显示框架之Choreographer

Android为了提供一个稳定的帧率输出机制，让软件层和硬件层可以以共同的频率一起工作，引入了 Vsync + TripleBuffer + Choreographer 的刷帧机制。

引入Choreographer

Choreographer 的引入，主要是配合 Vsync ，给上层 App 的渲染提供一个稳定的 Message 处理的时机，也就是 Vsync 到来的时候 ，会唤醒Choreographer 来做 App 的绘制操作。

Choreographer作用

Choreographer 扮演 Android 渲染链路中承上启下的角色。

承上：负责接收和处理 App 的各种更新消息和回调，等到 Vsync 到来的时候统一处理。比如集中处理 Input(主要是 Input 事件的处理) 、Animation(动画相关)、Traversal(包括 measure、layout、draw 等操作) ，判断卡顿掉帧情况，记录 CallBack 耗时等

启下：负责请求和接收 Vsync 信号，通过申请和接收vsync来驱动app刷新。

简单理解，Choreographer + SurfaceFlinger + Vsync + TripleBuffer这一套从上到下的机制，保证了 Android App 可以以一个稳定的帧率运行，减少帧率波动带来的不适感。下面从源码一探究竟Choreographer 的工作逻辑。

Choreographer初始化

文件：frameworks/base/core/java/android/view/ViewRootImpl.java

public ViewRootImpl(Context context, Display display, IWindowSession session,
            boolean useSfChoreographer) {
       ...
       // ViewRootImpl 初始化的时候创建Choreographer
        mChoreographer = useSfChoreographer
                ? Choreographer.getSfInstance() : Choreographer.getInstance();
       ...

}

文件：frameworks/base/core/java/android/view/Choreographer.java

// Thread local storage for the choreographer.
    private static final ThreadLocal<Choreographer> sThreadInstance =
            new ThreadLocal<Choreographer>() {
        @Override
        protected Choreographer initialValue() {
           // 获取当前线程的looper
            Looper looper = Looper.myLooper();
            if (looper == null) {
                throw new IllegalStateException("The current thread must have a looper!");
            }
           // 创建Choreographer 对象，source类型为app
            Choreographer choreographer = new Choreographer(looper, VSYNC_SOURCE_APP);
            if (looper == Looper.getMainLooper()) {
                mMainInstance = choreographer;
            }
            return choreographer;
        }
    };

  private Choreographer(Looper looper, int vsyncSource) {
        mLooper = looper;
        // 初始化 FrameHandler
        mHandler = new FrameHandler(looper);
       // 初始化 DisplayEventReceiver
        mDisplayEventReceiver = USE_VSYNC
                ? new FrameDisplayEventReceiver(looper, vsyncSource)
                : null;
        mLastFrameTimeNanos = Long.MIN_VALUE;

        mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
        // 初始化 CallbacksQueues，存放5种类型的callback
        mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
        for (int i = 0; i <= CALLBACK_LAST; i++) {
            mCallbackQueues[i] = new CallbackQueue();
        }
        // b/68769804: For low FPS experiments.
        setFPSDivisor(SystemProperties.getInt(ThreadedRenderer.DEBUG_FPS_DIVISOR, 1));
    }

     // 实现handleMessage 方法
    private final class FrameHandler extends Handler {
        public FrameHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_DO_FRAME:
                    doFrame(System.nanoTime(), 0);
                    break;
                case MSG_DO_SCHEDULE_VSYNC:
                    doScheduleVsync();
                    break;
                case MSG_DO_SCHEDULE_CALLBACK:
                    doScheduleCallback(msg.arg1);
                    break;
            }
        }
    }

我们把FrameDisplayEventReceiver 的初始化单独拿出来分析。

FrameDisplayEventReceiver初始化

文件:frameworks/base/core/java/android/view/Choreographer.java 

private final class FrameDisplayEventReceiver extends DisplayEventReceiver
            implements Runnable {
        private boolean mHavePendingVsync;
        private long mTimestampNanos;
        private int mFrame;

        public FrameDisplayEventReceiver(Looper looper, int vsyncSource) {
           // 执行DisplayEventReceiver 的初始化
            super(looper, vsyncSource, CONFIG_CHANGED_EVENT_SUPPRESS);
        }

文件:frameworks/base/core/java/android/view/DisplayEventReceiver.java 

public DisplayEventReceiver(Looper looper, int vsyncSource, int configChanged) {
        if (looper == null) {
            throw new IllegalArgumentException("looper must not be null");
        }

        mMessageQueue = looper.getQueue();
        // 通过jni调到native层
        mReceiverPtr = nativeInit(new WeakReference<DisplayEventReceiver>(this), mMessageQueue,
                vsyncSource, configChanged);

        mCloseGuard.open("dispose");
    }

文件：frameworks/base/core/jni/android_view_DisplayEventReceiver.cpp

static jlong nativeInit(JNIEnv* env, jclass clazz, jobject receiverWeak,
        jobject messageQueueObj, jint vsyncSource, jint configChanged) {
    ...
     // NativeDisplayEventReceiver 继承 DisplayEventDispatcher， 执行DisplayEventDispatcher的初始化
    sp<NativeDisplayEventReceiver> receiver = new NativeDisplayEventReceiver(env,
            receiverWeak, messageQueue, vsyncSource, configChanged);
    // 执行DisplayEventDispatcher：：initialize
    status_t status = receiver->initialize();
    ...
}

文件：frameworks/native/libs/gui/DisplayEventDispatcher.cpp

DisplayEventDispatcher::DisplayEventDispatcher(const sp<Looper>& looper,
                                               ISurfaceComposer::VsyncSource vsyncSource,
                                               ISurfaceComposer::ConfigChanged configChanged)
      : mLooper(looper), mReceiver(vsyncSource, configChanged), mWaitingForVsync(false) {
    ALOGV("dispatcher %p ~ Initializing display event dispatcher.", this);
}

status_t DisplayEventDispatcher::initialize() {
    status_t result = mReceiver.initCheck();
    if (result) {
        ALOGW("Failed to initialize display event receiver, status=%d", result);
        return result;
    }

    if (mLooper != nullptr) {
        // 添加mReceiveFd 监控，同Surfaceflinger的Message添加fd类似，区别是looper在不同的线程，回调的方法也不一样
        int rc = mLooper->addFd(mReceiver.getFd(), 0, Looper::EVENT_INPUT, this, NULL);
        if (rc < 0) {
            return UNKNOWN_ERROR;
        }
    }

    return OK;
}

这里重要的方法是mLooper->addFd(mReceiver.getFd(), 0, Looper::EVENT_INPUT, this, NULL)， 添加mReceiveFd 监控。那什么时候唤醒底层的loop等待呢？就是执行EventThread:: dispatchEvent，会对mSendFd进行write操作，这里采用socket通信，当监听到mReceiveFd 有事件来时，进行唤醒，从而执行回调函数handleEvent。

文件：system/core/libutils/Looper.cpp

int Looper::addFd(int fd, int ident, int events, const sp<LooperCallback>& callback, void* data) {
...
   Request request;
   request.fd = fd;
   request.ident = ident;
   request.events = events;
   request.seq = mNextRequestSeq++;
   request.callback = callback;
   request.data = data;
...
   if (requestIndex < 0) {
            // 添加mReceiveFd 进行监控
            int epollResult = epoll_ctl(mEpollFd.get(), EPOLL_CTL_ADD, fd, &eventItem);
            if (epollResult < 0) {
                ALOGE("Error adding epoll events for fd %d: %s", fd, strerror(errno));
                return -1;
            }
            mRequests.add(fd, request);

...
}


int Looper::pollInner(int timeoutMillis) {
...
int eventCount = epoll_wait(mEpollFd.get(), eventItems, EPOLL_MAX_EVENTS, timeoutMillis);
...
for (size_t i = 0; i < mResponses.size(); i++) {
      ...
            // 执行addFd 时加入的回调函数
            int callbackResult = response.request.callback->handleEvent(fd, events, data);
      ...
    return result;
}

文件:frameworks/native/libs/gui/DisplayEventDispatcher.cpp 

int DisplayEventDispatcher::handleEvent(int, int events, void*) {
    if (events & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP)) {
        ALOGE("Display event receiver pipe was closed or an error occurred.  "
              "events=0x%x",
              events);
        return 0; // remove the callback
    }

    if (!(events & Looper::EVENT_INPUT)) {
        ALOGW("Received spurious callback for unhandled poll event.  "
              "events=0x%x",
              events);
        return 1; // keep the callback
    }

    // Drain all pending events, keep the last vsync.
    nsecs_t vsyncTimestamp;
    PhysicalDisplayId vsyncDisplayId;
    uint32_t vsyncCount;
    if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount)) {
        ALOGV("dispatcher %p ~ Vsync pulse: timestamp=%" PRId64
              ", displayId=%" ANDROID_PHYSICAL_DISPLAY_ID_FORMAT ", count=%d",
              this, ns2ms(vsyncTimestamp), vsyncDisplayId, vsyncCount);
        mWaitingForVsync = false;
        dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount);
    }

    return 1; // keep the callback
}

到这里初始化完成，总结下做了哪些事情：

1. 初始化DisplayEventDispatcher，添加mReceiveFd进行监控，为vsync事件到来做回调准备
2. 初始化CallbacksQueues，里面存放5种类型的callback
3. 初始化FrameHandler，实现了handleMessage 方法

APP请求Vsync

随着代码流程走到requestLayout(addView->setView->requestLayout->schduleTraversals)表示app要请求vsync刷帧，具体看下请求经过

文件：frameworks/base/core/java/android/view/ViewRootImpl.java 

void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
            // 添加CALLBACK_TRAVERSAL 类型的回调
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            notifyRendererOfFramePending();
            pokeDrawLockIfNeeded();
        }
    }

文件： frameworks/base/core/java/android/view/Choreographer.java 

private void postCallbackDelayedInternal(int callbackType,
            Object action, Object token, long delayMillis) {
        if (DEBUG_FRAMES) {
            Log.d(TAG, "PostCallback: type=" + callbackType
                    + ", action=" + action + ", token=" + token
                    + ", delayMillis=" + delayMillis);
        }

        synchronized (mLock) {
            final long now = SystemClock.uptimeMillis();
            final long dueTime = now + delayMillis;
            // mCallbackQueues[CALLBACK_TRAVERSAL] 队列添加一个回调
            mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);

            if (dueTime <= now) {
                // dueTime == now ，从这里进来
                scheduleFrameLocked(now);
            } else {
                Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
                msg.arg1 = callbackType;
                msg.setAsynchronous(true);
                mHandler.sendMessageAtTime(msg, dueTime);
            }
        }
    }

// CallbackRecord 是个单链表，有3个成员，dueTime，action，token和1个next指针
// 将回调加入到当前的链表中
public void addCallbackLocked(long dueTime, Object action, Object token) {
            CallbackRecord callback = obtainCallbackLocked(dueTime, action, token);
            CallbackRecord entry = mHead;
            if (entry == null) {
                mHead = callback;
                return;
            }
            if (dueTime < entry.dueTime) {
                callback.next = entry;
                mHead = callback;
                return;
            }
            while (entry.next != null) {
                if (dueTime < entry.next.dueTime) {
                    callback.next = entry.next;
                    break;
                }
                entry = entry.next;
            }
            entry.next = callback;
        }

// obtainCallbackLocked 的作用是如果当前pool里面没有回调，则新创建一个，如果有，则返回当前的回调并将mCallbackPool 指向下一个回调
private CallbackRecord obtainCallbackLocked(long dueTime, Object action, Object token) {
        CallbackRecord callback = mCallbackPool;
        if (callback == null) {
            callback = new CallbackRecord();
        } else {
            mCallbackPool = callback.next;
            callback.next = null;
        }
        callback.dueTime = dueTime;
        callback.action = action;
        callback.token = token;
        return callback;
    }

 private void scheduleFrameLocked(long now) {
        if (!mFrameScheduled) {
            mFrameScheduled = true;
            if (USE_VSYNC) {
                if (DEBUG_FRAMES) {
                    Log.d(TAG, "Scheduling next frame on vsync.");
                }

               // 判断looper是不是在主线程，这两个方法殊途同归，都会执行scheduleVsyncLocked
                if (isRunningOnLooperThreadLocked()) {
                    scheduleVsyncLocked();
                } else {
                    Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
                    msg.setAsynchronous(true);
                    mHandler.sendMessageAtFrontOfQueue(msg);
                }
          ...
        }
    }

private void scheduleVsyncLocked() {
        mDisplayEventReceiver.scheduleVsync();
    }

文件：frameworks/native/libs/gui/DisplayEventDispatcher.cpp

status_t DisplayEventDispatcher::scheduleVsync() {
    if (!mWaitingForVsync) {
        ...
        status_t status = mReceiver.requestNextVsync();
        if (status) {
            ALOGW("Failed to request next vsync, status=%d", status);
            return status;
        }

        mWaitingForVsync = true;
    }
    return OK;
}

文件：frameworks/native/libs/gui/DisplayEventReceiver.cpp 

status_t DisplayEventReceiver::requestNextVsync() {
    if (mEventConnection != nullptr) {
        mEventConnection->requestNextVsync();
        return NO_ERROR;
    }
    return NO_INIT;
}

文件：frameworks/native/services/surfaceflinger/Scheduler/EventThread.cpp 

void EventThreadConnection::requestNextVsync() {
    ATRACE_NAME("requestNextVsync");
    mEventThread->requestNextVsync(this);
}

void EventThread::requestNextVsync(const sp<EventThreadConnection>& connection) {
    if (connection->resyncCallback) {
        connection->resyncCallback();
    }

    std::lock_guard<std::mutex> lock(mMutex);

    if (connection->vsyncRequest == VSyncRequest::None) {
        connection->vsyncRequest = VSyncRequest::Single;
        mCondition.notify_all();
    }
}

跟着代码流程可以看到，最终调到app EventThread的requestNextVsync，将connection->vsyncRequest更新为Single，并唤醒mCondition的wait，用一副图表示。

app请求vsync流程.png

APP接收Vsync

sw-vsync分发需要具备两个条件：1. 有请求vsync的动作 2. TimerDispatch线程定时时间到，会make一个vsync event。在当前帧请求vsync，在下一帧创造vsync event后就会回调给app。

文件：frameworks/native/services/surfaceflinger/Scheduler/EventThread.cpp

void EventThread::threadMain(std::unique_lock<std::mutex>& lock) {
DisplayEventConsumers consumers;

    while (mState != State::Quit) {
        std::optional<DisplayEventReceiver::Event> event;

        // Determine next event to dispatch.
        if (!mPendingEvents.empty()) {
            event = mPendingEvents.front();
            mPendingEvents.pop_front();
      ...

       if (!consumers.empty()) {
             // 开始分发vsync event，如果是app eventthread就分发给app
            dispatchEvent(*event, consumers);
            consumers.clear();
        }
...
}

tatus_t EventThreadConnection::postEvent(const DisplayEventReceiver::Event& event) {
    ssize_t size = DisplayEventReceiver::sendEvents(&mChannel, &event, 1);
    return size < 0 ? status_t(size) : status_t(NO_ERROR);
}

文件：frameworks/native/libs/gui/DisplayEventReceiver.cpp
ssize_t DisplayEventReceiver::sendEvents(gui::BitTube* dataChannel,
        Event const* events, size_t count)
{
    return gui::BitTube::sendObjects(dataChannel, events, count);
}

文件：frameworks/native/libs/gui/BitTube.cpp
ssize_t BitTube::sendObjects(BitTube* tube, void const* events, size_t count, size_t objSize) {
    const char* vaddr = reinterpret_cast<const char*>(events);
    ssize_t size = tube->write(vaddr, count * objSize);

    // should never happen because of SOCK_SEQPACKET
    LOG_ALWAYS_FATAL_IF((size >= 0) && (size % static_cast<ssize_t>(objSize)),
                        "BitTube::sendObjects(count=%zu, size=%zu), res=%zd (partial events were "
                        "sent!)",
                        count, objSize, size);

    // ALOGE_IF(size<0, "error %d sending %d events", size, count);
    return size < 0 ? size : size / static_cast<ssize_t>(objSize);
}

ssize_t BitTube::write(void const* vaddr, size_t size) {
    ssize_t err, len;
    do {
        // 对端mReceiveFd，会唤醒epoll_wait
        len = ::send(mSendFd, vaddr, size, MSG_DONTWAIT | MSG_NOSIGNAL);
        // cannot return less than size, since we're using SOCK_SEQPACKET
        err = len < 0 ? errno : 0;
    } while (err == EINTR);
    return err == 0 ? len : -err;
}

对端mReceiveFd 被添加到epllo fd进行监听，当事件来临时就会执行回调函数handleEvent

文件：frameworks/native/libs/gui/DisplayEventDispatcher.cpp
int DisplayEventDispatcher::handleEvent(int, int events, void*) {
    ...
    nsecs_t vsyncTimestamp;
    PhysicalDisplayId vsyncDisplayId;
    uint32_t vsyncCount;
    // 得到发送过来的vsync event
    if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount)) {
        ALOGV("dispatcher %p ~ Vsync pulse: timestamp=%" PRId64
              ", displayId=%" ANDROID_PHYSICAL_DISPLAY_ID_FORMAT ", count=%d",
              this, ns2ms(vsyncTimestamp), vsyncDisplayId, vsyncCount);
        mWaitingForVsync = false;
        // 处理vsync event
        dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount);
    }

    return 1; // keep the callback
}

bool DisplayEventDispatcher::processPendingEvents(nsecs_t* outTimestamp,
                                                  PhysicalDisplayId* outDisplayId,
                                                  uint32_t* outCount) {
    bool gotVsync = false;
    DisplayEventReceiver::Event buf[EVENT_BUFFER_SIZE];
    ssize_t n;
    // 获取发送过来的vsync event
    while ((n = mReceiver.getEvents(buf, EVENT_BUFFER_SIZE)) > 0) {
        ALOGV("dispatcher %p ~ Read %d events.", this, int(n));
        for (ssize_t i = 0; i < n; i++) {
            const DisplayEventReceiver::Event& ev = buf[i];
            switch (ev.header.type) {
                case DisplayEventReceiver::DISPLAY_EVENT_VSYNC:
                    // Later vsync events will just overwrite the info from earlier
                    // ones. That's fine, we only care about the most recent.
                    gotVsync = true;
                    *outTimestamp = ev.header.timestamp;
                    *outDisplayId = ev.header.displayId;
                    *outCount = ev.vsync.count;
                    break;
               ...
    return gotVsync;
}

文件：frameworks/base/core/jni/android_view_DisplayEventReceiver.cpp

void NativeDisplayEventReceiver::dispatchVsync(nsecs_t timestamp, PhysicalDisplayId displayId,
                                               uint32_t count) {
    JNIEnv* env = AndroidRuntime::getJNIEnv();

    ScopedLocalRef<jobject> receiverObj(env, jniGetReferent(env, mReceiverWeakGlobal));
    if (receiverObj.get()) {
        ALOGV("receiver %p ~ Invoking vsync handler.", this);
        // 从jni调到java方法
        env->CallVoidMethod(receiverObj.get(),
                gDisplayEventReceiverClassInfo.dispatchVsync, timestamp, displayId, count);
        ALOGV("receiver %p ~ Returned from vsync handler.", this);
    }

    mMessageQueue->raiseAndClearException(env, "dispatchVsync");
}

文件：frameworks/base/core/java/android/view/DisplayEventReceiver.java
 private void dispatchVsync(long timestampNanos, long physicalDisplayId, int frame) {
        onVsync(timestampNanos, physicalDisplayId, frame);
    }

文件：frameworks/base/core/java/android/view/Choreographer.java
public void onVsync(long timestampNanos, long physicalDisplayId, int frame) {
            ...
            mTimestampNanos = timestampNanos;
            mFrame = frame;
            // obtain 会new message，callback为FrameDisplayEventReceiver对象，实现了Runnable的方法
            Message msg = Message.obtain(mHandler, this);
            msg.setAsynchronous(true);
            // 发送message，唤醒loop wait
            mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
        }

文件：frameworks/base/core/java/android/os/Message.java 
public static Message obtain(Handler h, Runnable callback) {
        Message m = obtain();
        m.target = h;
        m.callback = callback;

        return m;
    }

文件：frameworks/base/core/java/android/os/Handler.java
  public void dispatchMessage(@NonNull Message msg) {
        if (msg.callback != null) {
            // callback 不为空，走handleCallback
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

 private static void handleCallback(Message message) {
        // 执行FrameDisplayEventReceiver的run方法
        message.callback.run();
    }

文件：frameworks/base/core/java/android/view/Choreographer.java 
 public void run() {
            mHavePendingVsync = false;
             // 处理 App 的各种更新消息和回调
            doFrame(mTimestampNanos, mFrame);
        }
    }

从链路来看，最后会调到doFrame来处理 App 的各种更新消息和回调，用个图总结下。

APP接收Vsync流程.png

APP处理Vsync

app接收到vsync信号后，就开始执行各种更新消息和回调，主要的处理逻辑在doFrame函数。

文件：frameworks/base/core/java/android/view/Choreographer.java
void doFrame(long frameTimeNanos, int frame) {
        // 处理掉帧逻辑，暂且不看
        ...
        try {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
            AnimationUtils.lockAnimationClock(frameTimeNanos / TimeUtils.NANOS_PER_MS);
            // 记录input开始时间
            mFrameInfo.markInputHandlingStart();
            // 回调input方法
            doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
            //记录动画开始时间
            mFrameInfo.markAnimationsStart();
             //回调动画方法   
            doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
            doCallbacks(Choreographer.CALLBACK_INSETS_ANIMATION, frameTimeNanos);
             // 记录Traversal开始时间
            mFrameInfo.markPerformTraversalsStart();
              // 回调Traversal 方法，重点看下
            doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);

            doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
        } finally {
            AnimationUtils.unlockAnimationClock();
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }

        if (DEBUG_FRAMES) {
            final long endNanos = System.nanoTime();
            Log.d(TAG, "Frame " + frame + ": Finished, took "
                    + (endNanos - startNanos) * 0.000001f + " ms, latency "
                    + (startNanos - frameTimeNanos) * 0.000001f + " ms.");
        }
    }

 void doCallbacks(int callbackType, long frameTimeNanos) {
            ...
            //取出对应的callback，还记得前面是通过addCallbackLocked将callback加入到链表里面，现将对应的callback取出
            callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
                    now / TimeUtils.NANOS_PER_MS);
            if (callbacks == null) {
                return;
            }
            mCallbacksRunning = true;

            ...

        try {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
            for (CallbackRecord c = callbacks; c != null; c = c.next) {
                if (DEBUG_FRAMES) {
                    Log.d(TAG, "RunCallback: type=" + callbackType
                            + ", action=" + c.action + ", token=" + c.token
                            + ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
                }
                // 执行CallbackRecord的run方法
                c.run(frameTimeNanos);
            }
        ...
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

 private static final class CallbackRecord {
        public CallbackRecord next;
        public long dueTime;
        public Object action; // Runnable or FrameCallback
        public Object token;

        @UnsupportedAppUsage
        public void run(long frameTimeNanos) {
            if (token == FRAME_CALLBACK_TOKEN) {
                ((FrameCallback)action).doFrame(frameTimeNanos);
            } else {
                // 执行mTraversalRunnable的run方法，在scheduleTraversals里面通过postCallback加入callback链表
                ((Runnable)action).run();
            }
        }
    }

文件：frameworks/base/core/java/android/view/ViewRootImpl.java 
final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            //最后回调到这个方法，处理measure、layout、draw 
            doTraversal();
        }
    }

 private void performTraversals() {
...
 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
 performLayout(lp, mWidth, mHeight);
...
 performDraw();
...
}

可以看到当app接收到vsync信号后就开始处理各种回调的方法，由于有些内容并不是很熟悉，也就没能继续分析下去，文章只是分析了大致的流程，具体的细节遇到问题再分析。
文章从app请求vsync，eventThread分发vsync，再到app处理vsync，分析了大致的流程，也印证了文章开始说的Choreographer作用： 1. 负责请求和接收 Vsync 信号 2. 负责接收和处理 App 的各种更新消息和回调，等到 Vsync 到来的时候统一处理。