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<? extends Handler> 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？