个Looper当然就是在ActivityThread中prepare的那个。顺一下此时的关系：Handler中保存了ActivityThread中的Looper，并从该Looper中获取了MessageQueue；调用obtainMessage，实质上是创建了一个Message对象，并将Message对象的target设置为现在的Handler；调用Message.sendToTarget()实际是调用了Message.target.sendMessage()，即Handler.sendMessage，而Handler.sendMessage会来到enqueueMessage方法，在这个方法中调用MessageQueue.enqueueMessage将消息压缩刚开始我们获取的那个MessageQueue。
此时，，再来看看Looper.loop()是怎么将消息回调到handleMessage中的：
for (;;) {Message msg = queue.next(); // might block…msg.target.dispatchMessage(msg);…msg.recycle(); }继续看看Handler.dispatchMessage():public void dispatchMessage(Message msg) {if (msg.callback != null) {handleCallback(msg);} else {if (mCallback != null) {if (mCallback.handleMessage(msg)) {return;}}handleMessage(msg);}}转来转去，终于把Message回调到了handleMessage！！！从源码中，我们也能看出， sendToTarget()方法并不是真正的将Message发送出去，而是将Message压入了MessageQueue中，真正去回调handleMessage还是在Looper.loop()中完成的。
还深深埋在心底，要除去，怕是不能活命。
