Handler消息传递机制

demo

一.Handler消息传递机制解决的问题

线程通信

耗时工作(从服务器拉去数据)	更新UI
子线程(避免方式ANR,就是主线程卡死)	主线程

要想在子线程做完耗时工作后及时更新UI，就必须使用Handler消息传递机制。

二. Handler工作流程

1. Handler.post/sendMessage 方法最终都会调用enqueueMessage将消息放入消息队列

   
   image
   

   2.在子线程中放入消息，在主线程中取出消息

   
   link

三. 源码分析

enqueueMessage

将消息插入到消息队列的正确位置

    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;
            }

Looper

queue.next();从消息对列中取出消息，如果没有消息就阻塞(主线程进入休眠)。
msg.target就是Handler
msg.target.dispatchMessage(msg)调用Handler的dispatchMessage处理消息
Looper的参考

 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;
            }

            try {
                msg.target.dispatchMessage(msg);
            }
        }
    }

Handler

处理消息
mCallback是Handler带参构造函数new Handler(new Handler.Callback()）传入的。

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