android的UI开发工程师指引

不管是MFC，还是linux，还是android，UI开发都是如下两大核心机制：

第一个是消息循环，第二个是界面组织结构。

围绕着这些，衍生出来的OpenGL，SurfaceView，SurfaceFinger等都是为这两大机制服务的。

打一个比方。

消息循环是UI中的发动机。

界面组织结构就是UI的设计结构。

而其他的东西，则是建立在这些基础之上的。

理解这两大块儿，那么android的UI基础就学得差不多了。这个时候可以结合一些例子，来做一些真正有意义的开发，例如UI特效啊。自定义动画啊。。。。也可以顺便把动画机制给理解吃透。

接下来就学一下Canvas，SurfaceFlinger，Matrix，来做一些特效。

如果想更深入地学习。那么学习一下OpenGL。

再想深入的话，学习一下JNI编程。

再深入的话，把java虚拟机给了解一下，也许对于提高程序效率帮助很大。

本篇就介绍一下消息队列：

android_os_MessageQueue.cpp

MessageQueue.java

Looper.cpp (frameworks\base\native\android) 2369 2011/12/12

Looper.java (frameworks\base\core\java\android\os) 8874 2011/12/12
Handler.java (frameworks\base\core\java\android\os) 23620 2011/12/12

Activity中的事件默认都是在UI线程中发生的。

这意味着Activity中的任何一个函数执行完之后，都要回到消息队列，这个节点。handleMessage结束之后，就会再次去消息队列查看消息。这跟windows上开发的消息队列的概念是一致的。

1.入队：

入队的时候，按照Message.when的大小进行排序。如果时间相同，那么按照入队的先后进行排序。

如果入队的时候，时间戳为0，那么就激活消息管道。否则不激活等超时。

MessageQueue.java

final boolean enqueueMessage(Message msg, long when) {
if (msg.isInUse()) {
throw new AndroidRuntimeException(msg
+ " This message is already in use.");
}
if (msg.target == null && !mQuitAllowed) {
throw new RuntimeException("Main thread not allowed to quit");
}
final boolean needWake;
synchronized (this) {
if (mQuiting) {
RuntimeException e = new RuntimeException(
msg.target + " sending message to a Handler on a dead thread");
Log.w("MessageQueue", e.getMessage(), e);
return false;
} else if (msg.target == null) {
mQuiting = true;
}

msg.when = when;
//Log.d("MessageQueue", "Enqueing: " + msg);
Message p = mMessages;
if (p == null || when == 0 || when < p.when) {
msg.next = p;
mMessages = msg;
needWake = mBlocked; // new head, might need to wake up
} else {
Message prev = null;
while (p != null && p.when <= when) {
prev = p;
p = p.next;
}
msg.next = prev.next;
prev.next = msg;
needWake = false; // still waiting on head, no need to wake up
}
}
if (needWake) {
nativeWake(mPtr);
}
return true;
}

2. 遍历

遍历的时候，按照队列头部的时间戳（为0，则立即调用，否则等待超时），进行poll函数调用。

final Message next() {
int pendingIdleHandlerCount = -1; // -1 only during first iteration
int nextPollTimeoutMillis = 0;

for (;;) {
if (nextPollTimeoutMillis != 0) {
Binder.flushPendingCommands();
}
nativePollOnce(mPtr, nextPollTimeoutMillis);

synchronized (this) {
// Try to retrieve the next message.  Return if found.
final long now = SystemClock.uptimeMillis();
final Message msg = mMessages;
if (msg != null) {
final long when = msg.when;
if (now >= when) {
mBlocked = false;
mMessages = msg.next;
msg.next = null;
if (false) Log.v("MessageQueue", "Returning message: " + msg);
msg.markInUse();
return msg;
} else {
nextPollTimeoutMillis = (int) Math.min(when - now, Integer.MAX_VALUE);
}
} else {
nextPollTimeoutMillis = -1;
}

// If first time, then get the number of idlers to run.
if (pendingIdleHandlerCount < 0) {
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;
}
}

3.。 下面看一下looper的loop函数

Looper.java (frameworks\base\core\java\android\os) 8874 2011/12/12

public static void loop() {
Looper me = myLooper();
if (me == null) {
throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
}
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();

while (true) {
Message msg = queue.next(); // might block
if (msg != null) {
if (msg.target == null) {
// No target is a magic identifier for the quit message.
return;
}

long wallStart = 0;
long threadStart = 0;

// 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);
wallStart = SystemClock.currentTimeMicro();
threadStart = SystemClock.currentThreadTimeMicro();
}

msg.target.dispatchMessage(msg);

if (logging != null) {
long wallTime = SystemClock.currentTimeMicro() - wallStart;
long threadTime = SystemClock.currentThreadTimeMicro() - threadStart;

logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
if (logging instanceof Profiler) {
((Profiler) logging).profile(msg, wallStart, wallTime,
threadStart, threadTime);
}
}

// 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.recycle();
}
}
}

sendMessage调用的queMessage，这与windows很不相同。

5. 一个小小的设计，则是效率上N倍以上的提升。

另外在UI开发中invalidate到最后的时候，就是一个sendMessage，而不是直接调用traversal方法。为的就是异步处理，防止一个循环周期中调用多次invalidate。这样可以给程序一个联合rect的机会。

代码如下：

注意mTraversalScheduled这个成员，是理解本机制的关键：

ViewRoot.java

void invalidate() {
mDirty.set(0, 0, mWidth, mHeight);
scheduleTraversals();
}

public void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;

//noinspection ConstantConditions
if (ViewDebug.DEBUG_LATENCY && mLastTraversalFinishedTimeNanos != 0) {
final long now = System.nanoTime();
Log.d(TAG, "Latency: Scheduled traversal, it has been "
+ ((now - mLastTraversalFinishedTimeNanos) * 0.000001f)
+ "ms since the last traversal finished.");
}

sendEmptyMessage(DO_TRAVERSAL);
}
}

完毕。这是我对UI开发的一些基础性的理解，请扔砖。也希望能抛砖引玉吧。

6.next函数一开始就调用了poolInner。来扫描一下用户事件，并直接调用到onTouchEvent等：

int Looper::pollInner(int timeoutMillis) {
#if DEBUG_POLL_AND_WAKE
LOGD("%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
LOGD("%p ~ pollOnce - next message in %lldns, adjusted timeout: timeoutMillis=%d",
this, mNextMessageUptime - now, timeoutMillis);
#endif
}

// Poll.
int result = ALOOPER_POLL_WAKE;
mResponses.clear();
mResponseIndex = 0;

#ifdef LOOPER_STATISTICS
nsecs_t pollStartTime = systemTime(SYSTEM_TIME_MONOTONIC);
#endif

#ifdef LOOPER_USES_EPOLL
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
#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_t requestedCount = mRequestedFds.size();
int eventCount = poll(mRequestedFds.editArray(), requestedCount, timeoutMillis);
#endif

// Acquire lock.
mLock.lock();

// Check for poll error.
if (eventCount < 0) {
if (errno == EINTR) {
goto Done;
}
LOGW("Poll failed with an unexpected error, errno=%d", errno);
result = ALOOPER_POLL_ERROR;
goto Done;
}

// Check for poll timeout.
if (eventCount == 0) {
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - timeout", this);
#endif
result = ALOOPER_POLL_TIMEOUT;
goto Done;
}

// Handle all events.
#if DEBUG_POLL_AND_WAKE
LOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);
#endif

#ifdef LOOPER_USES_EPOLL
for (int i = 0; i < eventCount; i++) {
int fd = eventItems[i].data.fd;
uint32_t epollEvents = eventItems[i].events;
if (fd == mWakeReadPipeFd) {
if (epollEvents & EPOLLIN) {
awoken();
} else {
LOGW("Ignoring unexpected epoll events 0x%x on wake read pipe.", epollEvents);
}
} else {
ssize_t requestIndex = mRequests.indexOfKey(fd);
if (requestIndex >= 0) {
int events = 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);
}
}
}
Done: ;
#else
for (size_t i = 0; i < requestedCount; i++) {
const struct pollfd& requestedFd = mRequestedFds.itemAt(i);

short pollEvents = 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 {
int events = 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.
mPolling = false;
if (mWaiters != 0) {
mAwake.broadcast();
}
#endif

#ifdef LOOPER_STATISTICS
nsecs_t pollEndTime = systemTime(SYSTEM_TIME_MONOTONIC);
mSampledPolls += 1;
if (timeoutMillis == 0) {
mSampledZeroPollCount += 1;
mSampledZeroPollLatencySum += pollEndTime - pollStartTime;
} else if (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

// Invoke pending message callbacks.
mNextMessageUptime = 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
sp<MessageHandler> handler = messageEnvelope.handler;
Message message = messageEnvelope.message;
mMessageEnvelopes.removeAt(0);
mSendingMessage = true;
mLock.unlock();

#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
LOGD("%p ~ pollOnce - sending message: handler=%p, what=%d",
this, handler.get(), message.what);
#endif
handler->handleMessage(message);
} // release handler

mLock.lock();
mSendingMessage = false;
result = ALOOPER_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++) {
const Response& response = mResponses.itemAt(i);
ALooper_callbackFunc callback = response.request.callback;
if (callback) {
int fd = response.request.fd;
int events = response.events;
void* data = response.request.data;
#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS
LOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",
this, callback, fd, events, data);
#endif
int callbackResult = callback(fd, events, data);
if (callbackResult == 0) {
removeFd(fd);
}
result = ALOOPER_POLL_CALLBACK;
}
}
return result;
}

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