一,概述
这几天一直在看《Android开发艺术探索》，对于我这个Android还没有学多久的人，说实话难度还是有大。其中Android消息机制这一章，看了半天，总算流程大致走了一遍，虽说不是都弄明白了，但是感觉收获还是不小。首先借用别人的一张Handler消息机制流程图（http://www.jianshu.com/p/c3459c13deff）
Android消息机制主要就是指Handler运行机制，所附带的MessageQueue和Looper的工作过程，接下来将从这三方面一一介绍。
二，Handler
相信在做开发时，Handler经常会被使用，他是Android消息机制的上层接口，打交道的机会自然不会少。它的使用过程就是将一个任务切换到Handler所在线程中去，而刚开始很多初学者，都会认为Handler只能在主线程中创建，然后在子线程中执行耗时任务，发送消息通知Handler更新UI。其实不然，首先看Handler的构造方法，Handler的构造方法有多种重载，通常我们在主线程中构建的Handler通常会跳到这个构造方法
public Handler(Callback callback, boolean async) {
。。。
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
。。。。
}在这个构造方法中我们可以看见如果mLooper为null，就会抛出Can’t create handler inside thread that has not called Looper.prepare()异常。从这里我们就可以得出一个结论如果一个线程中Looper为null，就会报错。那为什么主线程可以直接实例化Handler?来来，容贫道一一道来，首先看ActivityThread的main方法，
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");
}是不是很熟悉main方法，这不是我们学java天天写的主函数吗？好吧，它就是主线程的入口函数，接下来可以看到Looper.prepareMainLooper()，打开prepareMainLooper()方法我们看见如下：
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
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));
}上面两个方法主要就做了两件事：
1 .实例化了一个Looper，注入给sThreadLocal
2 .myLooper()赋给sMainLooper
至于ThreadLocal以后再说，暂且不讨论，大家可以将它理解为一个数据存储类，且每个线程只有一个存储变量副本。loop()后面在介绍Looper()再介绍
从上面可知主线程在初始化时就实例化了Looper，所以，主线程中可以直接实例化Handler（）。从上面可以知道，如果我们想实例化Handler，就必须先有Looper，故，Handler在子线程中也可以创建。那么我们该如何在子线程中创建Handler了，下面给出例子：
new Thread(){
@Override
public void run() {
//为当前线程创建一个Looper
Looper.prepare();
handler = new Handler(){
@Override
public void handleMessage(Message msg) {
}
};
handler.sendEmptyMessageDelayed(0, 1000);
//开启消息循环
Looper.loop();
}
}.start();好了，构造方法讲完了，接下来就是消息的发送，我们一般可以通过两种方法发送消息。
1.send….. 一般在handMessage中处理消息
2.post….. 一般在runable中处理消息
post….底层也是调用send…..发送消息，典型执行过程如下
//1.
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
//2.
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
//3.
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);
}
//4.
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}可以看到消息最终通过enqueueMessage投递到消息队列queue，其中msg.target = this就是指的消息处理者自己，后面Looper中会使用，这里先介绍下。
三，MessageQueue
MessageQueue的中文翻译就是消息队列，顾名思义，它的内部提供了一组消息，以队列的形式对外提供插入和删除操作。虽然叫消息队列，但它的内部是通过单链表的形式提供插入和删除操作。这里插入和删除是enqueueMessag和next，其中enqueueMessag往消息队列底部插入一条消息，next()则是取出消息，并将取出的消息移除回收掉，且next是一个阻塞试的死循环机制，只要有消息插入，将进行next取出。下面大概看下源代码
boolean enqueueMessage(Message msg, long when) {
。。。
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
return false;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;
} else {
// Inserted within the middle of the queue. Usually we don't have to wake
// up the event queue unless there is a barrier at the head of the queue
// and the message is the earliest asynchronous message in the queue.
needWake = mBlocked && p.target == null && msg.isAsynchronous();
Message prev;
for (;;) {
prev = p;
p = p.next;
if (p == null || when < p.when) {
break;
}
if (needWake && p.isAsynchronous()) {
needWake = false;
}
}
msg.next = p; // invariant: p == prev.next
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if (needWake) {
nativeWake(mPtr);
}
}
return true;
}主要做了单链表的插入，代码细节就不多看了，接下来看下next()
Message next() {
// Return here if the message loop has already quit and been disposed.
// This can happen if the application tries to restart a looper after quit
// which is not supported.
final long ptr = mPtr;
if (ptr == 0) {
return null;
}
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);
synchronized (this) {
// Try to retrieve the next message. Return if found.
final long now = SystemClock.uptimeMillis();
Message prevMsg = null;
Message msg = mMessages;
if (msg != null && msg.target == null) {
// Stalled by a barrier. Find the next asynchronous message in the queue.
do {
prevMsg = msg;
msg = msg.next;
} while (msg != null && !msg.isAsynchronous());
}
if (msg != null) {
if (now < msg.when) {
// Next message is not ready. Set a timeout to wake up when it is ready.
nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
} else {
// Got a message.
mBlocked = false;
if (prevMsg != null) {
prevMsg.next = msg.next;
} else {
mMessages = msg.next;
}
msg.next = null;
if (DEBUG) Log.v(TAG, "Returning message: " + msg);
msg.markInUse();
return msg;
}
} else {
// No more messages.
nextPollTimeoutMillis = -1;
}
// Process the quit message now that all pending messages have been handled.
if (mQuitting) {
dispose();
return null;
}
// If first time idle, then get the number of idlers to run.
// Idle handles only run if the queue is empty or if the first message
// in the queue (possibly a barrier) is due to be handled in the future.
if (pendingIdleHandlerCount < 0
&& (mMessages == null || now < mMessages.when)) {
pendingIdleHandlerCount = mIdleHandlers.size();
}
if (pendingIdleHandlerCount <= 0) {
// No idle handlers to run. Loop and wait some more.
mBlocked = true;
continue;
}
if (mPendingIdleHandlers == null) {
mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for (int i = 0; i < pendingIdleHandlerCount; i++) {
final IdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] = null; // release the reference to the handler
boolean keep = false;
try {
keep = idler.queueIdle();
} catch (Throwable t) {
Log.wtf(TAG, "IdleHandler threw exception", t);
}
if (!keep) {
synchronized (this) {
mIdleHandlers.remove(idler);
}
}
}
// Reset the idle handler count to 0 so we do not run them again.
pendingIdleHandlerCount = 0;
// While calling an idle handler, a new message could have been delivered
// so go back and look again for a pending message without waiting.
nextPollTimeoutMillis = 0;
}
}这段代码可以看出， 它是一个无限循环的消息，会无限的检测是否有消息。当有消息到来时，next会取出消息，并从队列里面取出。
四，Looper
首先看下其构造方法
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
在构造方法中实例化了MessageQueue。通常MessageQueue只负责消息的插入和读取，不负责消息的处理，而Looper是一个无限的消息循环，它负责从消息队列中取出消息并发给Handler处理，看下刚刚没有介绍的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
final Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
final long traceTag = me.mTraceTag;
if (traceTag != 0) {
Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
}
try {
msg.target.dispatchMessage(msg);
} finally {
if (traceTag != 0) {
Trace.traceEnd(traceTag);
}
}
......
}可以看出它是一个无限的消息循环，Message msg = queue.next()程序会阻塞在这里，如果没有消息到来，当looper检测到消息时会取出msg，然后执行msg.target.dispatchMessage(msg); 刚刚我们说到msg.target就是handler所以最后执行的是handler的dispatchMessage()方法，看下它的实现：
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}它的执行过程可以用张图概述下：
(稍微有点丑，将就下)
可以看出，平常我们直接实例化Handler(),就只会调用最后的handleMessage(msg)，在handler中handleMessage是一个空实现，需要我们在调用时手动实现。而另外一个CallBack是一个内部接口，它的实现如下
/**
* 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);
}那么它的意义是什么？ 从接口上面注释就可以看见，它的实现是为了不需要去派生Handler的子类，日常生活中通常是直接实例化Handler,并重写handleMessage，但是有时候，可以这样写
private Handler handler = new Handler(new Handler.Callback() {
@Override
public boolean handleMessage(Message msg) {
return false;
}
});返回值就是一个Boolean型变量，从那个图我们可以看见返回true就是自己消费了，handler的handleMessage不需要处理，false，就是要调用系统的handleMessage处理，这就是它的执行过程。
总结
好了，到了这里，它的基本过程以及涉及的三个部分Handler,MessageQueue，Looper都介绍了一遍，稍微再总结下吧
1 .Handler负责在子线程中发消息，消息会到MessageQueue底部。
2 .MessageQueue负责将消息加到队列，以及取出消息并回收掉。
3 .Looper负责检测MessageQueue是否有新消息，如果有就取出消息，然后发给消息发送者处理，此时线程已经从子线程切换到了另一个线程（通常是主线程）。
整个过程大致就是这，看到结果还是蛮容易理解的，不过内部执行过程确实很复杂（发个消息也不容易啊）！ 好了，就到这里了！！！！！
