本文内容基于《Android开发艺术探索》，有兴趣的同学可以买本书，值得一看。
1.Handler工作原理
Handler主要任务是发送和接收处理消息，发送消息可以通过post或者send相关方法来实现，我们先来看一下Handler类中post和send方式的代码实现
public final boolean post(Runnable r){
return sendMessageDelayed(getPostMessage(r), 0);
}
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}
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);
}
return queue.enqueueMessage(msg, uptimeMillis);
}
我们可以发现post还是通过调用send方式来发送消息的，发送消息只是通过queue.enqueueMessage(msg, uptimeMillis);向消息队列中插入一条消息，MessageQueue会把消息交给Looper，Looper再把消息交给Handler的dispatchMessage方法处理，具体过程会在下面分析，现在我们来看一下dispatchMessage的实现
public void dispatchMessage(Message msg) {
if (msg.callback != null) { //如果消息已设置callback,则调用该callback函数
handleCallback(msg);
} else {
if (mCallback != null) { //如果Handler已设置callback,则调用该callback处理消息
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg); //默认为空函数，用户可重载处理自定义消息
}
}
从上面代码可以看出，首先检查Message的callback是否为null，不为null则交给handleCallback方法执行，Message的callback是一个Runnable对象，实际上就是post传入的Runable对象(可查看Handler的getPostMessage()方法)，handleCallback中直接调用callback的run方法，handleCallback实现如下
private static void handleCallback(Message message) {
message.callback.run();
}
如果msg.callback为null，即消息是通过send方式发送的，则会再判断mCallback是否为null，不为null则调用mCallback的handleMessage方法，Callback是Handler类中的一个接口，我们可以在中通过Handler handler = new Handler(callback)来创建handler并实现Callback来处理消息。
public interface Callback {
public boolean handleMessage(Message msg);
}
如果mCallback为null则会调用Handler的handleMessage方法来处理消息，这是我们最常用的方式。
2.MessageQueue工作原理
接下来分析一下MessageQueue的工作原理，MessageQueue中通过enqueueMessage()方法来插入消息，通过next()方法来取出数据。enqueueMessage方法实现如下
boolean enqueueMessage(Message msg, long when) {
...
synchronized (this) {
...
msg.markInUse();
msg.when = when;
Message p = mMessages;
boolean needWake;
if (p == null || when == 0 || when < p.when) { //如果消息队列为空或者当前消息为发送时间最早的消息
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked; //mBlocked=true表示线程已被挂起
} 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); //唤醒线程
}
}
return true;
}
通过上面代码我们可以看出MessageQueue是通过单链表的方式循环遍历找到p.next为空的Message对象来插入消息，接下来看看next方法的实现
Message next() {
...
int pendingIdleHandlerCount = -1; //空闲的handler个数。只有在第一次循环的时候值为-1。
int nextPollTimeoutMillis = 0; //下次轮询时间，如果当前消息队列中没有消息，它要等待，为0，表示不等待，不为0则表示等待消息。
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) { //判断时间是否可以处理这个消息，如果符合条件，把消息返回传给looper处理。否则，算出需要等待时间，等待到该时间，然后执行
// 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 (false) Log.v("MessageQueue", "Returning message: " + msg);
return msg;
}
} else {
// 如果msg为null则吧nextPollTimeoutMillis赋值为－1，表示等到下一个消息
nextPollTimeoutMillis = -1;
}
...
}
...
}
}
可以看出next方法中有一个无限循环的代码块在获取message，如果有新消息则将详细从链表中移除并返回这条消息，如果消息队列中没有消息那么next方法会一直阻塞在这里。
3.Looper工作原理
Looper主要作用是循环从MessageQueue取出消息处理，如果没有新的消息则会阻塞，它的构造方法中会创建一个MessageQueue对象，并获取当前现成对象的引用
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
Handler的创建必须在含有Looper的线程中，否则会报错，Looper的创建可调用Looper.prepare()，主线程中会在ActivityThread的main()方法中调用Looper.prepareMainLooper()方法为主线程创建Looper对象，所以在主线程中创建Handler对象是不需要我们显示的创建Looper对象，在工作线程中创建线程如下所示
new Thread() {
@Override
public void run() {
Looper.prepare(); //创建Looper对象
Handler handler = new Handler(); //创建Handler对象
Looper.loop(); //开启循环
}
}
只有点用loop()方法消息循环才会起作用，loop方法的实现如下
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) { //只有queue.next()返回为null才会跳出循环，MessageQueue只有退出next才会返回null，否则有新消息则会返回消息，没有则会阻塞
// 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();
}
}
loop()中是一个死循环，只有queue.next()返回为null才会跳出循环，MessageQueue只有退出next()才会返回null，否则有新消息则会返回消息，没有消息则会阻塞。获取到新消息会掉用msg.target.dispatchMessage(msg);来处理消息，msg.target是发送这条消息的Handler对象，这样就达到了谁发送的消息谁处理的效果。