我们使用handler，可以实现主线程和子线程之间的相互通信，然后子线程和子线程之间的通信，如果不清楚，基本用法请先参考我的这篇博客
Android之用Handler实现主线程和子线程互相通信以及子线程和子线程之间的通信 http://blog.csdn.net/u011068702/article/details/75577005
我们看下Looper.java这个类，它在安卓android.os包下，我们看这个类的一开始的注释
* <p>This is a typical example of the implementation of a Looper thread,
* using the separation of {@link #prepare} and {@link #loop} to create an
* initial Handler to communicate with the Looper.
*
* <pre>
* class LooperThread extends Thread {
* public Handler mHandler;
*
* public void run() {
* Looper.prepare();
*
* mHandler = new Handler() {
* public void handleMessage(Message msg) {
* // process incoming messages here
* }
* };
*
* Looper.loop();
* }
* }</pre>
很明显，在一个线程里面需要使用Handler之前需要Looper.prepare(),但是我们平时在主线程更新UI的时候，为什么没有看到这行代码呢？
我们看下ActivityThread.java这个类，它在安卓包名android.app目录下，我们知道ActivityThread.java这个类是安卓程序的入口，我们看下main函数的代码
public static void main(String[] args) {
SamplingProfilerIntegration.start();
// CloseGuard defaults to true and can be quite spammy. We
// disable it here, but selectively enable it later (via
// StrictMode) on debug builds, but using DropBox, not logs.
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
Security.addProvider(new AndroidKeyStoreProvider());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
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"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
我们可以看到有Looper.prepareMainLooper()函数，我们点击进去
public static void prepareMainLooper() {
prepare(false);
synchronized (Looper.class) {
if (sMainLooper != null) {
throw new IllegalStateException("The main Looper has already been prepared.");
}
sMainLooper = myLooper();
}
}
然后看到了prepare(false)函数，我们再点击这个函数
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));
}
我们可以看到这里就调用prepare函数，所以主线程不需要调用Looper.prepare()函数，然后我们也可以看到这里有行这个代码
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
在Looper.java类中，我们的Looper保存在ThreadLocal里面
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();用ThreadLocal修饰的变量，可以理解为只有当前线程可以改变这个参数，其它线程不可以改变这个参数，如果你对ThreadLocal不清楚，单独可以先看下我这篇博客的简单使用
java之ThreadLocal简单使用总结 http://blog.csdn.net/u011068702/article/details/75770226
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}上面的代码写得很清楚了，如果当前的sThreadLocal对象里面有个Looper对象，那么就会抛出异常，而且英文也提示了，所以，一个线程为什么只能有一个Looper对象的原因，所以如果在程序里面，每个线程调用2次Looper.prepare()就会报错，我们再看这行代码
sThreadLocal.set(new Looper(quitAllowed));
点击Looper的构造函数
private Looper(boolean quitAllowed) {
mQueue = new MessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
里面构建了一个MessageQueue对象，上面我们分析一个线程只有一个Looper对象，那么Looper对象只构建一个，也就意味着MessageQueue对象也只构建一次，所以一个线程也只有一个MessageQueue对象的原因。
在main函数里面，也调用了Looper.loop（）函数，后面分析这个方法
我们先看下Handler.java的构造方法，这个类在安卓 android.os这个目录下面
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());
}
}
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 = callback;
mAsynchronous = async;
}这里看出初始化变量，然后保存了当前线程中的Looper对象。
发送消息的时候我们一般都这样写
handler.sendMessage(message) 所以我们点击 sendMessage方法看下
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
再点击sendMessageDelayed(msg, 0);
public final boolean sendMessageDelayed(Message msg, long delayMillis)
{
if (delayMillis < 0) {
delayMillis = 0;
}
return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}
再点击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);
}
再点击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);
}这里msg.target就是Handler对象本身，再点击queue.enqueueMessage(msg, uptimeMillis)（在MessageQueue这个类里面）
boolean enqueueMessage(Message msg, long when) {
if (msg.target == null) {
throw new IllegalArgumentException("Message must have a target.");
}
if (msg.isInUse()) {
throw new IllegalStateException(msg + " This message is already in use.");
}
synchronized (this) {
if (mQuitting) {
IllegalStateException e = new IllegalStateException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", 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;
}
可以看出MessageQueue从而按照时间将所有的Message排序
然后我们不是最后还调用了Looper.loop()函数吗？点击进去
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the 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
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();
}
}Looper.loop()方法里起了一个死循环，不断的判断MessageQueue中的消息是否为空，如果为空则直接return掉，然后执行queue.next()方法，点击进去
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 (false) Log.v("MessageQueue", "Returning message: " + msg);
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;
}
}Message的出栈操作，里面可能对线程，并发控制做了一些限制等。获取到栈顶的Message对象，然后就执行这个函数
msg.target.dispatchMessage(msg);
我么知道msg.tartget对象就是handler对象，我们点击dispatchMessage(msg)函数
/**
* 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);
}
}
我们可以知道msg.callback != null的时候，就执行了handleCallback(msg)
private static void handleCallback(Message message) {
message.callback.run();
}
意味着这个Runable执行 run方法
然后还有就是也可能会执行到这里来
handleMessage(msg);
点击进去，
/**
* Subclasses must implement this to receive messages.
*/
public void handleMessage(Message msg) {
}
我们一般在主线程这样写接收消息
public Handler mHandlerCToP = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
switch(msg.what) {
case 0:
break;
default:
break;
}
}
};也就意味着把handleMessage()方法重写了，所以我们的代码，有地方发送消息，loop()不断分发消息，当收到了，如果接收到了，我们重写handleMessage就会掉到这个地方来，得到我需要的数据。
我们一般在子线程调用这个方法也可以来更新UI
runOnUiThread(new Runnable() {
@Override
public void run() {
}
});点击进去
public final void runOnUiThread(Runnable action) {
if (Thread.currentThread() != mUiThread) {
mHandler.post(action);
} else {
action.run();
}
}再点击mHandler.post(action) 方法
public final boolean post(Runnable r)
{
return sendMessageDelayed(getPostMessage(r), 0);
}点击进去
private static Message getPostMessage(Runnable r) {
Message m = Message.obtain();
m.callback = r;
return m;
}看到了吗？其实最后还是到了发送消息这里，一开始我们不是分析了Looper.loop()里面的dispatchMessage()方法吗？
我么知道msg.tartget对象就是handler对象，我们点击dispatchMessage(msg)函数
/**
* 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);
}
}很明显程序，会走handleCallback(msg);所以会调到handler.java里面的这个方法
private static void handleCallback(Message message) {
message.callback.run();
}这样就执行了这个Runnable
上面那个函数内部有handler.post()这个方法，已分析
点击view.post()方法
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
// Assume that post will succeed later
ViewRootImpl.getRunQueue().post(action);
return true;
} 可以发现其调用的就是activity中默认保存的handler对象的post方法
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