消息都是存放在一个消息队列中去，而消息循环线程就是围绕这个消息队列进入一个无限循环的，直到线程退出。如果队列中有消息，消息循环线程就会把它取出来，并分发给相应的Handler进行处理；如果队列中没有消息，消息循环线程就会进入空闲等待状态，等待下一个消息的到来。在编写Android应用程序时，当程序执行的任务比较繁重时，为了不阻塞UI主线程而导致ANR的发生，我们通常的做法的创建一个子线程来完成特定的任务。在创建子线程时，有两种选择，一种通过创建Thread对象来创建一个无消息循环的子线程；还有一种就是创建一个带有消息循环的子线程，而创建带有消息循环的子线程由于两种实现方法，一种是直接利用Android给我们封装好的HandlerThread类来直接生成一个带有消息循环的线程对象，另一种方法是在实现线程的run()方法内使用以下方式启动一个消息循环：

一、消息机制使用

通常消息都是有一个消息线程和一个Handler组成，下面我们看PowerManagerService中的一个消息Handler：

mHandlerThread = new ServiceThread(TAG, Process.THREAD_PRIORITY_DISPLAY, false /*allowIo*/); mHandlerThread.start(); mHandler = new PowerManagerHandler(mHandlerThread.getLooper());

这里的ServiceThread就是一个HandlerThread，创建Handler的时候，必须把HandlerThread的looper传进去，否则就是默认当前线程的looper。

而每个handler，大致如下：

private final class PowerManagerHandler extends Handler { public PowerManagerHandler(Looper looper) { super(looper, null, true /*async*/); } @Override public void handleMessage(Message msg) { switch (msg.what) { case MSG_USER_ACTIVITY_TIMEOUT: handleUserActivityTimeout(); break; case MSG_SANDMAN: handleSandman(); break; case MSG_SCREEN_BRIGHTNESS_BOOST_TIMEOUT: handleScreenBrightnessBoostTimeout(); break; case MSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT: checkWakeLockAquireTooLong(); Message m = mHandler.obtainMessage(MSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT); m.setAsynchronous(true); mHandler.sendMessageDelayed(m, WAKE_LOCK_ACQUIRE_TOO_LONG_TIMEOUT); break; } } }

二、消息机制原理

那我们先来看下HandlerThread的主函数run函数：

public void run() { mTid = Process.myTid(); Looper.prepare(); synchronized (this) { mLooper = Looper.myLooper();//赋值后notifyall，主要是getLooper函数返回的是mLooper notifyAll(); } Process.setThreadPriority(mPriority); onLooperPrepared(); Looper.loop(); mTid = -1; }

再来看看Lopper的prepare函数，最后新建了一个Looper对象，并且放在线程的局部变量中。

public static void prepare() { prepare(true); } 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)); }

Looper的构造函数中创建了MessageQueue

private Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mThread = Thread.currentThread(); }

我们再来看下MessageQueue的构造函数，其中nativeInit是一个native方法，并且把返回值保存在mPtr显然是用long型变量保存的指针

MessageQueue(boolean quitAllowed) { mQuitAllowed = quitAllowed; mPtr = nativeInit(); }

native函数中主要创建了NativeMessageQueue对象，并且把指针变量返回了。

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) { NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue(); if (!nativeMessageQueue) { jniThrowRuntimeException(env, "Unable to allocate native queue"); return 0; } nativeMessageQueue->incStrong(env); return reinterpret_cast<jlong>(nativeMessageQueue); }

NativeMessageQueue构造函数就是获取mLooper，如果没有就是新建一个Looper

NativeMessageQueue::NativeMessageQueue() : mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) { mLooper = Looper::getForThread(); if (mLooper == NULL) { mLooper = new Looper(false); Looper::setForThread(mLooper); } }

然后我们再看下Looper的构造函数，显示调用了eventfd创建了一个fd，eventfd它的主要是用于进程或者线程间的通信，我们可以看下这篇博客eventfd介绍

Looper::Looper(bool allowNonCallbacks) : mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false), mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false), mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) { mWakeEventFd = eventfd(0, EFD_NONBLOCK); LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd. errno=%d", errno); AutoMutex _l(mLock); rebuildEpollLocked(); }

2.1 c层创建epoll

我们再来看下rebuildEpollLocked函数，创建了epoll，并且把mWakeEventFd加入epoll，而且把mRequests的fd也加入epoll

void Looper::rebuildEpollLocked() { // Close old epoll instance if we have one. if (mEpollFd >= 0) { #if DEBUG_CALLBACKS ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this); #endif close(mEpollFd); } // Allocate the new 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); struct epoll_event eventItem; memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union eventItem.events = EPOLLIN; eventItem.data.fd = mWakeEventFd; int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem); LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance. errno=%d", errno); for (size_t i = 0; i < mRequests.size(); i++) { const Request& request = mRequests.valueAt(i); struct epoll_event eventItem; request.initEventItem(&eventItem); int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem); if (epollResult < 0) { ALOGE("Error adding epoll events for fd %d while rebuilding epoll set, errno=%d", request.fd, errno); } } }

继续回到HandlerThread的run函数，我们继续分析Looper的loop函数

public void run() { mTid = Process.myTid(); Looper.prepare(); synchronized (this) { mLooper = Looper.myLooper(); notifyAll(); } Process.setThreadPriority(mPriority); onLooperPrepared(); Looper.loop(); mTid = -1; }

我们看看Looper的loop函数：

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;//得到Looper的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这个函数会阻塞，阻塞主要是epoll_wait 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;//自己打的打印 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(); } }

MessageQueue类的next函数主要是调用了nativePollOnce函数，后面就是从消息队列中取出一个Message

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;//之前保留的指针 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);

下面我们主要看下nativePollOnce这个native函数，把之前的指针强制转换成NativeMessageQueue，然后调用其pollOnce函数

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj, jlong ptr, jint timeoutMillis) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); nativeMessageQueue->pollOnce(env, obj, timeoutMillis); }

2.2 c层epoll_wait阻塞

pollOnce函数，这个函数前面的while一般都没有只是处理了indent大于0的情况，这种情况一般没有，所以我们可以直接看pollInner函数

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) { int result = 0; for (;;) { while (mResponseIndex < mResponses.size()) { const Response& response = mResponses.itemAt(mResponseIndex++); int ident = response.request.ident; if (ident >= 0) { int fd = response.request.fd; int events = response.events; void* data = response.request.data; #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - returning signalled identifier %d: " "fd=%d, events=0x%x, data=%p", this, ident, fd, events, data); #endif if (outFd != NULL) *outFd = fd; if (outEvents != NULL) *outEvents = events; if (outData != NULL) *outData = data; return ident; } } if (result != 0) { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - returning result %d", this, result); #endif if (outFd != NULL) *outFd = 0; if (outEvents != NULL) *outEvents = 0; if (outData != NULL) *outData = NULL; return result; } result = pollInner(timeoutMillis); } }

pollInner函数主要就是调用epoll_wait阻塞，并且java层会计算每次阻塞的时间传到c层，等待有mWakeEventFd或者之前addFd的fd有事件过来，才会epoll_wait返回。

int Looper::pollInner(int timeoutMillis) { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis); #endif // Adjust the timeout based on when the next message is due. if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) { nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC); int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime); if (messageTimeoutMillis >= 0 && (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) { timeoutMillis = messageTimeoutMillis; } #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - next message in %" PRId64 "ns, adjusted timeout: timeoutMillis=%d", this, mNextMessageUptime - now, timeoutMillis); #endif } // Poll. int result = POLL_WAKE; mResponses.clear();//清空mResponses mResponseIndex = 0; // We are about to idle. mPolling = true; struct epoll_event eventItems[EPOLL_MAX_EVENTS]; int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);//epoll_wait主要线程阻塞在这，这个阻塞的时间也是有java层传过来的 // No longer idling. mPolling = false; // Acquire lock. mLock.lock(); // Rebuild epoll set if needed. if (mEpollRebuildRequired) { mEpollRebuildRequired = false; rebuildEpollLocked(); goto Done; } // Check for poll error. if (eventCount < 0) { if (errno == EINTR) { goto Done; } ALOGW("Poll failed with an unexpected error, errno=%d", errno); result = POLL_ERROR; goto Done; } // Check for poll timeout. if (eventCount == 0) { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - timeout", this); #endif result = POLL_TIMEOUT; goto Done; } // Handle all events. #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount); #endif for (int i = 0; i < eventCount; i++) { int fd = eventItems[i].data.fd; uint32_t epollEvents = eventItems[i].events; if (fd == mWakeEventFd) {//通知唤醒线程的事件 if (epollEvents & EPOLLIN) { awoken(); } else { ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents); } } else { ssize_t requestIndex = mRequests.indexOfKey(fd);//之前addFd的事件 if (requestIndex >= 0) { int events = 0; if (epollEvents & EPOLLIN) events |= EVENT_INPUT; if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT; if (epollEvents & EPOLLERR) events |= EVENT_ERROR; if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP; pushResponse(events, mRequests.valueAt(requestIndex));//放在mResponses中 } else { ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is " "no longer registered.", epollEvents, fd); } } } Done: ; // Invoke pending message callbacks. mNextMessageUptime = LLONG_MAX; while (mMessageEnvelopes.size() != 0) {// 这块主要是c层的消息，java层的消息是自己管理的 nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC); const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0); if (messageEnvelope.uptime <= now) { // Remove the envelope from the list. // We keep a strong reference to the handler until the call to handleMessage // finishes. Then we drop it so that the handler can be deleted *before* // we reacquire our lock. { // obtain handler sp<MessageHandler> handler = messageEnvelope.handler; Message message = messageEnvelope.message; mMessageEnvelopes.removeAt(0); mSendingMessage = true; mLock.unlock(); #if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d", this, handler.get(), message.what); #endif handler->handleMessage(message); } // release handler mLock.lock(); mSendingMessage = false; result = POLL_CALLBACK; } else { // The last message left at the head of the queue determines the next wakeup time. mNextMessageUptime = messageEnvelope.uptime; break; } } // Release lock. mLock.unlock(); // Invoke all response callbacks. for (size_t i = 0; i < mResponses.size(); i++) {//这是之前addFd的事件的处理，主要是遍历mResponses，然后调用其回调 Response& response = mResponses.editItemAt(i); if (response.request.ident == POLL_CALLBACK) { int fd = response.request.fd; int events = response.events; void* data = response.request.data; #if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p", this, response.request.callback.get(), fd, events, data); #endif // Invoke the callback. Note that the file descriptor may be closed by // the callback (and potentially even reused) before the function returns so // we need to be a little careful when removing the file descriptor afterwards. int callbackResult = response.request.callback->handleEvent(fd, events, data); if (callbackResult == 0) { removeFd(fd, response.request.seq); } // Clear the callback reference in the response structure promptly because we // will not clear the response vector itself until the next poll. response.request.callback.clear(); result = POLL_CALLBACK; } } return result; }

继续分析Looper的loop函数，可以增加自己的打印来调试代码，之前调用Message的target的dispatchMessage来分配消息

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;//自己的打印 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(); } }

2.3 增加调试打印

我们先来看自己添加打印，可以通过Lopper的setMessageLogging函数来打印

public void setMessageLogging(@Nullable Printer printer) { mLogging = printer; } Printer就是一个interface public interface Printer { /** * Write a line of text to the output. There is no need to terminate * the given string with a newline. */ void println(String x); }

2.4 java层消息分发处理

再来看消息的分发，先是调用Handler的obtainMessage函数

Message msg = mHandler.obtainMessage(MSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT); msg.setAsynchronous(true); mHandler.sendMessageDelayed(msg, WAKE_LOCK_ACQUIRE_TOO_LONG_TIMEOUT);

先看obtainMessage调用了Message的obtain函数

public final Message obtainMessage(int what) { return Message.obtain(this, what); }

Message的obtain函数就是新建一个Message，然后其target就是设置成其Handler

public static Message obtain(Handler h, int what) { Message m = obtain();//就是新建一个Message m.target = h; m.what = what; return m; }

我们再联系之前分发消息

msg.target.dispatchMessage(msg);最后就是调用Handler的dispatchMessage函数，最后在Handler中，最后会根据不同的情况对消息进行处理。

public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg);//这种就是用post形式发送，带Runnable的 } else { if (mCallback != null) {//这种是handler传参的时候就是传入了mCallback回调了 if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg);//最后就是在自己实现的handleMessage处理 } }

2.3 java层 消息发送

我们再看下java层的消息发送，主要也是调用Handler的sendMessage post之类函数，最终都会调用下面这个函数

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); }

我们再来看java层发送消息最终都会调用enqueueMessage函数

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this; if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }

最终在enqueueMessage中，把消息加入消息队列，然后需要的话就调用c层的nativeWake函数

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(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; }

我们看下这个native方法，最后也是调用了Looper的wake函数

static void android_os_MessageQueue_nativeWake(JNIEnv* env, jclass clazz, jlong ptr) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); nativeMessageQueue->wake(); } void NativeMessageQueue::wake() { mLooper->wake(); }

Looper类的wake，函数只是往mWakeEventfd中写了一些内容，这个fd只是通知而已，类似pipe，最后会把epoll_wait唤醒，线程就不阻塞了继续先发送c层消息，然后处理之前addFd的事件，然后处理java层的消息。

void Looper::wake() { #if DEBUG_POLL_AND_WAKE ALOGD("%p ~ wake", this); #endif uint64_t inc = 1; ssize_t nWrite = TEMP_FAILURE_RETRY(write(mWakeEventFd, &inc, sizeof(uint64_t))); if (nWrite != sizeof(uint64_t)) { if (errno != EAGAIN) { ALOGW("Could not write wake signal, errno=%d", errno); } } }

2.4 c层发送消息

在c层也是可以发送消息的，主要是调用Looper的sendMessageAtTime函数，参数有有一个handler是一个回调，我们把消息放在mMessageEnvelopes中。

void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler, const Message& message) { #if DEBUG_CALLBACKS ALOGD("%p ~ sendMessageAtTime - uptime=%" PRId64 ", handler=%p, what=%d", this, uptime, handler.get(), message.what); #endif size_t i = 0; { // acquire lock AutoMutex _l(mLock); size_t messageCount = mMessageEnvelopes.size(); while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) { i += 1; } MessageEnvelope messageEnvelope(uptime, handler, message); mMessageEnvelopes.insertAt(messageEnvelope, i, 1); // Optimization: If the Looper is currently sending a message, then we can skip // the call to wake() because the next thing the Looper will do after processing // messages is to decide when the next wakeup time should be. In fact, it does // not even matter whether this code is running on the Looper thread. if (mSendingMessage) { return; } } // release lock // Wake the poll loop only when we enqueue a new message at the head. if (i == 0) { wake(); } }

当在pollOnce中，在epoll_wait之后，会遍历mMessageEnvelopes中的消息，然后调用其handler的handleMessage函数

while (mMessageEnvelopes.size() != 0) { nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC); const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0); if (messageEnvelope.uptime <= now) { // Remove the envelope from the list. // We keep a strong reference to the handler until the call to handleMessage // finishes. Then we drop it so that the handler can be deleted *before* // we reacquire our lock. { // obtain handler sp<MessageHandler> handler = messageEnvelope.handler; Message message = messageEnvelope.message; mMessageEnvelopes.removeAt(0); mSendingMessage = true; mLock.unlock(); #if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d", this, handler.get(), message.what); #endif handler->handleMessage(message); } // release handler mLock.lock(); mSendingMessage = false; result = POLL_CALLBACK; } else { // The last message left at the head of the queue determines the next wakeup time. mNextMessageUptime = messageEnvelope.uptime; break; } }

有一个Looper_test.cpp文件，里面介绍了很多Looper的使用方法，我们来看下

sp<StubMessageHandler> handler = new StubMessageHandler(); mLooper->sendMessageAtTime(now + ms2ns(100), handler, Message(MSG_TEST1)); StubMessageHandler继承MessageHandler就必须实现handleMessage方法 class StubMessageHandler : public MessageHandler { public: Vector<Message> messages; virtual void handleMessage(const Message& message) { messages.push(message); } };

我们再顺便看下Message和MessageHandler类

struct Message { Message() : what(0) { } Message(int what) : what(what) { } /* The message type. (interpretation is left up to the handler) */ int what; }; /** * Interface for a Looper message handler. * * The Looper holds a strong reference to the message handler whenever it has * a message to deliver to it. Make sure to call Looper::removeMessages * to remove any pending messages destined for the handler so that the handler * can be destroyed. */ class MessageHandler : public virtual RefBase { protected: virtual ~MessageHandler() { } public: /** * Handles a message. */ virtual void handleMessage(const Message& message) = 0; };

2.5 c层addFd

我们也可以在Looper.cpp的addFd中增加fd放入线程epoll中，当fd有数据来我们也可以处理相应的数据，下面我们先来看下addFd函数，我们注意其中有一个callBack回调

int Looper::addFd(int fd, int ident, int events, Looper_callbackFunc callback, void* data) { return addFd(fd, ident, events, callback ? new SimpleLooperCallback(callback) : NULL, data); } int Looper::addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data) { #if DEBUG_CALLBACKS ALOGD("%p ~ addFd - fd=%d, ident=%d, events=0x%x, callback=%p, data=%p", this, fd, ident, events, callback.get(), data); #endif if (!callback.get()) { if (! mAllowNonCallbacks) { ALOGE("Invalid attempt to set NULL callback but not allowed for this looper."); return -1; } if (ident < 0) { ALOGE("Invalid attempt to set NULL callback with ident < 0."); return -1; } } else { ident = POLL_CALLBACK; } { // acquire lock AutoMutex _l(mLock); Request request; request.fd = fd; request.ident = ident; request.events = events; request.seq = mNextRequestSeq++; request.callback = callback; request.data = data; if (mNextRequestSeq == -1) mNextRequestSeq = 0; // reserve sequence number -1 struct epoll_event eventItem; request.initEventItem(&eventItem); ssize_t requestIndex = mRequests.indexOfKey(fd); if (requestIndex < 0) { int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem);//加入epoll if (epollResult < 0) { ALOGE("Error adding epoll events for fd %d, errno=%d", fd, errno); return -1; } mRequests.add(fd, request);//放入mRequests中 } else { int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_MOD, fd, & eventItem);//更新 if (epollResult < 0) { if (errno == ENOENT) { // Tolerate ENOENT because it means that an older file descriptor was // closed before its callback was unregistered and meanwhile a new // file descriptor with the same number has been created and is now // being registered for the first time. This error may occur naturally // when a callback has the side-effect of closing the file descriptor // before returning and unregistering itself. Callback sequence number // checks further ensure that the race is benign. // // Unfortunately due to kernel limitations we need to rebuild the epoll // set from scratch because it may contain an old file handle that we are // now unable to remove since its file descriptor is no longer valid. // No such problem would have occurred if we were using the poll system // call instead, but that approach carries others disadvantages. #if DEBUG_CALLBACKS ALOGD("%p ~ addFd - EPOLL_CTL_MOD failed due to file descriptor " "being recycled, falling back on EPOLL_CTL_ADD, errno=%d", this, errno); #endif epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, fd, & eventItem); if (epollResult < 0) { ALOGE("Error modifying or adding epoll events for fd %d, errno=%d", fd, errno); return -1; } scheduleEpollRebuildLocked(); } else { ALOGE("Error modifying epoll events for fd %d, errno=%d", fd, errno); return -1; } } mRequests.replaceValueAt(requestIndex, request); } } // release lock return 1; }

在pollOnce函数中，我们先寻找mRequests中匹配的fd，然后在pushResponse中新建一个Response，然后把Response和Request匹配起来。

} else { ssize_t requestIndex = mRequests.indexOfKey(fd); if (requestIndex >= 0) { int events = 0; if (epollEvents & EPOLLIN) events |= EVENT_INPUT; if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT; if (epollEvents & EPOLLERR) events |= EVENT_ERROR; if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP; pushResponse(events, mRequests.valueAt(requestIndex)); } else { ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is " "no longer registered.", epollEvents, fd); } }

下面我们就会遍历mResponses中的Response，然后调用其request中的回调

for (size_t i = 0; i < mResponses.size(); i++) { Response& response = mResponses.editItemAt(i); if (response.request.ident == POLL_CALLBACK) { int fd = response.request.fd; int events = response.events; void* data = response.request.data; #if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p", this, response.request.callback.get(), fd, events, data); #endif // Invoke the callback. Note that the file descriptor may be closed by // the callback (and potentially even reused) before the function returns so // we need to be a little careful when removing the file descriptor afterwards. int callbackResult = response.request.callback->handleEvent(fd, events, data); if (callbackResult == 0) { removeFd(fd, response.request.seq); } // Clear the callback reference in the response structure promptly because we // will not clear the response vector itself until the next poll. response.request.callback.clear(); result = POLL_CALLBACK; } }

同样我们再来看看Looper_test.cpp是如何使用的？

Pipe pipe; StubCallbackHandler handler(true); handler.setCallback(mLooper, pipe.receiveFd, Looper::EVENT_INPUT);

我们看下handler的setCallback函数

class CallbackHandler { public: void setCallback(const sp<Looper>& looper, int fd, int events) { looper->addFd(fd, 0, events, staticHandler, this);//就是调用了looper的addFd函数，并且回调 } protected: virtual ~CallbackHandler() { } virtual int handler(int fd, int events) = 0; private: static int staticHandler(int fd, int events, void* data) {//这个就是回调函数 return static_cast<CallbackHandler*>(data)->handler(fd, events); } }; class StubCallbackHandler : public CallbackHandler { public: int nextResult; int callbackCount; int fd; int events; StubCallbackHandler(int nextResult) : nextResult(nextResult), callbackCount(0), fd(-1), events(-1) { } protected: virtual int handler(int fd, int events) {//这个是通过回调函数再调到这里的 callbackCount += 1; this->fd = fd; this->events = events; return nextResult; } };

我们结合Looper的addFd一起来看，当callback是有的，我们新建一个SimpleLooperCallback

int Looper::addFd(int fd, int ident, int events, Looper_callbackFunc callback, void* data) { return addFd(fd, ident, events, callback ? new SimpleLooperCallback(callback) : NULL, data); }

这里的Looper_callbackFunc是一个typedef

typedef int (*Looper_callbackFunc)(int fd, int events, void* data);

我们再来看SimpleLooperCallback

class SimpleLooperCallback : public LooperCallback { protected: virtual ~SimpleLooperCallback(); public: SimpleLooperCallback(Looper_callbackFunc callback); virtual int handleEvent(int fd, int events, void* data); private: Looper_callbackFunc mCallback; };SimpleLooperCallback::SimpleLooperCallback(Looper_callbackFunc callback) : mCallback(callback) { } SimpleLooperCallback::~SimpleLooperCallback() { } int SimpleLooperCallback::handleEvent(int fd, int events, void* data) { return mCallback(fd, events, data); }

最后我们是调用callback->handleEvent(fd, events, data)，而callback就是SimpleLooperCallback，这里的data，之前传进来的就是CallbackHandler 的this指针

因此最后就是调用了staticHandler，而data->handler，就是this->handler，最后是虚函数就调用到了StubCallbackHandler 的handler函数中了。

当然我们也可以不用这么复杂，直接使用第二个addFd函数，当然callBack我们需要自己定义一个类来实现LooperCallBack类就行了，这样就简单多了。

int addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data);

2.6 java层addFd

一直以为只能在c层的Looper中才能addFd，原来在java层也通过jni做了这个功能。

我们可以在MessageQueue中的addOnFileDescriptorEventListener来实现这个功能

public void addOnFileDescriptorEventListener(@NonNull FileDescriptor fd, @OnFileDescriptorEventListener.Events int events, @NonNull OnFileDescriptorEventListener listener) { if (fd == null) { throw new IllegalArgumentException("fd must not be null"); } if (listener == null) { throw new IllegalArgumentException("listener must not be null"); } synchronized (this) { updateOnFileDescriptorEventListenerLocked(fd, events, listener); } }

我们再来看看OnFileDescriptorEventListener 这个回调

public interface OnFileDescriptorEventListener { public static final int EVENT_INPUT = 1 << 0; public static final int EVENT_OUTPUT = 1 << 1; public static final int EVENT_ERROR = 1 << 2; /** @hide */ @Retention(RetentionPolicy.SOURCE) @IntDef(flag=true, value={EVENT_INPUT, EVENT_OUTPUT, EVENT_ERROR}) public @interface Events {} @Events int onFileDescriptorEvents(@NonNull FileDescriptor fd, @Events int events); }

接着调用了updateOnFileDescriptorEventListenerLocked函数

private void updateOnFileDescriptorEventListenerLocked(FileDescriptor fd, int events, OnFileDescriptorEventListener listener) { final int fdNum = fd.getInt$(); int index = -1; FileDescriptorRecord record = null; if (mFileDescriptorRecords != null) { index = mFileDescriptorRecords.indexOfKey(fdNum); if (index >= 0) { record = mFileDescriptorRecords.valueAt(index); if (record != null && record.mEvents == events) { return; } } } if (events != 0) { events |= OnFileDescriptorEventListener.EVENT_ERROR; if (record == null) { if (mFileDescriptorRecords == null) { mFileDescriptorRecords = new SparseArray<FileDescriptorRecord>(); } record = new FileDescriptorRecord(fd, events, listener);//fd保存在FileDescriptorRecord对象 mFileDescriptorRecords.put(fdNum, record);//mFileDescriptorRecords然后保存在 } else { record.mListener = listener; record.mEvents = events; record.mSeq += 1; } nativeSetFileDescriptorEvents(mPtr, fdNum, events);//调用native函数 } else if (record != null) { record.mEvents = 0; mFileDescriptorRecords.removeAt(index); } }

native最后调用了NativeMessageQueue的setFileDescriptorEvents函数

static void android_os_MessageQueue_nativeSetFileDescriptorEvents(JNIEnv* env, jclass clazz, jlong ptr, jint fd, jint events) { NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr); nativeMessageQueue->setFileDescriptorEvents(fd, events); }

setFileDescriptorEvents函数，这个addFd就是调用的第二个addFd，因此我们可以肯定NativeMessageQueue继承了LooperCallback

void NativeMessageQueue::setFileDescriptorEvents(int fd, int events) { if (events) { int looperEvents = 0; if (events & CALLBACK_EVENT_INPUT) { looperEvents |= Looper::EVENT_INPUT; } if (events & CALLBACK_EVENT_OUTPUT) { looperEvents |= Looper::EVENT_OUTPUT; } mLooper->addFd(fd, Looper::POLL_CALLBACK, looperEvents, this, reinterpret_cast<void*>(events)); } else { mLooper->removeFd(fd); } }

果然是，需要实现handleEvent函数

class NativeMessageQueue : public MessageQueue, public LooperCallback { public: NativeMessageQueue(); virtual ~NativeMessageQueue(); virtual void raiseException(JNIEnv* env, const char* msg, jthrowable exceptionObj); void pollOnce(JNIEnv* env, jobject obj, int timeoutMillis); void wake(); void setFileDescriptorEvents(int fd, int events); virtual int handleEvent(int fd, int events, void* data);

handleEvent就是在looper中epoll_wait之后，当我们增加的fd有数据就会调用这个函数

int NativeMessageQueue::handleEvent(int fd, int looperEvents, void* data) { int events = 0; if (looperEvents & Looper::EVENT_INPUT) { events |= CALLBACK_EVENT_INPUT; } if (looperEvents & Looper::EVENT_OUTPUT) { events |= CALLBACK_EVENT_OUTPUT; } if (looperEvents & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP | Looper::EVENT_INVALID)) { events |= CALLBACK_EVENT_ERROR; } int oldWatchedEvents = reinterpret_cast<intptr_t>(data); int newWatchedEvents = mPollEnv->CallIntMethod(mPollObj, gMessageQueueClassInfo.dispatchEvents, fd, events); //调用回调 if (!newWatchedEvents) { return 0; // unregister the fd } if (newWatchedEvents != oldWatchedEvents) { setFileDescriptorEvents(fd, newWatchedEvents); } return 1; }

最后在java的MessageQueue中的dispatchEvents就是在jni层反调过来的，然后调用之前注册的回调函数

// Called from native code. private int dispatchEvents(int fd, int events) { // Get the file descriptor record and any state that might change. final FileDescriptorRecord record; final int oldWatchedEvents; final OnFileDescriptorEventListener listener; final int seq; synchronized (this) { record = mFileDescriptorRecords.get(fd);//通过fd得到FileDescriptorRecord if (record == null) { return 0; // spurious, no listener registered } oldWatchedEvents = record.mEvents; events &= oldWatchedEvents; // filter events based on current watched set if (events == 0) { return oldWatchedEvents; // spurious, watched events changed } listener = record.mListener; seq = record.mSeq; } // Invoke the listener outside of the lock. int newWatchedEvents = listener.onFileDescriptorEvents(//listener回调 record.mDescriptor, events); if (newWatchedEvents != 0) { newWatchedEvents |= OnFileDescriptorEventListener.EVENT_ERROR; } // Update the file descriptor record if the listener changed the set of // events to watch and the listener itself hasn't been updated since. if (newWatchedEvents != oldWatchedEvents) { synchronized (this) { int index = mFileDescriptorRecords.indexOfKey(fd); if (index >= 0 && mFileDescriptorRecords.valueAt(index) == record && record.mSeq == seq) { record.mEvents = newWatchedEvents; if (newWatchedEvents == 0) { mFileDescriptorRecords.removeAt(index); } } } } // Return the new set of events to watch for native code to take care of. return newWatchedEvents; }

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