承接上一篇Handler系列一,上篇主要总结了Handler如何通信,这篇来介绍Handler怎么通信。

  • Handler的通信机制
  • Handler,Looper,MessageQueue如何关联

Handler 通信机制

Handler 通信机制

  • 创建Handler,并采用当前线程的Looper创建消息循环系统;
  • Handler通过sendMessage(Message)或Post(Runnable)发送消息,调用enqueueMessage把消息插入到消息链表中;
  • Looper循环检测消息队列中的消息,若有消息则取出该消息,并调用该消息持有的handler的dispatchMessage方法,回调到创建Handler线程中重写的handleMessage里执行。

Handler 如何关联 Looper、MessageQueue

Handler及其关联的类图

Handler及其关联的类图

以上类图可以快速帮助我们理清Handler与Looper、MessageQueue的关系,以下从源码的角度慢慢分析:

1、Handler 发送消息

上一段很熟悉的代码:

 Message msg =Message.obtain();  //从全局池中返回一个message实例,避免多次创建message(如new Message)
 msg.obj = data;
 msg.what=1;   //标志消息的标志
 handler.sendMessage(msg);    

从sendMessageQueue开始追踪,函数调用关系:sendMessage -> sendMessageDelayed ->sendMessageAtTime,在sendMessageAtTime中,携带者传来的message与Handler的mQueue一起通过enqueueMessage进入队列了。

对于postRunnable而言,通过post投递该runnable,调用getPostMessage,通过该runnable构造一个message,再通过 sendMessageDelayed投递,接下来和sendMessage的流程一样了。

2、消息入队列

在enqueueMessage中,通过MessageQueue入队列,并为该message的target赋值为当前的handler对象,记住msg.target很重要,之后Looper取出该消息时,还需要由msg.target.dispatchMessage回调到该handler中处理消息。

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

在MessageQueue中,由Message的消息链表进行入队列

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;
}

3、Looper 处理消息

再说处理消息之前,先看Looper是如何构建与获取的:
  • 构造Looper时,构建消息循环队列,并获取当前线程

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
          mThread = Thread.currentThread();
    }
    
  • 但该函数是私有的,外界不能直接构造一个Looper,而是通过Looper.prepare来构造的:

    public static void prepare() {

      prepare(true);

    }

    private static void prepare(boolean quitAllowed) {

    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));

    }

  • 这里创建Looper,并把Looper对象保存在sThreadLocal中,那sThreadLocal是什么呢?

    static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

    它是一个保存Looper的TheadLocal实例,而ThreadLocal是线程私有的数据存储类,可以来保存线程的Looper对象,这样Handler就可以通过ThreadLocal来保存于获取Looper对象了。

  • TheadLocal 如何保存与获取Looper?

    public void set(T value) {

    Thread currentThread = Thread.currentThread();
    Values values = values(currentThread);
    if (values == null) {
        values = initializeValues(currentThread);
    }
    values.put(this, value);

    }

    public T get() {

    // Optimized for the fast path.
    Thread currentThread = Thread.currentThread();
    Values values = values(currentThread);
    if (values != null) {
        Object[] table = values.table;
        int index = hash & values.mask;
        if (this.reference == table[index]) {
            return (T) table[index + 1];
        }
    } else {
        values = initializeValues(currentThread);
    }
    
    return (T) values.getAfterMiss(this);

    }

在 set 中都是通过 values.put 保存当前线程的 Looper 实例,通过 values.getAfterMiss(this)获取,其中putgetAfterMiss都有keyvalue,都是由Value对象的table数组保存的,那么在table数组里怎么存的呢?

  table[index] = key.reference;
  table[index + 1] = value;

很显然在数组中,前一个保存着ThreadLocal对象引用的索引,后一个存储传入的Looper实例。

接下来看Looper在loop中如何处理消息

loop中,一个循环,通过next取出MessageQueue中的消息

  • 若取出的消息为null,则结束循环,返回。

    • 设置消息为空,可以通过MessageQueue的quit和quitSafely方法通知消息队列退出。
  • 若取出的消息不为空,则通过msg.target.dispatchMessage回调到handler中去。
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();
    }
}

4、handler处理消息

Looper把消息回调到handler的dispatchMessage中进行消息处理:

  • 若该消息有callback,即通过Post(Runnable)的方式投递消息,因为在投递runnable时,把runnable对象赋值给了message的callback
  • 若handler的mCallback不为空,则交由通过callback创建handler方式去处理。
  • 否则,由最常见创建handler对象的方式,在重写handlerMessage中处理。
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}    

总结

以一个时序图来总结handler的消息机制,包含上述如何关联Looper和MessageQueue的过程。

Handler-Looper-MessageQueue时序图

Handler-Looper-MessageQueue时序图

本文发表于个人博客:http://lavnfan.github.io/,欢迎指教。


lavnFan
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Android开发程序员,Github主页:[链接]。


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