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1. 为什么Android会设计Handler去更新UI
Handler根本上是为了解决多线程之间引发的并发问题，在ActivityThread中，要是有多个子线程在没有加锁的情况下更新UI，有可能引发UI显示错乱的现象，但要是对更新UI的操作进行类似synchronized加锁机制的话，会造成性能下降，而Handler允许多线程向一个MessageQueue中押入Message，在UIThread中通过轮询的方式取出Message，并进行对应的处理，这样就可以避免UI显示错乱的问题，又不会降低性能。
2.Handler的内部机制---Looper，MessageQueue
（1）.Looper：在一个Handler对象被创建时，内部自动关联了一个Looper对象，而Looper的内部会包含一个消息队列（MessageQueue对象），所有的给该Handler对象发送的Message，都会被加入到这个MessageQueue， 在Looper被创建之后，会调用Looper.loop()方法，looper()方法内部开始了一个for(;;)的死循环，不断的从MessageQueue中读取消息，如果取到Message，则处理消息，没有消息则处于阻塞状态。
（2）.MessageQueue，消息队列为存放消息的容器
首先，查看ActivityThread.java源码,在ActivityThread的main()方法中可以看到这么一行代码：
Looper.prepareMainLooper();
public static void prepareMainLooper() {
prepare();
setMainLooper(myLooper());
myLooper().mQueue.mQuitAllowed = false;
}public static void prepare() {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper());
}
可以看出，prepare()方法中，首先是从sThreadLocal中通过get()判断是否为null，若为null，则会new一个Looper对象，并存储在sThreadLocal中，查看new Looper()方法的源码：
private Looper() {
mQueue = new MessageQueue();
mRun = true;
mThread = Thread.currentThread();
}
// sThreadLocal.get() will return null unless you've called prepare(). static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<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);
}
从Looper()的构造函数中看到，MessageQueue是在Looper被创建的时候创建的，用于接收发送给对应Handler对象的Message对象。
下面看一下Handler被new创建的源码：
public Handler() {
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 = null;
}
可以看到有两行重要的代码：
mLooper = Looper.myLooper(); mQueue = mLooper.mQueue;
/**
* Return the Looper object associated with the current thread. Returns
* null if the calling thread is not associated with a Looper.
*/
public static Looper myLooper() {
return sThreadLocal.get();
}
Handler在被创建的时候拿到消息队列，便可以通过如handleMessage()方法向该消息队列中添加请求，而在ActivityThread创建Message Queue的时候，就已经在主线程中通过Looper.loop()的方法开启了一个for死循环来轮询Message，这样就把UIThread中的Looper和Handler相关联起来了，以后在子线程中调用 handler.sendMessage()方法的时候，其实质就是将Message加载到了UIThread的MessageQueue中，而在UIThread中已经有一个死循环轮询并在UIThread中处理Message了。
下面我们跟踪一下子线程中的handler.sendMessage(0x1)方法,一层层的最终到了boolean sendMessageAtTime(Message, long)方法：
public boolean sendMessageAtTime(Message msg, long uptimeMillis)
{
boolean sent = false;
MessageQueue queue = mQueue;
if (queue != null) {
msg.target = this;
sent = queue.enqueueMessage(msg, uptimeMillis);
}
else {
RuntimeException e = new RuntimeException(
this + " sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
return sent;
}
下面看看Looper.loop()方法：
public static void loop() {
Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
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();
while (true) {
Message msg = queue.next(); // might block
if (msg != null) {
if (msg.target == null) {
// No target is a magic identifier for the quit message.
return;
}
long wallStart = 0;
long threadStart = 0;
// 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);
wallStart = SystemClock.currentTimeMicro();
threadStart = SystemClock.currentThreadTimeMicro();
}
msg.target.dispatchMessage(msg);
if (logging != null) {
long wallTime = SystemClock.currentTimeMicro() - wallStart;
long threadTime = SystemClock.currentThreadTimeMicro() - threadStart;
logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
if (logging instanceof Profiler) {
((Profiler) logging).profile(msg, wallStart, wallTime,
threadStart, threadTime);
}
}
// 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();
}
}
}
［注意］其中msg.target为当前Message对应的Handler。
而Handler的dispatchMessage(Message)方法：
/**
* 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);
}
}
［总结］
UIThread在被初始化创建的时候，会首先通过prepare()方法为当前线程创建一个Looper对象，并存储在当前线程的ThreadLocal中，Looper的创建，其内部会自动创建一个消息队列（MessageQueue）的对象mQueue，在UIThread中创建一个Handler对象uiHandler的时候，uiHandler会从当前线程（UIThread）的ThreadLocal中拿到Looper对象，从而获得ActivityThread为UIThread创建的MessageQueue对象mQueue，当子线程通过uiHandler发送Message的时候，会首先将Message的target指向当前的uiHandler，并将此Message对象压入到mQueue中，而在ActivityThread创建Looper成功之后，便会通过Looper.loop()方法以死循环的方式轮询mQueue中的消息，当获得一个不为null的Message对象时，就会将该Message发送给Message对应的target（target为Handler对象）处理，target回调我们自定义实现的Callback.handleMessage(msg)和handleMessage(msg)方法处理消息。
Handler机制详述2---Looper,MessageQueue
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原文地址：http://blog.csdn.net/tianmaxingkong_/article/details/51341035
