扫盲细节，源码解析消息队列和Looper

1. 什么是消息队列
消息队列在Android中对应MessageQueue这个类，消息队列中存放了大量的消息（Message）

2.什么是消息
消息（Message）代表一个行为（what）或者一串动作（Runnable），有两处会用到Message：Handler和Messenger

3.什么是Handler
Handler主要用来在子线程更新UI和线程间通信

4.什么是Looper
Looper负责从消息队列中循环的取出消息然后把消息交给目标处理

5.如何让子线程有Looper从而正常使用Handler？
在线程的run方法中加入如下两句：

Looper.prepare();

Looper.loop();

HandlerThread就是带有Looper的线程。

想用线程的Looper来创建Handler，很简单，Handler handler = new Handler(thread.getLooper())，有了Looper，就可以在子线程中创建Handler了。

消息队列和Looper的工作机制
一个Handler会有一个Looper，一个Looper会有一个消息队列，Looper的作用就是循环的遍历消息队列，如果有新消息，就把新消息交给它的目标处理。每当我们用Handler来发送消息，消息就会被放入消息队列中，然后Looper就会取出消息发送给它的目标target。一般一个消息的target是发送这个消息的Handler，这样Looper就会把消息交给Handler处理，这个时候Handler的dispatchMessage方法就会被调用，一般情况最终会调用Handler的handleMessage来处理消息，用handleMessage来处理消息是我们常用的方式。

看源码：

1. Handler发送消息

public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }
    
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }
    
    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        return enqueueMessage(queue, msg, uptimeMillis);
    }
    
    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        //这里msg被加入消息队列queue
        return queue.enqueueMessage(msg, uptimeMillis);
    }

2.Looper的工作过程

public static void loop() {
final Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
//从Looper中取出消息队列
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();

//死循环，循环的取消息，没有新消息就会阻塞
for (;;) {
Message msg = queue.next(); // might block 这里会被阻塞，如果没有新消息
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);
}

//将消息交给target处理，这个target就是Handler类型
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.recycle();
}
}

3.Handler如何处理消息

/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}

/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
//这个方法很简单，直接调用msg.callback.run();
handleCallback(msg);
} else {
//如果我们设置了callback会由callback来处理消息
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
//否则消息就由这里来处理，这是我们最常用的处理方式
handleMessage(msg);
}
}

 

msg.callback和mCallback是啥东西？
/*package*/ Runnable callback;   

现在已经很明确了，msg.callback是个Runnable，什么时候会设置这个callback：handler.post(runnable)，接着往下看，一目了然，就是常用的post(Runnable callback)方法。

/**
* Callback interface you can use when instantiating a Handler to avoid
* having to implement your own subclass of Handler.
*
* @param msg A {@link android.os.Message Message} object
* @return True if no further handling is desired
*/
public interface Callback {
public boolean handleMessage(Message msg);
}
final Callback mCallback;

而mCallback是个接口，可以这样来设置 Handler handler = new Handler(callback)，这个callback的意义是什么呢，代码里面的注释已经说了，可以让你不用创建Handler的子类但是还能照样处理消息，我们知道了，不创建Handler的子类也可以处理消息。

HandlerThread简介

@Override
public void run() {
mTid = Process.myTid();
Looper.prepare();
synchronized (this) {
mLooper = Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid = -1;
}

HandlerThread继承自Thread，其在run方法内部为自己创建了一个Looper，使用上HandlerThread和普通的Thread不一样，无法执行常见的后台操作，只能用来处理新消息，这是因为Looper.loop()是死循环。

在子线程创建Handler报错根本原因是啥？

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());
}
}
//获取当前线程的Looper
mLooper = Looper.myLooper();
//报错的根本原因是：当前线程没有Looper
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;
}

如何避免：在UI线程使用Handler或者给子线程加上Looper。