MessageQueue如何做到阻塞的?
我们都知道如果想在子线程中实现更新Ui可以使用handler,那么handler是怎样做到的呢？下面我们通过源码来分析下
Handler简例如下：
private Handler myHandler=new MyHandler();
class MyHandler extends Handler{
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
//更新Ui
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
new Thread(new Runnable() {
@Override
public void run() {
myHandler.sendEmptyMessage(1);
}
});
}
我们先分析下这里Handler最终调用的构造
public Handler(Callback callback, boolean async) {
省略....
//重要的就这行了
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;
}
这里我们获取到了一个Looper实例，
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
看注释我们可以知道它会返回一个与当前线程关联的Looper，如果子线程没有关联Looper就调用这个方法的话会返回null，因为MyHandler是在主线程中创建的，所以这里会返回一个关联主线程的Looper,问题来了，Looper的其它方法我们都没调用sThreadLocal是在哪设置了值呢？
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));
}
/**
* Initialize the current thread as a looper, marking it as an
* application's main looper. The main looper for your application
* is created by the Android environment, so you should never need
* to call this function yourself. See also: {@link #prepare()}
*/
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
Looper里我们发现有一个prepareMainLooper（）方法，通过注释主大概可以知道主线程的looper就是在这初始化的，还提示你一定不要掉这个方法，因为它是Android系统的事，那么到底在哪调用了它呢？我们查找它的调用地方会发现一个ActivityThread的地方调用了它
public static void main(String[] args) {
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();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
这个ActivityThread.main()可以认为是程序的入口，它是系统调用的，先调用prepareMainLooper（）再调用loop（）消息循环，在调用prepareMainLooper()会最终 给sThreadLocal赋值，sThreadLocal.set(new Looper(quitAllowed));那么这个Looper的构造函数到底做了啥？
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
它会生成一个消息队列以及获取当前的线程。prepareMainLooper（）跑完后就会跑Looper.loop（）方法，这里强调下ActivityThread.main()里调用loop()方法是在主线程，所以loop()里的代码都是主线程里执行
/**
* 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;
}
省略...
final long start = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
final long end;
try {
msg.target.dispatchMessage(msg);
end = (slowDispatchThresholdMs == 0) ? 0 : SystemClock.uptimeMillis();
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
省略...
msg.recycleUnchecked();
}
}
loop()做的事主要的开启消息循环不断的从消息队列MessageQueue中取出消息，然后用msg.target.dispatchMessage(msg)进行消息分发，最后消息回收。这个msg.target其实就是MyHandler
/**
* 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);
}
}
到了这里我们看到了 handleMessage(msg);它就是我们重写的方法这样处理消息就是程序员自己的事了，因为loop()是在ActivityThread.main()里启动的所以是主线程，所以handlerMessage(msg)就是在Ui线程中处理事情的，这样更新UI就没问题了。
现在又有问题，消息队列MessageQueue里的消息是怎么来的呢?它怎样做到阻塞的呢？
myHandler.sendEmptyMessage(1);最终会跑到enqueueMessage()里
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
msg.target就是在这赋值的这里是MyHandler,接着把消息入队列(实现是Message单向链表，1->2>3.....,前面的可以拿到后面的, Message里有个next变量，这就相当于指针指向下一个)，所以在子线程里做的只是将一条消息存入消息队列中就完事了。因为loop()开启的消息循环所以有消息来了就跑到msg.target.dispatchMessage(msg)，接着又循环queue.next(），问题来了现在queue里已经没消息了，就会阻塞，那不会包ANR吗？？？（https://www.jianshu.com/p/8c829dc15950）
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