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在开始这篇文章之前，。首先，我们在总结前两篇文章Handler， Looper和MessageQueue像一些关键点：
0）在创建线程Handler之前，你必须调用Looper.prepare(), 创建一个线程局部变量Looper，然后调用Looper.loop() 进入轮循。
1）当Handler创建之后，就能够调用Handler的sendMessageAtTime方法发送消息。而实际上是调用MessageQueue的enqueueMessage方法。将相应的消息放入消息队列。
2）每个线程都仅仅有一个Looper，这个Looper负责对MessageQueue进行轮循，当获取到Message。就调用handler.dispatchMessage进行分发。
从上面这三点，我们就能够大概地看出Handler的使用流程了。
今天我们就先从消息開始的地方讲起。就是Handler的 enqueueMessage 方法了，代码例如以下：
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
msg.target = this;
if (mAsynchronous) {
msg.setAsynchronous(true);
}
return queue.enqueueMessage(msg, uptimeMillis);
}1）将msg.target 设置成当前Handler对象
2）调用MessageQueue的enqueueMessage方法
所以。事实上就是在这里。将Message对象放到消息队列中去的。
说到MessageQueue，我们首先要明确这个消息队列事实上是一个链表的结构，一个串一个的。
而其队列的初始化并非在 java层做的。而是在JNI层利用C++实现的。
我们能够看看其定义的几个native方法，例如以下：
private native static long nativeInit();
private native static void nativeDestroy(long ptr);
private native static void nativePollOnce(long ptr, int timeoutMillis);
private native static void nativeWake(long ptr);
private native static boolean nativeIsIdling(long ptr); MessageQueue(boolean quitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();
}在这里，我们并不进入其在JNI层的代码，水太深了。
我们还是从Java层来看吧。在MessageEnqueue的 enqueueMesage方法中。基本的代码例如以下：
synchronized (this) {
...
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;
}
...
}
既然消息已经放到队列中去了。那么下一步就是在Looper的轮循操作中去获取消息，然后将消息进行分发。我们能够在Looper 的loop方法中看到其调用了MessageQueue的next方法。
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
Printer logging = me.mLogging;
if (logging != null) {
logging.println(">>>>> Dispatching to " + msg.target + " " +
msg.callback + ": " + msg.what);
}
msg.target.dispatchMessage(msg);
Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;
for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
// We can assume mPtr != 0 because the loop is obviously still running.
// The looper will not call this method after the loop quits.
nativePollOnce(mPtr, 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 (false) Log.v("MessageQueue", "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("MessageQueue", "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;
}
}
1）是一个 for(;;)循环
2）仅仅有当获取 message的时候或者mQuitting为true的时候才会跳出循环。
3）在获取消息的时候，会依据 Message.when 字段来进行推断
从以上几点，我们就能够大概了解为什么说在 loop方法中，next方法有可能会堵塞。由于它就是一个无限的轮循操作呀。
好吧，到这里之后，我们大概知道了下面两件事情：
1）在Handler的sendMessageAtTime方法调用MessageQueue的 enqueueMessage方法，将消息放入队列。
2）在Looper的looop方法。调用MessageQueue的next方法。将消息取出队列。
接下来第三步，非常显然，就是调用handler的dispatchMessage方法了，例如以下：
msg.target.dispatchMessage(msg);
public void dispatchMessage(Message msg) {
if (msg.callback != null) {
handleCallback(msg);
} else {
if (mCallback != null) {
if (mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}1）msg.callback
2) mCallback
3 ) handleMessage
首先， msg.callback是什么？
在Message类中。我们能够看到
Runnable callback;
还有其构造函数：
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
所以在dispatchMessage中，首先就是推断是否有对Message设置了Runnable的callback，假设有。就运行这个callback方法，例如以下：
private static void handleCallback(Message message) {
message.callback.run();
} public interface Callback {
public boolean handleMessage(Message msg);
}而普通情况下，仅仅有我们想去继承Handler类。实现自己定义的Handler的时候，我们才会去实现这个接口，而当此接口返回true的时候，Handler默认的handleMessage方法就不会再被调用了。反之，则依旧会调用。
最后，就是我们最普通的handleMessage方法了。也就是我们在实现一个最普通的handler的时候所实现的方法了。
相同。没有样例怎么能够呢，请看代码：
class LooperThread extends Thread {
public Handler mHandler;
public void run() {
Looper.prepare();
mHandler = new Handler() {
public void handleMessage(Message msg) {
Log.v("Test", "Id of LooperThread : " + Thread.currentThread().getId());
switch (msg.what) {
case MSG_ID_1:
Log.v("Test", "Toast called from Handler.sendMessage()");
break;
case MSG_ID_2:
String str = (String) msg.obj;
Log.v("Test", str);
break;
}
}
};
Looper.loop();
}
}
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.v("Test", "Id of MainThread : " + Thread.currentThread().getId());
LooperThread looperThread = new LooperThread();
looperThread.start();
while(looperThread.mHandler == null){
}
Message message = Message.obtain(looperThread.mHandler, new Runnable() {
@Override
public void run() {
Log.v("Test", "Message.callack()");
}
});
message.what = MSG_ID_1;
message.sendToTarget();
looperThread.mHandler.post(new Runnable() {
@Override
public void run() {
Log.v("Test", "Handler.callack()");
}
});
}10-28 11:27:49.328: V/Test(22009): Id of MainThread : 1 10-28 11:27:49.328: V/Test(22009): Message.callack() 10-28 11:27:49.328: V/Test(22009): Handler.callack()
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原文地址：http://www.cnblogs.com/mengfanrong/p/4638343.html
