Android Handler消息机制初探(2)

Android Handler消息机制初探(2)

面试被问到一个问题：
Handler处理Message的delay原理：

Hanlder.java

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);}queueMessage(msg, uptimeMillis);}

MessageQueue.java:

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);le();return false;}msg.markInUse();msg.when = when;Message p = mMessages;boolean needWake;//如果mMessage执行时间比msg晚则优先处理if (p == null || when == 0 || when < p.when) {// New head, wake up the event queue if  = p;mMessages = msg;needWake = mBlocked;} else {//将Message插入正确的队列的位置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;}} = p; // invariant: p ==  = msg;}// We can assume mPtr != 0 because mQuitting is false.if (needWake) {nativeWake(mPtr);}}return true;}

完成了入队操作

出队：

Message next() {.......int pendingIdleHandlerCount = -1; // -1 only during first iterationint 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 = ;} 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.//Message还没有准备好，时间没到nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);} else {// Got a message.mBlocked = false;if (prevMsg != null) { = ;} else {mMessages = ;} = null;if (DEBUG) Log.v(TAG, "Returning message: " + msg);msg.markInUse();return msg;}} else {// No PollTimeoutMillis = -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 = Array(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 handlerboolean keep = false;try {keep = idler.queueIdle();} catch (Throwable t) {Log.wtf(TAG, "IdleHandler threw exception", t);}if (!keep) {synchronized (this) {ve(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 PollTimeoutMillis = 0;}}

nativePollOnce(ptr, nextPollTimeoutMillis);调用了Looper::pollInner

Looper.java:

int Looper::pollInner(int timeoutMillis) {
#if DEBUG_POLL_AND_WAKEALOGD("%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_WAKEALOGD("%p ~ pollOnce - next message in %" PRId64 "ns, adjusted timeout: timeoutMillis=%d",this, mNextMessageUptime - now, timeoutMillis);
#endif}// Poll.int result = POLL_WAKE;mResponses.clear();mResponseIndex = 0;// We are about to idle.mPolling = true;struct epoll_event eventItems[EPOLL_MAX_EVENTS];//调用了epoll_wait()int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);// 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: %s", strerror(errno));result = POLL_ERROR;goto Done;}// Check for poll timeout.if (eventCount == 0) {
#if DEBUG_POLL_AND_WAKEALOGD("%p ~ pollOnce - timeout", this);
#endifresult = POLL_TIMEOUT;goto Done;}// Handle all events.
#if DEBUG_POLL_AND_WAKEALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
#endiffor (int i = 0; i < eventCount; i++) {int fd = eventItems[i].data.fd;uint32_t epollEvents = eventItems[i].events;if (fd == mWakeEventFd) {if (epollEvents & EPOLLIN) {awoken();} else {ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);}} 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);}}}
Done: ;// Invoke pending message callbacks.//处理等待的messagemNextMessageUptime = LLONG_MAX;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//Message出队列sp<MessageHandler> handler = messageEnvelope.handler;Message message = veAt(0);mSendingMessage = true;mLock.unlock();#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKSALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d",this, (), message.what);
#endifhandler->handleMessage(message);} // release handlermLock.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++) {Response& response = mResponses.editItemAt(i);if (quest.ident == POLL_CALLBACK) {int fd = quest.fd;int events = response.events;void* data = quest.data;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKSALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",this, (), 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 = quest.callback->handleEvent(fd, events, data);if (callbackResult == 0) {removeFd(fd, quest.seq);}// Clear the callback reference in the response structure promptly because we// will not clear the response vector itself until the quest.callback.clear();result = POLL_CALLBACK;}}return result;
}int Looper::pollAll(int timeoutMillis, int* outFd, int* outEvents, void** outData) {if (timeoutMillis <= 0) {int result;do {result = pollOnce(timeoutMillis, outFd, outEvents, outData);} while (result == POLL_CALLBACK);return result;} else {nsecs_t endTime = systemTime(SYSTEM_TIME_MONOTONIC)+ milliseconds_to_nanoseconds(timeoutMillis);for (;;) {int result = pollOnce(timeoutMillis, outFd, outEvents, outData);if (result != POLL_CALLBACK) {return result;}nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);timeoutMillis = toMillisecondTimeoutDelay(now, endTime);if (timeoutMillis == 0) {return POLL_TIMEOUT;}}}
}

参考
.html

得到一个信息：Native层的优先级比Java高