消息机制概述
Android应用程序的每一个线程在启动时，都可以首先在内部创建一个消息队列，然后再进入一个无限循环中，不断检查它的消息队列是否有新的消息需要处理，如果有新的消息需要处理，那么线程就会将它从消息队列中取出来，并且对它进行处理；否则线程就会进入睡眠等待状态，直到有新的消息需要处理为止，这样就可以通过消息来驱动Android应用程序的执行。
消息机制组成部分
Android系统主要通过Messagequeue，Looper，Handler三个类来实现Android应用程序的消息处理机制：
1.Messagequeue：描述消息队列
2.Looper：创建消息队列，以及进入消息循环
3.Handler：用来发送消息和处理消息
消息处理流程
1、程序启动的时候，主线程会创建一个Looper对象。Looper对象初始化一个MessageQueue，然后调用loop()方法循环去读取消息。
2、初始化Handler的时候，在Handler的构造函数内部，会获取当前线程的Looper对象，进而获取MessageQueue对象。由此可见，想要操作UI的Handler必须在主线程中创建。否则会提示你：【”Can’tcreate handler inside thread that has not called Looper.prepare()”】
3、调用Handler的相关方法时，会获取Message对象，将消息对象的target指向当前handler对象，然后放到消息队列中。
4、loop()工作中，会从消息队列中获取一个个的消息，调用handle的dispatchMessage(msg)分发处理。
5、Message内部维护一个消息池，用来回收缓存message对象。
6、Looper相当于一个发动机，MessageQueue相当于流水线，Message相当于一个个的物品，而Handler就相当于工人。
消息循环机制机构图
消息循环机制java层与c++层关系图
创建线程消息队列
1.创建Java层Looper对象：
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privatestaticvoidprepare(booleanquitAllowed) {
if(sThreadLocal.get() !=null) {
thrownewRuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(newLooper(quitAllowed));//创建looper对象
}
2.Looper对象内部创建一个MessageQueue对象（mQueue）：
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privateLooper(booleanquitAllowed) {
//Looper在创建的时候会创建一个MessageQueue对象
mQueue =newMessageQueue(quitAllowed);
mThread = Thread.currentThread();
}
3.调用MessageQueue的nativeInit方法创建一个NativeMessageQueue对象：
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MessageQueue(booleanquitAllowed) {
mQuitAllowed = quitAllowed;
mPtr = nativeInit();//..
}
4.nativeInit方法返回NativeMessageQueue地址给mPtr：
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staticjlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {
//在C++层通过此方法创建一个NativeMessageQueue对象
NativeMessageQueue* nativeMessageQueue =newNativeMessageQueue();
if(!nativeMessageQueue) {
jniThrowRuntimeException(env,"Unable to allocate native queue");
return0;
}
nativeMessageQueue->incStrong(env);
//返回nativeMessageQueue地址给Java层；
returnreinterpret_cast(nativeMessageQueue);
}
5.NativeMessageQueue创建时内部创建一个C++层Looper(Native)对象：
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NativeMessageQueue::NativeMessageQueue() : mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {
//NativeMessageQueue创建时会创建一个Looper(Native)对象
mLooper = Looper::getForThread();
if(mLooper == NULL) {
mLooper =newLooper(false);
Looper::setForThread(mLooper);
}
}
6.Looper(Native)创建时内部创建一个管道通过两个文件描述符管理它：
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Looper::Looper(boolallowNonCallbacks) : mAllowNonCallbacks(allowNonCallbacks), mResponseIndex(0) {
intwakeFds[2];
intresult = pipe(wakeFds);//创建一个管道
LOG_ALWAYS_FATAL_IF(result != 0,"Could not create wake pipe. errno=%d", errno);
mWakeReadPipeFd = wakeFds[0];//读端文件描述符
mWakeWritePipeFd = wakeFds[1];//写端文件描述符
result = fcntl(mWakeReadPipeFd, F_SETFL, O_NONBLOCK);
LOG_ALWAYS_FATAL_IF(result != 0,"Could not make wake read pipe non-blocking. errno=%d",errno);
result = fcntl(mWakeWritePipeFd, F_SETFL, O_NONBLOCK);
LOG_ALWAYS_FATAL_IF(result != 0,"Could not make wake write pipe non-blocking. errno=%d",errno);
#ifdef LOOPER_USES_EPOLL
// Allocate the epoll instance and register the wake pipe.
mEpollFd = epoll_create(EPOLL_SIZE_HINT);//..
LOG_ALWAYS_FATAL_IF(mEpollFd < 0,"Could not create epoll instance. errno=%d", errno);
structepoll_event eventItem;
memset(& eventItem, 0,sizeof(epoll_event));// zero out unused members of data field union
eventItem.events = EPOLLIN;
eventItem.data.fd = mWakeReadPipeFd;
//将文件描述符放在epoll中进行管理
result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeReadPipeFd, & eventItem);
LOG_ALWAYS_FATAL_IF(result != 0,"Could not add wake read pipe to epoll instance. errno=%d",errno);
#else
// Add the wake pipe to the head of the request list with a null callback.
structpollfd requestedFd;
requestedFd.fd = mWakeReadPipeFd;
requestedFd.events = POLLIN;
mRequestedFds.push(requestedFd);
Request request;
request.fd = mWakeReadPipeFd;
request.callback = NULL;
request.ident = 0;
request.data = NULL;
mRequests.push(request);
mPolling =false;
mWaiters = 0;
#endif
#ifdef LOOPER_STATISTICS
mPendingWakeTime = -1;
mPendingWakeCount = 0;
mSampledWakeCycles = 0;
mSampledWakeCountSum = 0;
mSampledWakeLatencySum = 0;
mSampledPolls = 0;
mSampledZeroPollCount = 0;
mSampledZeroPollLatencySum = 0;
mSampledTimeoutPollCount = 0;
mSampledTimeoutPollLatencySum = 0;
#endif
}
消息循环过程
1.Looper获取当前线程MessageQueue并循环调用它的next方法检查是否有新消息需要处理：
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publicstaticvoidloop() {
finalLooper me = myLooper();//获取当前线程looper
if(me ==null) {
thrownewRuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
finalMessageQueue queue = me.mQueue;//获取当前线程MessageQueue
// 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();
finallongident = Binder.clearCallingIdentity();
for(;;) {//不断检查是否有新消息需要处理
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指向一个Handler对象，调用Handler的dispatchMessage方法分发消息
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.
finallongnewIdent = 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();
}
}
2.MessageQueue的next方法中调用nativePollOnce函数检查当前线程的消息队列中是否有新消息要处理，如果有消息会存在mMessage中并进行处理：
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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.
finallongptr = mPtr;
if(ptr ==0) {
returnnull;
}
intpendingIdleHandlerCount = -1;// -1 only during first iteration
intnextPollTimeoutMillis =0;//当前线程需要进入睡眠等待状态的时间
for(;;) {//不断调用成员函数nativePollOnce来检查当前线程的消息队列是否有新消息要处理
if(nextPollTimeoutMillis !=0) {
Binder.flushPendingCommands();
}
nativePollOnce(ptr, nextPollTimeoutMillis);//..
synchronized(this) {
// Try to retrieve the next message. Return if found.
finallongnow = SystemClock.uptimeMillis();
Message prevMsg =null;
Message msg = mMessages;//当前线程需要处理的消息
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();
returnmsg;
}
}else{
// No more messages.
nextPollTimeoutMillis = -1;//没有消息就睡觉
}
// Process the quit message now that all pending messages have been handled.
if(mQuitting) {
dispose();
returnnull;
}
// 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 =newIdleHandler[Math.max(pendingIdleHandlerCount,4)];
}
mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
}
// Run the idle handlers.
// We only ever reach this code block during the first iteration.
for(inti =0; i < pendingIdleHandlerCount; i++) {
finalIdleHandler idler = mPendingIdleHandlers[i];
mPendingIdleHandlers[i] =null;//release the reference to the handler
booleankeep =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;
}
}
3.在nativePollOnce函数中调用Looper(Native)的pollOnce函数不断检查是否有新消息要处理：
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staticvoidandroid_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,jlong ptr, jint timeoutMillis) {
//通过ptr找到NativeMessageQueue
NativeMessageQueue* nativeMessageQueue =reinterpret_cast(ptr);
//调用nativeMessageQueue对象的pollOnce函数检查当前线程是否有新消息
nativeMessageQueue->pollOnce(env, obj, timeoutMillis);
}
4.在pollOnce函数中调用polllnner函数(返回值不等于0即有新消息)：
[cpp]view plaincopy
voidNativeMessageQueue::pollOnce(JNIEnv* env, jobject pollObj,inttimeoutMillis) {
mPollEnv = env;
mPollObj = pollObj;
//调用Looper(Native)的pollOnce函数检查当前线程是否有新消息要处理
mLooper->pollOnce(timeoutMillis);
mPollObj = NULL;
mPollEnv = NULL;
if(mExceptionObj) {
env->Throw(mExceptionObj);
env->DeleteLocalRef(mExceptionObj);
mExceptionObj = NULL;
}
}
----------------------------------------------------------分割线------------------------------------------------------------------
[java]view plaincopy
intLooper::pollOnce(inttimeoutMillis,int* outFd,int* outEvents,void** outData) {
intresult =0;
for(;;) {//不断调用pollInner方法检查是否有新消息
while(mResponseIndex < mResponses.size()) {
constResponse& response = mResponses.itemAt(mResponseIndex++);
if(! response.request.callback) {
#ifDEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - returning signalled identifier %d: "
"fd=%d, events=0x%x, data=%p",this,
response.request.ident, response.request.fd,
response.events, response.request.data);
#endif
if(outFd != NULL) *outFd = response.request.fd;
if(outEvents != NULL) *outEvents = response.events;
if(outData != NULL) *outData = response.request.data;
returnresponse.request.ident;
}
}
if(result !=0) {
#ifDEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - returning result %d",this, result);
#endif
if(outFd != NULL) *outFd =0;
if(outEvents != NULL) *outEvents = NULL;
if(outData != NULL) *outData = NULL;
returnresult;
}
result = pollInner(timeoutMillis);//如果有新消息返回值不等于0
}
}
5.polllnner函数中调用awoken方法把管道中的旧数据清理掉：
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intLooper::pollInner(inttimeoutMillis) {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - waiting: timeoutMillis=%d",this, timeoutMillis);
#endif
intresult = ALOOPER_POLL_WAKE;
mResponses.clear();
mResponseIndex = 0;
#ifdef LOOPER_STATISTICS
nsecs_t pollStartTime = systemTime(SYSTEM_TIME_MONOTONIC);
#endif
#ifdef LOOPER_USES_EPOLL
structepoll_event eventItems[EPOLL_MAX_EVENTS];
inteventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
boolacquiredLock =false;
#else
// Wait for wakeAndLock() waiters to run then set mPolling to true.
mLock.lock();
while(mWaiters != 0) {
mResume.wait(mLock);
}
mPolling =true;
mLock.unlock();
size_trequestedCount = mRequestedFds.size();
inteventCount = poll(mRequestedFds.editArray(), requestedCount, timeoutMillis);
#endif
if(eventCount < 0) {
if(errno == EINTR) {
gotoDone;
}
LOGW("Poll failed with an unexpected error, errno=%d", errno);
result = ALOOPER_POLL_ERROR;
gotoDone;
}
if(eventCount == 0) {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - timeout",this);
#endif
result = ALOOPER_POLL_TIMEOUT;
gotoDone;
}
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - handling events from %d fds",this, eventCount);
#endif
#ifdef LOOPER_USES_EPOLL
for(inti = 0; i < eventCount; i++) {
intfd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
//判断发生IO事件的文件描述符是否与当前线程所关联的管道的mWakeReadPipeFd一致
if(fd == mWakeReadPipeFd) {
if(epollEvents & EPOLLIN) {
awoken();
}else{
LOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
}
}else{
if(! acquiredLock) {
mLock.lock();
acquiredLock =true;
}
ssize_t requestIndex = mRequests.indexOfKey(fd);
if(requestIndex >= 0) {
intevents = 0;
if(epollEvents & EPOLLIN) events |= ALOOPER_EVENT_INPUT;
if(epollEvents & EPOLLOUT) events |= ALOOPER_EVENT_OUTPUT;
if(epollEvents & EPOLLERR) events |= ALOOPER_EVENT_ERROR;
if(epollEvents & EPOLLHUP) events |= ALOOPER_EVENT_HANGUP;
pushResponse(events, mRequests.valueAt(requestIndex));
}else{
LOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "
"no longer registered.", epollEvents, fd);
}
}
}
if(acquiredLock) {
mLock.unlock();
}
Done: ;
#else
for(size_ti = 0; i < requestedCount; i++) {
conststructpollfd& requestedFd = mRequestedFds.itemAt(i);
shortpollEvents = requestedFd.revents;
if(pollEvents) {
if(requestedFd.fd == mWakeReadPipeFd) {
if(pollEvents & POLLIN) {
awoken();
}else{
LOGW("Ignoring unexpected poll events 0x%x on wake read pipe.", pollEvents);
}
}else{
intevents = 0;
if(pollEvents & POLLIN) events |= ALOOPER_EVENT_INPUT;
if(pollEvents & POLLOUT) events |= ALOOPER_EVENT_OUTPUT;
if(pollEvents & POLLERR) events |= ALOOPER_EVENT_ERROR;
if(pollEvents & POLLHUP) events |= ALOOPER_EVENT_HANGUP;
if(pollEvents & POLLNVAL) events |= ALOOPER_EVENT_INVALID;
pushResponse(events, mRequests.itemAt(i));
}
if(--eventCount == 0) {
break;
}
}
}
Done:
// Set mPolling to false and wake up the wakeAndLock() waiters.
mLock.lock();
mPolling =false;
if(mWaiters != 0) {
mAwake.broadcast();
}
mLock.unlock();
#endif
#ifdef LOOPER_STATISTICS
nsecs_t pollEndTime = systemTime(SYSTEM_TIME_MONOTONIC);
mSampledPolls += 1;
if(timeoutMillis == 0) {
mSampledZeroPollCount += 1;
mSampledZeroPollLatencySum += pollEndTime - pollStartTime;
}elseif(timeoutMillis > 0 && result == ALOOPER_POLL_TIMEOUT) {
mSampledTimeoutPollCount += 1;
mSampledTimeoutPollLatencySum += pollEndTime - pollStartTime
- milliseconds_to_nanoseconds(timeoutMillis);
}
if(mSampledPolls == SAMPLED_POLLS_TO_AGGREGATE) {
LOGD("%p ~ poll latency statistics: %0.3fms zero timeout, %0.3fms non-zero timeout",this,
0.000001f *float(mSampledZeroPollLatencySum) / mSampledZeroPollCount,
0.000001f *float(mSampledTimeoutPollLatencySum) / mSampledTimeoutPollCount);
mSampledPolls = 0;
mSampledZeroPollCount = 0;
mSampledZeroPollLatencySum = 0;
mSampledTimeoutPollCount = 0;
mSampledTimeoutPollLatencySum = 0;
}
#endif
for(size_ti = 0; i < mResponses.size(); i++) {
constResponse& response = mResponses.itemAt(i);
if(response.request.callback) {
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
LOGD("%p ~ pollOnce - invoking callback: fd=%d, events=0x%x, data=%p",this,
response.request.fd, response.events, response.request.data);
#endif
intcallbackResult = response.request.callback(
response.request.fd, response.events, response.request.data);
if(callbackResult == 0) {
removeFd(response.request.fd);
}
result = ALOOPER_POLL_CALLBACK;
}
}
returnresult;
}
6.awoken方法的实现：
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voidLooper::awoken() {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ awoken",this);
#endif
#ifdef LOOPER_STATISTICS
if(mPendingWakeCount == 0) {
LOGD("%p ~ awoken: spurious!",this);
}else{
mSampledWakeCycles += 1;
mSampledWakeCountSum += mPendingWakeCount;
mSampledWakeLatencySum += systemTime(SYSTEM_TIME_MONOTONIC) - mPendingWakeTime;
mPendingWakeCount = 0;
mPendingWakeTime = -1;
if(mSampledWakeCycles == SAMPLED_WAKE_CYCLES_TO_AGGREGATE) {
LOGD("%p ~ wake statistics: %0.3fms wake latency, %0.3f wakes per cycle",this,
0.000001f *float(mSampledWakeLatencySum) / mSampledWakeCycles,
float(mSampledWakeCountSum) / mSampledWakeCycles);
mSampledWakeCycles = 0;
mSampledWakeCountSum = 0;
mSampledWakeLatencySum = 0;
}
}
#endif
charbuffer[16];
ssize_t nRead;
do{
nRead = read(mWakeReadPipeFd, buffer,sizeof(buffer));//将管道中数据读出来
}while((nRead == -1 && errno == EINTR) || nRead ==sizeof(buffer));
}
线程消息发送过程
1.通过调用sendMessageXXX方法将消息发送到一个消息队列中：
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publicbooleansendMessageAtTime(Message msg,longuptimeMillis)
{
booleansent =false;
MessageQueue queue = mQueue;
if(queue !=null) {
msg.target =this;
sent = queue.enqueueMessage(msg, uptimeMillis);//将消息发送到消息队列中
}
else{
RuntimeException e =newRuntimeException(
this+" sendMessageAtTime() called with no mQueue");
Log.w("Looper", e.getMessage(), e);
}
returnsent;
}
2.调用enqueueMessage方法将消息插入到消息队列(队头或队中)：
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booleanenqueueMessage(Message msg,longwhen) {
if(msg.target ==null) {
thrownewIllegalArgumentException("Message must have a target.");
}
if(msg.isInUse()) {
thrownewIllegalStateException(msg +" This message is already in use.");
}
synchronized(this) {
if(mQuitting) {
IllegalStateException e =newIllegalStateException(
msg.target +" sending message to a Handler on a dead thread");
Log.w(TAG, e.getMessage(), e);
msg.recycle();
returnfalse;
}
msg.markInUse();
msg.when = when;
Message p = mMessages;
booleanneedWake;
if(p ==null|| when ==0|| when < p.when) {
// New head, wake up the event queue if blocked.
msg.next = p;
mMessages = msg;
needWake = mBlocked;//记录了当前线程是否处于睡眠等待状态
}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，将消息插入到消息队列中
prev.next = msg;
}
// We can assume mPtr != 0 because mQuitting is false.
if(needWake) {
nativeWake(mPtr);//如果线程正在睡眠，那么将其唤醒
}
}
returntrue;
3.如果线程处于睡眠状态调用NativeMessageQueue的nativeWake方法，唤醒他：
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staticvoidandroid_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) {
NativeMessageQueue *nativeMessageQueue =reinterpret_cast(ptr);
//通过ptr找到NativeMessageQueue,并调用它的wake方法唤醒目标线程，让它处理消息
nativeMessageQueue->wake();
}
4.nativeWake方法中调用NativeMessageQueue的wake方法：
[cpp]view plaincopy
voidNativeMessageQueue::wake() {
mLooper->wake();//调用Looper(Native)的wake方法
}
5.wake方法中调用Looper(Native)的wake方法，向管道写入字符：
[cpp]view plaincopy
voidLooper::wake() {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ wake",this);
#endif
#ifdef LOOPER_STATISTICS
// FIXME: Possible race with awoken() but this code is for testing only and is rarely enabled.
if(mPendingWakeCount++ == 0) {
mPendingWakeTime = systemTime(SYSTEM_TIME_MONOTONIC);
}
#endif
ssize_t nWrite;
do{
nWrite = write(mWakeWritePipeFd,"W", 1);//向管道中写入字符，唤醒线程。
}while(nWrite == -1 && errno == EINTR);
if(nWrite != 1) {
if(errno != EAGAIN) {
LOGW("Could not write wake signal, errno=%d", errno);
}
}
}
线程消息处理过程
1.通过Looper类的loop方法获取消息：
[java]view plaincopy
publicstaticvoidloop() {
finalLooper me = myLooper();//获取当前线程looper
if(me ==null) {
thrownewRuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
finalMessageQueue queue = me.mQueue;//获取当前线程MessageQueue
// 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();
finallongident = Binder.clearCallingIdentity();
for(;;) {//不断检查是否有新消息需要处理
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指向一个Handler对象，调用Handler的dispatchMessage方法分发消息
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.
finallongnewIdent = 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();
}
}
2.通过Handler类的dispatchMessage分发消息：
[java]view plaincopy
//分发Looper传来的消息
publicvoiddispatchMessage(Message msg) {
if(msg.callback !=null) {
handleCallback(msg);
}else{
if(mCallback !=null) {
if(mCallback.handleMessage(msg)) {
return;
}
}
handleMessage(msg);
}
}
3.通过Handler的成员函数handleMessage处理消息：
[java]view plaincopy
//通过重写handleMessage方法处理消息
publicvoidhandleMessage(Message msg) { }
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