结合源码分析android的消息机制

描述

结合几个问题去看源码。

1.Handler, MessageQueue, Message, Looper, LocalThread这5者在android的消息传递过程中扮演了什么样的角色？

2.一个线程中可以有多个Handler吗？多个Looper呢？

3.整个消息处理过程，消息是怎么流动的？

4.为什么只有UI线程可以更改UI？（就凭他叫UI线程？其实也可以叫主线程或者ActivityThread）

开始看源码

1.View的绘制都是从ViewRootImpl这个类开始，我们在这个类中找找，为什么异步线程不能更新UI，发现了这个方法

void checkThread() {
    if (mThread != Thread.currentThread()) {
        throw new CalledFromWrongThreadException(
                "Only the original thread that created a view hierarchy can touch its views.");
    }
}

其中mThread就是大名鼎鼎的UI线程，这段代码说明不是UI线程就直接抛异常，相当粗暴。接着我们发现

@Override
public void requestFitSystemWindows() {
    checkThread();
    mApplyInsetsRequested = true;
    scheduleTraversals();
}

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }
}

还有更多就不一一列举了，对UI的操作都调用了这个检查的方法，所以非UI线程不能更新UI。这样做有什么好处？大概有2点

a.防止耗时操作导致的ANR，严重影响体验

b.UI控件不是线程安全的，多线程访问会出问题

2.接着看Handler,当我们new出一个Handler时，系统在干什么。

public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Class klass = getClass();
        if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                (klass.getModifiers() & Modifier.STATIC) == 0) {
            Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                klass.getCanonicalName());
        }
    }

    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

这个构造器最直接。首先需要一个Looper，哪里来的？在子线程中，我们通常要执行Looper.prepare()

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));
}

发现系统新建了一个Looper对象，并且存在了threadLocal中。当Looper创建的时候，同时创建了消息队列

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

当handler构造时，调用Looper.myLooper()方法，从threadLocal中取出新建的Looper对象。如果你没用调用Looper.myLooper()，执行下边代码就遇到那个熟悉的异常了。

if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread that has not called Looper.prepare()");
    }

在主线程中没用调用，为什么没有报错？因为ActivityThread帮你调了

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (sMainThreadHandler == null) {
    sMainThreadHandler = thread.getHandler();
}

if (false) {
    Looper.myLooper().setMessageLogging(new
            LogPrinter(Log.DEBUG, "ActivityThread"));
}

// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();

看Looper.prepareMainLooper()和Looper.loop()都调了。

3.再看下Handler的post和send方法，Message怎么被Handler送进MessageQueue的。发现最后他们都进了这个方法

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

通过enqueueMessage方法将消息体也就是Message对象压入MessageQueue中。

4，然后调用Looper.loop()，开始处理消息

/**
 * Run the message queue in this thread. Be sure to call
 * {@link #quit()} to end the loop.
 */
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();
    }
}

一个死循环，然后不断通过queue.next()方法把Message从MessageQueue中取出来处理

5.将消息取出来后，通过调用msg.target.dispatchMessage(msg)来处理消息。msg.target指的是Handler对象，来具体看下：

/**
 * Handle system messages here.
 */
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

这个方法说明当有Runnable对象(handler.post(new Runnable())设置)时,消息交给handleCallback()处理

private static void handleCallback(Message message) {
    message.callback.run();
}

直接调用了Runnable的run方法。这也解释了handler.post(new Runnable())时没有新建线程的问题。当有mCallBack（new Handler(new CallBack())时设置），调用callBack的方法handlerMessage()。都没有，则调用Handler的handlerMessage方法。

6.自此消息基本从 产生->加入队列->处理 过程基本走通了。

总结

1.很多细节没讲到，主要是从源码角度分析整个过程

2.handler可以在任意线程发送消息，这些消息会被添加到关联的MessageQueue上

3.消息最终会交给产生handler的线程处理，所以子线程可以持handler的引用更新UI

4.MessageQueue和Looper和当前线程相关

5.handler可以建多个，Looper只能有一个

5.ThreadLocal是个存储类，从他保存新建的Looper对象可以看出，以后细说

遗留问题

1.handler的回调中明明打印的当前是子线程，为什么还是可以更新UI？

2.但是调用延时的方法后又不行了？

实验代码：

Log.e("---1---", String.valueOf(Thread.currentThread())
        + "\nThreadID:" + Thread.currentThread().getId());
new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            Looper.prepare();
            Handler handler = new Handler() {
                @Override
                public void handleMessage(Message msg) {
                    Log.e("---3---", String.valueOf(Thread.currentThread())
                    + "\nThreadID:" + Thread.currentThread().getId());
                    btn.setText("handlerMessage");
                }
            };
            Log.e("---2---", String.valueOf(Thread.currentThread())
                    + "\nThreadID:" + Thread.currentThread().getId());
            handler.sendEmptyMessage(0);
                      handler.sendEmptyMessageDelayed(0, 2000);
            handler.post(new Runnable() {
                @Override
                public void run() {
                    Log.e("---4---", String.valueOf(Thread.currentThread())
                            + "\nThreadID:" + Thread.currentThread().getId());
                    btn.setBackgroundColor(Color.YELLOW);
                }
            });
            Looper.loop();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}).start();

结果：

01-07 14:55:40.688 21362-21362/com.empty.animationtest E/---1---: Thread[main,5,main]

ThreadID:1

01-07 14:55:40.698 21362-21394/com.empty.animationtest E/---2---: Thread[Thread-37592,5,main]

                                                                 ThreadID:37592

01-07 14:55:40.698 21362-21394/com.empty.animationtest E/---3---: Thread[Thread-37592,5,main]

                                                                 ThreadID:37592

01-07 14:55:40.698 21362-21394/com.empty.animationtest E/---4---: Thread[Thread-37592,5,main]

                                                                 ThreadID:37592

01-07 14:55:42.698 21362-21394/com.empty.animationtest E/---3---: Thread[Thread-37592,5,main]

                                                                 ThreadID:37592

01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: 01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err:     at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6294)

01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err:     at android.view.ViewRootImpl.invalidateChildInParent(ViewRootImpl.java:878)

01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err:     at android.view.ViewGroup.invalidateChild(ViewGroup.java:4344)

01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err:     at android.view.View.invalidate(View.java:10957)

01-07 14:55:42.698 21362-21394/com.empty.animationtest W/System.err:     at android.view.View.invalidate(View.java:10912) 

结果分析：

执行到3，4确实是子线程，也完成了UI的更新。而延时的第二个3，不能成功更新UI，为什么？