18.Handler消息机制时序图

msg

此处取消息，需要一直循环获取，如果还没到达时间，就休眠，还有一个同步屏障的概念需要注意

Looper.loop()

sendMessage(msg)

mLooper、mQueue

sThreadLocal.set(Looper)

queue.next()

handleMessage()

msg.target.dispatchMessage(msg)

MessageQueue()

Looper.myLooper()

sendMessageAtTime

MessageQueue

new Looper()

mQueue

ActivityThread

Looper.prepareMainLooper();

mThread = Thread.currentThread()

ThreadLocal

main()

sThreadLocal.get()

Handler

Looper

XxxActivity

new Handler()

mHandler