前言
Android应用的UI界面需要多个角色共同协作才能完成渲染上屏，最终呈现到屏幕上被用户看到。其中包括App进程的测量、布局和绘制，SurfaceFlinger进程的渲染数据合成等。而App进程何时开始测量、布局和绘制，SurfaceFlinger进程何时开始数据的合成和上屏，决定了Android系统能否有条不紊地刷新屏幕内容，给用户提供流畅的使用体验。
为了协调App进程的视图数据生产和SurfaceFlinger进程的视图数据消费处理，Android系统引入Choreographer对App进程的布局绘制工作和SurfaceFlinger进程的数据合成工作进行调度，减少因为Android应用绘制渲染和屏幕刷新之间不同步导致的屏幕撕裂（tearing）问题。
本文从Choreographer的创建流程和Choreographer的VSYNC请求、分发以及处理两个方面来分析Choreographer机制的工作原理。
说明：本文中的源码对应的Android版本是Android 13。
Choreographer的创建流程
先给出Choreographer的创建时机：Android中启动一个Activity时，在SystemServer进程在ActivityTaskSupervisor#realStartActivityLocked方法中通过Binder跨进程调用到App进程，在App进程调用了IApplicationThread#scheduleTransaction，最终执行完Activity#onResume方法之后会调用ViewManager#addView，最终创建了ViewRootImpl实例，并在ViewRootImpl的构造函数中完成了Choreographer实例创建，并作为ViewRootImpl的成员变量持有。
SystemServer进程收到App进程启动Activity的Binder请求后，在ActivityTaskSupervisor#realStartActivityLocked方法中封装LaunchActivityItem和ResumeActivityItem并通过ClientLifecycleManager#scheduleTransaction调度执行任务，最终还是通过App进程的IApplicationThread（Binder实例）跨进程调用到App进程的ApplicationThread#scheduleTransaction方法。
java
// com.android.server.wm.ActivityTaskSupervisor#realStartActivityLocked
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc, boolean andResume, boolean checkConfig) throws RemoteException {
// ...
// Create activity launch transaction.
final ClientTransaction clientTransaction = ClientTransaction.obtain(proc.getThread(), r.token);
// ...
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent), System.identityHashCode(r), r.info, mergedConfiguration.getGlobalConfiguration(), mergedConfiguration.getOverrideConfiguration(), r.compat, r.getFilteredReferrer(r.launchedFromPackage), task.voiceInteractor, proc.getReportedProcState(), r.getSavedState(), r.getPersistentSavedState(), results, newIntents, r.takeOptions(), isTransitionForward, proc.createProfilerInfoIfNeeded(), r.assistToken, activityClientController, r.shareableActivityToken, r.getLaunchedFromBubble(), fragmentToken));
// Set desired final state.
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(isTransitionForward);
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// Schedule transaction.
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
// ...
}
// android.app.ClientTransactionHandler#scheduleTransaction
void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
final IApplicationThread client = transaction.getClient();
transaction.schedule();
if (!(client instanceof Binder)) {
// If client is not an instance of Binder - it's a remote call and at this point it is
// safe to recycle the object. All objects used for local calls will be recycled after
// the transaction is executed on client in ActivityThread.
transaction.recycle();
}
}
// android.app.servertransaction.ClientTransaction#schedule
/**
* 当transaction初始化之后开始调度执行，将会被发送给client端并按照以下顺序进行处理：
* 1. 调用preExecute(ClientTransactionHandler)
* 2. 调度transaction对应的消息
* 3. 调用TransactionExecutor#execute(ClientTransaction)
*/
public void schedule() throws RemoteException {
mClient.scheduleTransaction(this); // mClient为App进程通过Binder通信传递的Binder句柄IApplicationThread。
}- 此时调用链已经传递到App进程，由
ActivityThread的ApplicationThread类型的成员变量进行处理，而ApplicationThread#scheduleTransaction方法直接调用了外部类ActivityThread的scheduleTransaction方法，相当于直接转发给ActivityThread#scheduleTransaction。而ActivityThread#scheduleTransaction通过主线程的Handler对象mH向主线程的MessageQueue中插入了一个EXECUTE_TRANSACTION消息。此时，App进程在主线程中处理EXECUTE_TRANSACTION消息。
java
/**
* 负责管理应用进程中的主线程任务，按照SystemServer进程请求的那样对任务进行调度和执行。
*/
public final class ActivityThread extends ClientTransactionHandler
implements ActivityThreadInternal {
@UnsupportedAppUsage
final ApplicationThread mAppThread = new ApplicationThread();
@UnsupportedAppUsage
final Looper mLooper = Looper.myLooper();
@UnsupportedAppUsage
final H mH = new H();
// An executor that performs multi-step transactions.
private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);
// ...
private class ApplicationThread extends IApplicationThread.Stub {
// ...
@Override
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
ActivityThread.this.scheduleTransaction(transaction);
}
// ...
}
// from the class ClientTransactionHandler that is the base class of ActivityThread
/** Prepare and schedule transaction for execution. */
void scheduleTransaction(ClientTransaction transaction) {
transaction.preExecute(this);
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}
private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
if (DEBUG_MESSAGES) {
Slog.v(TAG,
"SCHEDULE " + what + " " + mH.codeToString(what) + ": " + arg1 + " / " + obj);
}
Message msg = Message.obtain();
msg.what = what;
msg.obj = obj;
msg.arg1 = arg1;
msg.arg2 = arg2;
if (async) {
msg.setAsynchronous(true);
}
// 主线程的Handler
mH.sendMessage(msg);
}
class H extends Handler {
public void handleMessage(Message msg) {
// ...
switch (msg.what) {
// ...
case EXECUTE_TRANSACTION:
final ClientTransaction transaction = (ClientTransaction) msg.obj;
mTransactionExecutor.execute(transaction);
if (isSystem()) {
// Client transactions inside system process are recycled on the client side
// instead of ClientLifecycleManager to avoid being cleared before this
// message is handled.
transaction.recycle();
}
// TODO(lifecycler): Recycle locally scheduled transactions.
break;
// ...
}
// ...
}
}- 主线程调用
TransactionExecutor#execute方法处理消息，并在executeCallbacks方法中执行了ResumeActivityItem
java
/**
* Class that manages transaction execution in the correct order.
*/
public class TransactionExecutor {
// ...
/**
* Resolve transaction.
* First all callbacks will be executed in the order they appear in the list. If a callback
* requires a certain pre- or post-execution state, the client will be transitioned accordingly.
* Then the client will cycle to the final lifecycle state if provided. Otherwise, it will
* either remain in the initial state, or last state needed by a callback.
*/
public void execute(ClientTransaction transaction) {
// ...
// 实际执行生命周期任务的地方
executeCallbacks(transaction);
executeLifecycleState(transaction);
mPendingActions.clear();
// ...
}
/** Cycle through all states requested by callbacks and execute them at proper times. */
@VisibleForTesting
public void executeCallbacks(ClientTransaction transaction) {
// 取出之前SystemServer进程放入的LaunchActivityItem和ResumeActivityItem
final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
if (callbacks == null || callbacks.isEmpty()) {
// No callbacks to execute, return early.
return;
}
if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Resolving callbacks in transaction");
final IBinder token = transaction.getActivityToken();
ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
// In case when post-execution state of the last callback matches the final state requested
// for the activity in this transaction, we won't do the last transition here and do it when
// moving to final state instead (because it may contain additional parameters from server).
final ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();
final int finalState = finalStateRequest != null ? finalStateRequest.getTargetState()
: UNDEFINED;
// Index of the last callback that requests some post-execution state.
final int lastCallbackRequestingState = lastCallbackRequestingState(transaction);
final int size = callbacks.size();
for (int i = 0; i < size; ++i) {
final ClientTransactionItem item = callbacks.get(i);
if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Resolving callback: " + item);
final int postExecutionState = item.getPostExecutionState();
final int closestPreExecutionState = mHelper.getClosestPreExecutionState(r,
item.getPostExecutionState());
if (closestPreExecutionState != UNDEFINED) {
cycleToPath(r, closestPreExecutionState, transaction);
}
// 执行LaunchActivityItem和ResumeActivityItem的execute方法
item.execute(mTransactionHandler, token, mPendingActions);
item.postExecute(mTransactionHandler, token, mPendingActions);
if (r == null) {
// Launch activity request will create an activity record.
r = mTransactionHandler.getActivityClient(token);
}
if (postExecutionState != UNDEFINED && r != null) {
// Skip the very last transition and perform it by explicit state request instead.
final boolean shouldExcludeLastTransition =
i == lastCallbackRequestingState && finalState == postExecutionState;
cycleToPath(r, postExecutionState, shouldExcludeLastTransition, transaction);
}
}
}
// ...
}- 最终调用到了
ActivityThread#handleResumeActivity方法，执行了resume，并调用的addView将DecorView和WindowManagerImpl进行关联。
java
/**
* Request to move an activity to resumed state.
* @hide
*/
public class ResumeActivityItem extends ActivityLifecycleItem {
private static final String TAG = "ResumeActivityItem";
// ...
@Override
public void execute(ClientTransactionHandler client, ActivityClientRecord r,
PendingTransactionActions pendingActions) {
Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
// client是android.app.ActivityThread
client.handleResumeActivity(r, true /* finalStateRequest */, mIsForward,
"RESUME_ACTIVITY");
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}
// ...
}
// android.app.ActivityThread#handleResumeActivity
@Override
public void handleResumeActivity(ActivityClientRecord r, boolean finalStateRequest,
boolean isForward, String reason) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
// 执行Activity的Resume方法
if (!performResumeActivity(r, finalStateRequest, reason)) {
return;
}
// ...
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (r.mPreserveWindow) {
a.mWindowAdded = true;
r.mPreserveWindow = false;
// Normally the ViewRoot sets up callbacks with the Activity
// in addView->ViewRootImpl#setView. If we are instead reusing
// the decor view we have to notify the view root that the
// callbacks may have changed.
ViewRootImpl impl = decor.getViewRootImpl();
if (impl != null) {
impl.notifyChildRebuilt();
}
}
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
// 将DecorView和WindowManagerImpl进行关联
wm.addView(decor, l);
} else {
// The activity will get a callback for this {@link LayoutParams} change
// earlier. However, at that time the decor will not be set (this is set
// in this method), so no action will be taken. This call ensures the
// callback occurs with the decor set.
a.onWindowAttributesChanged(l);
}
}
// If the window has already been added, but during resume
// we started another activity, then don't yet make the
// window visible.
} else if (!willBeVisible) {
if (localLOGV) Slog.v(TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
// ...
Looper.myQueue().addIdleHandler(new Idler());
}- 在
addView方法中创建了ViewRootImpl实例，并将ViewRootImpl实例和DecorView实例进行维护，最后调用setView将DecorView和ViewRootImpl进行关联，后续由ViewRootImpl作为桥梁来间接和DecorView进行交互。
java
/**
* Provides low-level communication with the system window manager for
* operations that are not associated with any particular context.
*
* This class is only used internally to implement global functions where
* the caller already knows the display and relevant compatibility information
* for the operation. For most purposes, you should use {@link WindowManager} instead
* since it is bound to a context.
*
* @see WindowManagerImpl
* @hide
*/
public final class WindowManagerGlobal {
@UnsupportedAppUsage
private final ArrayList<View> mViews = new ArrayList<View>();
@UnsupportedAppUsage
private final ArrayList<ViewRootImpl> mRoots = new ArrayList<ViewRootImpl>();
@UnsupportedAppUsage
private final ArrayList<WindowManager.LayoutParams> mParams = new ArrayList<WindowManager.LayoutParams>();
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow, int userId) {
// ...
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
// ...
IWindowSession windowlessSession = null;
// ...
// 创建ViewRootImpl实例
if (windowlessSession == null) {
root = new ViewRootImpl(view.getContext(), display);
} else {
root = new ViewRootImpl(view.getContext(), display, windowlessSession);
}
view.setLayoutParams(wparams);
// 维护DecorView、ViewRootImpl实例
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
// do this last because it fires off messages to start doing things
try {
// 调用setView将DecorView和ViewRootImpl进行关联，后续由ViewRootImpl作为桥梁来间接和DecorView进行交互
root.setView(view, wparams, panelParentView, userId);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
if (index >= 0) {
removeViewLocked(index, true);
}
throw e;
}
}
}
// ...
}- 最终，我们来到了
ViewRootImpl的构造函数，在ViewRootImpl的构造函数中创建了Choreographer实例，并作为ViewRootImpl的成员变量持有了，这样ViewRootImpl就作为桥梁在App应用层和Android系统之间进行双向通信。接下来看下Choreographer是如何请求VSYNC信号以及如何分发VSYNC信号，最终实现App的UI不断刷新到屏幕上。
java
public final class ViewRootImpl implements ViewParent,
View.AttachInfo.Callbacks, ThreadedRenderer.DrawCallbacks,
AttachedSurfaceControl {
private static final String TAG = "ViewRootImpl";
public ViewRootImpl(@UiContext Context context, Display display, IWindowSession session,
boolean useSfChoreographer) {
// ...
// 创建Choreographer实例
mChoreographer = useSfChoreographer ? Choreographer.getSfInstance() : Choreographer.getInstance();
// ...
}
// ...
}
/**
* Coordinates the timing of animations, input and drawing.
* <p>
* The choreographer receives timing pulses (such as vertical synchronization)
* from the display subsystem then schedules work to occur as part of rendering
* the next display frame.
* </p><p>
* Applications typically interact with the choreographer indirectly using
* higher level abstractions in the animation framework or the view hierarchy.
* Here are some examples of things you can do using the higher-level APIs.
* </p>
* <ul>
* <li>To post an animation to be processed on a regular time basis synchronized with
* display frame rendering, use {@link android.animation.ValueAnimator#start}.</li>
* <li>To post a {@link Runnable} to be invoked once at the beginning of the next display
* frame, use {@link View#postOnAnimation}.</li>
* <li>To post a {@link Runnable} to be invoked once at the beginning of the next display
* frame after a delay, use {@link View#postOnAnimationDelayed}.</li>
* <li>To post a call to {@link View#invalidate()} to occur once at the beginning of the
* next display frame, use {@link View#postInvalidateOnAnimation()} or
* {@link View#postInvalidateOnAnimation(int, int, int, int)}.</li>
* <li>To ensure that the contents of a {@link View} scroll smoothly and are drawn in
* sync with display frame rendering, do nothing. This already happens automatically.
* {@link View#onDraw} will be called at the appropriate time.</li>
* </ul>
* <p>
* However, there are a few cases where you might want to use the functions of the
* choreographer directly in your application. Here are some examples.
* </p>
* <ul>
* <li>If your application does its rendering in a different thread, possibly using GL,
* or does not use the animation framework or view hierarchy at all
* and you want to ensure that it is appropriately synchronized with the display, then use
* {@link Choreographer#postFrameCallback}.</li>
* <li>... and that's about it.</li>
* </ul>
* <p>
* Each {@link Looper} thread has its own choreographer. Other threads can
* post callbacks to run on the choreographer but they will run on the {@link Looper}
* to which the choreographer belongs.
* </p>
*/
public final class Choreographer {
private static final String TAG = "Choreographer";
// Thread local storage for the choreographer.
private static final ThreadLocal<Choreographer> sThreadInstance =
new ThreadLocal<Choreographer>() {
@Override
protected Choreographer initialValue() {
Looper looper = Looper.myLooper();
if (looper == null) {
throw new IllegalStateException("The current thread must have a looper!");
}
Choreographer choreographer = new Choreographer(looper, VSYNC_SOURCE_APP);
if (looper == Looper.getMainLooper()) {
mMainInstance = choreographer;
}
return choreographer;
}
};
private Choreographer(Looper looper, int vsyncSource) {
mLooper = looper;
mHandler = new FrameHandler(looper);
mDisplayEventReceiver = USE_VSYNC
? new FrameDisplayEventReceiver(looper, vsyncSource)
: null;
mLastFrameTimeNanos = Long.MIN_VALUE;
mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
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));
}
/**
* Gets the choreographer for the calling thread. Must be called from
* a thread that already has a {@link android.os.Looper} associated with it.
*
* @return The choreographer for this thread.
* @throws IllegalStateException if the thread does not have a looper.
*/
public static Choreographer getInstance() {
return sThreadInstance.get();
}
}从上面的分析中，我们知道了Choreographer的创建时机是在Activity#onResume执行之后，可见Android系统这么设计是出于Activity是Android应用中承载UI的容器，只有容器创建之后，才需要创建Choreographer来调度VSYNC信号，最终开启一帧帧的界面渲染和刷新。
那么会不会在每启动一个Activity之后都会创建一个Choreographer实例呢？答案是不会的，因为从Choreographer的构造过程可以知道，Choreographer的创建是通过ThreadLocal实现的，所以Choreographer是线程单例的，所以主线程只会创建一个Choreographer实例。
那么是不是任何一个线程都可以创建Choreographer实例呢？答案是只有创建了Looper的线程才能创建Choreographer实例，原因是Choreographer会通过Looper进行线程切换，至于为什么线程切换将会在下面进行分析回答。
VSYNC信号的调度分发流程
下面我们结合源码分析下，Choreographer是如何调度VSYNC信号的，调度之后又是如何接收VSYNC信号的，接收到VSYNC信号之后又是怎么处理的。
首先，我们看下Choreographer的构造函数做了哪些事情来实现VSYNC信号的调度分发。首先，基于主线程的Looper创建了FrameHandler用于线程切换，保证是在主线程请求调度VSYNC信号以及在主线程处理接收到的VSYNC信号。接着，创建了FrameDisplayEventReceiver用于请求和接收VSYNC信号。最后，创建了CallbackQueue类型的数组，用于接收业务层投递的各种类型的任务。
java
private Choreographer(Looper looper, int vsyncSource) {
mLooper = looper;
// 负责线程切换
mHandler = new FrameHandler(looper);
// 负责请求和接收VSYNC信号
mDisplayEventReceiver = USE_VSYNC
? new FrameDisplayEventReceiver(looper, vsyncSource)
: null;
mLastFrameTimeNanos = Long.MIN_VALUE;
mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
// 存储业务提交的任务，四种任务类型
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));
}
java
private final class CallbackQueue {
private CallbackRecord mHead;
// ...
}
// 链表结构
private static final class CallbackRecord {
public CallbackRecord next;
public long dueTime;
/** Runnable or FrameCallback or VsyncCallback object. */
public Object action;
/** Denotes the action type. */
public Object token;
// ...
}下面结合源码看下FrameDisplayEventReceiver的创建过程，可以看到FrameDisplayEventReceiver继承自DisplayEventReceiver，并在构造函数中调用了DisplayEventReceiver的构造函数。因此，我们继续跟下DisplayEventReceiver的构造函数。
java
private final class FrameDisplayEventReceiver extends DisplayEventReceiver implements Runnable {
private boolean mHavePendingVsync;
private long mTimestampNanos;
private int mFrame;
private VsyncEventData mLastVsyncEventData = new VsyncEventData();
// 直接调用DisplayEventReceiver的构造函数
public FrameDisplayEventReceiver(Looper looper, int vsyncSource) {
super(looper, vsyncSource, 0);
}
@Override
public void onVsync(long timestampNanos, long physicalDisplayId, int frame, VsyncEventData vsyncEventData) {
try {
long now = System.nanoTime();
if (timestampNanos > now) {
timestampNanos = now;
}
if (mHavePendingVsync) {
Log.w(TAG, "Already have a pending vsync event. There should only be "
+ "one at a time.");
} else {
mHavePendingVsync = true;
}
mTimestampNanos = timestampNanos;
mFrame = frame;
mLastVsyncEventData = vsyncEventData;
// 发送异步消息到主线程
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
// 在主线程执行
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame, mLastVsyncEventData);
}
}DisplayEventReceiver的构造函数中将主线程的MessageQueue取出，之后调用了nativeInit方法并传递了主线程的MessageQueue，并将自身作为参数也一起传入nativeInit方法。
java
/**
* Provides a low-level mechanism for an application to receive display events
* such as vertical sync.
*
* The display event receive is NOT thread safe. Moreover, its methods must only
* be called on the Looper thread to which it is attached.
*
* @hide
*/
public abstract class DisplayEventReceiver {
/**
* Creates a display event receiver.
*
* @param looper The looper to use when invoking callbacks.
* @param vsyncSource The source of the vsync tick. Must be on of the VSYNC_SOURCE_* values.
* @param eventRegistration Which events to dispatch. Must be a bitfield consist of the
* EVENT_REGISTRATION_*_FLAG values.
*/
public DisplayEventReceiver(Looper looper, int vsyncSource, int eventRegistration) {
if (looper == null) {
throw new IllegalArgumentException("looper must not be null");
}
mMessageQueue = looper.getQueue();
mReceiverPtr = nativeInit(new WeakReference<DisplayEventReceiver>(this), mMessageQueue,
vsyncSource, eventRegistration);
}
private static native long nativeInit(WeakReference<DisplayEventReceiver> receiver,
MessageQueue messageQueue, int vsyncSource, int eventRegistration);
}而nativeInit是一个native方法，具体逻辑是通过C++实现的，代码位于android_view_DisplayEventReceiver.cpp中。其中关键的部分是NativeDisplayEventReceiver的创建以及调用initialize方法进行初始化。
cpp
// frameworks/base/core/jni/android_view_DisplayEventReceiver.cpp
static jlong nativeInit(JNIEnv* env, jclass clazz, jobject receiverWeak, jobject vsyncEventDataWeak, jobject messageQueueObj, jint vsyncSource, jint eventRegistration, jlong layerHandle) {
// 获取native层的MessageQueue对象
sp<MessageQueue> messageQueue = android_os_MessageQueue_getMessageQueue(env, messageQueueObj);
if (messageQueue == NULL) {
jniThrowRuntimeException(env, "MessageQueue is not initialized.");
return 0;
}
// 创建native层的DisplayEventReceiver，即NativeDisplayEventReceiver
sp<NativeDisplayEventReceiver> receiver = new NativeDisplayEventReceiver(env, receiverWeak, vsyncEventDataWeak, messageQueue, vsyncSource, eventRegistration, layerHandle);
// 调用initialize进行初始化
status_t status = receiver->initialize();
if (status) {
String8 message;
message.appendFormat("Failed to initialize display event receiver. status=%d", status);
jniThrowRuntimeException(env, message.c_str());
return 0;
}
receiver->incStrong(gDisplayEventReceiverClassInfo.clazz); // retain a reference for the object
return reinterpret_cast<jlong>(receiver.get());
}
// 父类是DisplayEventDispatcher
class NativeDisplayEventReceiver : public DisplayEventDispatcher {
public:
NativeDisplayEventReceiver(JNIEnv* env, jobject receiverWeak, jobject vsyncEventDataWeak, const sp<MessageQueue>& messageQueue, jint vsyncSource, jint eventRegistration, jlong layerHandle);
void dispose();
protected:
virtual ~NativeDisplayEventReceiver();
private:
jobject mReceiverWeakGlobal;
jobject mVsyncEventDataWeakGlobal;
sp<MessageQueue> mMessageQueue;
void dispatchVsync(nsecs_t timestamp, PhysicalDisplayId displayId, uint32_t count, VsyncEventData vsyncEventData) override;
void dispatchHotplug(nsecs_t timestamp, PhysicalDisplayId displayId, bool connected) override;
void dispatchHotplugConnectionError(nsecs_t timestamp, int errorCode) override;
void dispatchModeChanged(nsecs_t timestamp, PhysicalDisplayId displayId, int32_t modeId,
nsecs_t renderPeriod) override;
void dispatchFrameRateOverrides(nsecs_t timestamp, PhysicalDisplayId displayId,
std::vector<FrameRateOverride> overrides) override;
void dispatchNullEvent(nsecs_t timestamp, PhysicalDisplayId displayId) override {}
void dispatchHdcpLevelsChanged(PhysicalDisplayId displayId, int connectedLevel,
int maxLevel) override;
};
NativeDisplayEventReceiver::NativeDisplayEventReceiver(JNIEnv* env, jobject receiverWeak,
jobject vsyncEventDataWeak,
const sp<MessageQueue>& messageQueue,
jint vsyncSource, jint eventRegistration,
jlong layerHandle)
// 父类构造函数
: DisplayEventDispatcher(
messageQueue->getLooper(),
static_cast<gui::ISurfaceComposer::VsyncSource>(vsyncSource),
static_cast<gui::ISurfaceComposer::EventRegistration>(eventRegistration),
layerHandle != 0 ? sp<IBinder>::fromExisting(reinterpret_cast<IBinder*>(layerHandle)) : nullptr
),
// Java层的receiver
mReceiverWeakGlobal(env->NewGlobalRef(receiverWeak)),
mVsyncEventDataWeakGlobal(env->NewGlobalRef(vsyncEventDataWeak)),
mMessageQueue(messageQueue) {
ALOGV("receiver %p ~ Initializing display event receiver.", this);
}首先看下NativeDisplayEventReceiver对象的创建，NativeDisplayEventReceiver的父类是DisplayEventDispatcher。查看源码可以判断出DisplayEventDispatcher是用于分发VSYNC、Hotplug等信号的，而DisplayEventDispatcher内部会创建DisplayEventReceiver对象用于接收SurfaceFlinger进程发送过来的信号。
cpp
// frameworks/native/libs/gui/DisplayEventDispatcher.cpp
DisplayEventDispatcher::DisplayEventDispatcher(const sp<Looper>& looper,
gui::ISurfaceComposer::VsyncSource vsyncSource,
EventRegistrationFlags eventRegistration,
const sp<IBinder>& layerHandle)
: mLooper(looper),
mReceiver(vsyncSource, eventRegistration, layerHandle), // DisplayEventReceiver
mWaitingForVsync(false),
mLastVsyncCount(0),
mLastScheduleVsyncTime(0) {
ALOGV("dispatcher %p ~ Initializing display event dispatcher.", this);
}
// frameworks/native/libs/gui/DisplayEventReceiver.cpp
DisplayEventReceiver::DisplayEventReceiver(gui::ISurfaceComposer::VsyncSource vsyncSource, EventRegistrationFlags eventRegistration, const sp<IBinder>& layerHandle) {
// 获取SurfaceFlinger的代理对象
sp<gui::ISurfaceComposer> sf(ComposerServiceAIDL::getComposerService());
if (sf != nullptr) {
mEventConnection = nullptr;
// 创建一个与SurfaceFlinger进程中的EventTread-app线程的vsyncSource信号连接
binder::Status status = sf->createDisplayEventConnection(vsyncSource, static_cast<gui::ISurfaceComposer::EventRegistration>(eventRegistration.get()), layerHandle, &mEventConnection);
if (status.isOk() && mEventConnection != nullptr) {
// 创建成功之后，构造一个BitTube对象，并将上面创建好的连接的读端描述拷贝过来，用于后续VSYNC信号的监听
mDataChannel = std::make_unique<gui::BitTube>();
// 拷贝SurfaceFlinger进程中创建的Scoket读端描述符
status = mEventConnection->stealReceiveChannel(mDataChannel.get());
if (!status.isOk()) {
ALOGE("stealReceiveChannel failed: %s", status.toString8().c_str());
mInitError = std::make_optional<status_t>(status.transactionError());
mDataChannel.reset();
mEventConnection.clear();
}
} else {
ALOGE("DisplayEventConnection creation failed: status=%s", status.toString8().c_str());
}
}
}
// frameworks/native/services/surfaceflinger/Scheduler/EventThread.cpp
sp<EventThreadConnection> EventThread::createEventConnection(EventRegistrationFlags eventRegistration) const {
auto connection = sp<EventThreadConnection>::make(const_cast<EventThread*>(this), IPCThreadState::self()->getCallingUid(), eventRegistration);
if (FlagManager::getInstance().misc1()) {
const int policy = SCHED_FIFO;
connection->setMinSchedulerPolicy(policy, sched_get_priority_min(policy));
}
return connection;
}
// 创建了BitTube，内部创建了Socket
EventThreadConnection::EventThreadConnection(EventThread* eventThread, uid_t callingUid, EventRegistrationFlags eventRegistration)
: mOwnerUid(callingUid),
mEventRegistration(eventRegistration),
mEventThread(eventThread),
mChannel(gui::BitTube::DefaultSize) {}
从源码中可以看到，BitTube是用于接收SurfaceFlinger进程发送过来的信号的，从BitTube的类文件可以看出，BitTube内部是通过Socket来实现跨进程发送信号的。
cpp
// frameworks/native/libs/gui/BitTube.cpp
static const size_t DEFAULT_SOCKET_BUFFER_SIZE = 4 * 1024;
BitTube::BitTube(size_t bufsize) {
init(bufsize, bufsize);
}
void BitTube::init(size_t rcvbuf, size_t sndbuf) {
int sockets[2];
if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sockets) == 0) {
size_t size = DEFAULT_SOCKET_BUFFER_SIZE;
setsockopt(sockets[0], SOL_SOCKET, SO_RCVBUF, &rcvbuf, sizeof(rcvbuf));
setsockopt(sockets[1], SOL_SOCKET, SO_SNDBUF, &sndbuf, sizeof(sndbuf));
// since we don't use the "return channel", we keep it small...
setsockopt(sockets[0], SOL_SOCKET, SO_SNDBUF, &size, sizeof(size));
setsockopt(sockets[1], SOL_SOCKET, SO_RCVBUF, &size, sizeof(size));
fcntl(sockets[0], F_SETFL, O_NONBLOCK);
fcntl(sockets[1], F_SETFL, O_NONBLOCK);
mReceiveFd.reset(sockets[0]);
mSendFd.reset(sockets[1]);
} else {
mReceiveFd.reset();
ALOGE("BitTube: pipe creation failed (%s)", strerror(errno));
}
}
base::unique_fd BitTube::moveReceiveFd() {
return std::move(mReceiveFd);
}总结一下，Choreographer的创建流程：
VSYNC信号的请求
对于刷新率为60Hz的屏幕来说，一般是每16.67ms产生一个VSYNC信号，但是每个App进程不一定会每个VSYNC信号都会接收，而是根据上层业务的实际需要进行VSYNC信号的监听和接收。这样设计的好处是可以按需触发App进程的渲染流程，降低不必要的渲染所带来的功耗。
上层业务一般会通过validate或者requestLayout方法来发起一次绘制请求，最终这个绘制请求会被转换成CallbackRecord放入对应类型的CallbackQueue中。
java
public final class Choreographer {
// ...
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
// 计算任务执行的具体时间戳
final long dueTime = now + delayMillis;
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) { // 如果不需要延迟执行的话，则立即请求调度VSYNC信号
scheduleFrameLocked(now);
} else { // 否则通过延迟消息来请求调度VSYNC信号
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
private void scheduleFrameLocked(long now) {
if (!mFrameScheduled) {
mFrameScheduled = true;
if (USE_VSYNC) {
// 检查当前线程是否为主线程，如果是主线程，直接请求调度VSYNC信号，否则向主线程发送异步消息来请求调度VSYNC信号
if (isRunningOnLooperThreadLocked()) {
scheduleVsyncLocked();
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
msg.setAsynchronous(true);
mHandler.sendMessageAtFrontOfQueue(msg);
}
} else {
final long nextFrameTime = Math.max(
mLastFrameTimeNanos / TimeUtils.NANOS_PER_MS + sFrameDelay, now);
if (DEBUG_FRAMES) {
Log.d(TAG, "Scheduling next frame in " + (nextFrameTime - now) + " ms.");
}
Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, nextFrameTime);
}
}
}
@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
private void scheduleVsyncLocked() {
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#scheduleVsyncLocked");
// 通过调用FrameDisplayEventReceiver的scheduleVsync方法请求VSYNC信号
mDisplayEventReceiver.scheduleVsync();
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
// ...
}最终调用了native方法向SurfaceFlinger进程进行通信，请求分发VSYNC信号到App进程。之前创建Choreographer的过程中，创建了DisplayEventReceiver并通过JNI调用到了native层，在native层创建了NativeDisplayEventReceiver对象之后，将其返回了Java层，并保存在了Java层的DisplayEventReceiver对象中。Java层通过JNI调用到native层并将之前创建的NativeDisplayEventReceiver指针传回了native层，这样就可以在native层找到之前创建好的NativeDisplayEventReceiver对象，并调用其scheduleVsync方法，最终通过mEventConnection完成跨进程请求，
java
// android.view.DisplayEventReceiver#scheduleVsync
@UnsupportedAppUsage
public void scheduleVsync() {
if (mReceiverPtr == 0) {
Log.w(TAG, "Attempted to schedule a vertical sync pulse but the display event "
+ "receiver has already been disposed.");
} else {
nativeScheduleVsync(mReceiverPtr);
}
}
// frameworks/base/core/jni/android_view_DisplayEventReceiver.cpp
static void nativeScheduleVsync(JNIEnv* env, jclass clazz, jlong receiverPtr) {
sp<NativeDisplayEventReceiver> receiver =
reinterpret_cast<NativeDisplayEventReceiver*>(receiverPtr);
status_t status = receiver->scheduleVsync();
if (status) {
String8 message;
message.appendFormat("Failed to schedule next vertical sync pulse. status=%d", status);
jniThrowRuntimeException(env, message.c_str());
}
}
// frameworks/native/libs/gui/DisplayEventDispatcher.cpp
status_t DisplayEventDispatcher::scheduleVsync() {
if (!mWaitingForVsync) {
ALOGV("dispatcher %p ~ Scheduling vsync.", this);
// Drain all pending events.
nsecs_t vsyncTimestamp;
PhysicalDisplayId vsyncDisplayId;
uint32_t vsyncCount;
VsyncEventData vsyncEventData;
if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount, &vsyncEventData)) {
ALOGE("dispatcher %p ~ last event processed while scheduling was for %" PRId64 "", this,
ns2ms(static_cast<nsecs_t>(vsyncTimestamp)));
}
// 请求下一个VSYNC信号
status_t status = mReceiver.requestNextVsync();
if (status) {
ALOGW("Failed to request next vsync, status=%d", status);
return status;
}
mWaitingForVsync = true;
mLastScheduleVsyncTime = systemTime(SYSTEM_TIME_MONOTONIC);
}
return OK;
}
// frameworks/native/libs/gui/DisplayEventReceiver.cpp
status_t DisplayEventReceiver::requestNextVsync() {
if (mEventConnection != nullptr) {
mEventConnection->requestNextVsync();
return NO_ERROR;
}
return mInitError.has_value() ? mInitError.value() : NO_INIT;
}VSYNC信号的分发
当SurfaceFlinger进程通过Socket通知App进程VSYNC信号到达之后，App进程的handleEvent方法将会被调用，最终通过JNI调用到Java层的FrameDisplayEventReceiver#dispatchVsync方法。
cpp
// frameworks/native/libs/gui/DisplayEventDispatcher.cpp
int DisplayEventDispatcher::handleEvent(int, int events, void*) {
if (events & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP)) {
return 0; // remove the callback
}
if (!(events & Looper::EVENT_INPUT)) {
return 1; // keep the callback
}
// Drain all pending events, keep the last vsync.
nsecs_t vsyncTimestamp;
PhysicalDisplayId vsyncDisplayId;
uint32_t vsyncCount;
VsyncEventData vsyncEventData;
if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount, &vsyncEventData)) {
mWaitingForVsync = false;
mLastVsyncCount = vsyncCount;
dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount, vsyncEventData);
}
if (mWaitingForVsync) {
const nsecs_t currentTime = systemTime(SYSTEM_TIME_MONOTONIC);
const nsecs_t vsyncScheduleDelay = currentTime - mLastScheduleVsyncTime;
if (vsyncScheduleDelay > WAITING_FOR_VSYNC_TIMEOUT) {
mWaitingForVsync = false;
dispatchVsync(currentTime, vsyncDisplayId /* displayId is not used */,
++mLastVsyncCount, vsyncEventData /* empty data */);
}
}
return 1; // keep the callback
}
// frameworks/base/core/jni/android_view_DisplayEventReceiver.cpp
void NativeDisplayEventReceiver::dispatchVsync(nsecs_t timestamp, PhysicalDisplayId displayId, uint32_t count, VsyncEventData vsyncEventData) {
JNIEnv* env = AndroidRuntime::getJNIEnv();
ScopedLocalRef<jobject> receiverObj(env, GetReferent(env, mReceiverWeakGlobal));
ScopedLocalRef<jobject> vsyncEventDataObj(env, GetReferent(env, mVsyncEventDataWeakGlobal));
if (receiverObj.get() && vsyncEventDataObj.get()) {
env->SetIntField(vsyncEventDataObj.get(), gDisplayEventReceiverClassInfo.vsyncEventDataClassInfo.preferredFrameTimelineIndex, syncEventData.preferredFrameTimelineIndex);
env->SetIntField(vsyncEventDataObj.get(), gDisplayEventReceiverClassInfo.vsyncEventDataClassInfo.frameTimelinesLength, vsyncEventData.frameTimelinesLength);
env->SetLongField(vsyncEventDataObj.get(), gDisplayEventReceiverClassInfo.vsyncEventDataClassInfo.frameInterval, vsyncEventData.frameInterval);
ScopedLocalRef<jobjectArray> frameTimelinesObj(env, reinterpret_cast<jobjectArray>(env->GetObjectField(vsyncEventDataObj.get(), gDisplayEventReceiverClassInfo.vsyncEventDataClassInfo.frameTimelines)));
for (size_t i = 0; i < vsyncEventData.frameTimelinesLength; i++) {
VsyncEventData::FrameTimeline& frameTimeline = vsyncEventData.frameTimelines[i];
ScopedLocalRef<jobject>
frameTimelineObj(env, env->GetObjectArrayElement(frameTimelinesObj.get(), i));
env->SetLongField(frameTimelineObj.get(),
gDisplayEventReceiverClassInfo.frameTimelineClassInfo.vsyncId,
frameTimeline.vsyncId);
env->SetLongField(frameTimelineObj.get(),
gDisplayEventReceiverClassInfo.frameTimelineClassInfo
.expectedPresentationTime,
frameTimeline.expectedPresentationTime);
env->SetLongField(frameTimelineObj.get(),
gDisplayEventReceiverClassInfo.frameTimelineClassInfo.deadline,
frameTimeline.deadlineTimestamp);
}
// 最终调用到了Java层的dispatchVsync
env->CallVoidMethod(receiverObj.get(), gDisplayEventReceiverClassInfo.dispatchVsync, timestamp, displayId.value, count);
ALOGV("receiver %p ~ Returned from vsync handler.", this);
}
mMessageQueue->raiseAndClearException(env, "dispatchVsync");
}
java
// android.view.DisplayEventReceiver
// Called from native code.
@SuppressWarnings("unused")
private void dispatchVsync(long timestampNanos, long physicalDisplayId, int frame,
VsyncEventData vsyncEventData) {
onVsync(timestampNanos, physicalDisplayId, frame, vsyncEventData);
}
// android.view.Choreographer.FrameDisplayEventReceiver
@Override
public void onVsync(long timestampNanos, long physicalDisplayId, int frame, VsyncEventData vsyncEventData) {
try {
long now = System.nanoTime();
if (timestampNanos > now) {
timestampNanos = now;
}
if (mHavePendingVsync) {
Log.w(TAG, "Already have a pending vsync event. There should only be "
+ "one at a time.");
} else {
mHavePendingVsync = true;
}
mTimestampNanos = timestampNanos;
mFrame = frame;
mLastVsyncEventData = vsyncEventData;
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true); // 异步消息，利用之前插入的同步屏障来加速消息的处理
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame, mLastVsyncEventData);
}最终调用到了Choreographer#doFrame方法，到了这里就开始了新的一帧的处理，开始下一帧的数据准备工作。
VSYNC信号的处理
App进程收到VSYNC信号之后就会调用doFrame方法开始新的一帧数据的准备工作，其中还会计算卡顿时间，即VSYNC信号到达之后多久才被主线程处理，等待时间过长会导致无法在一帧时间内完成数据准备的工作，最终导致用户看到的视觉效果不够流畅。
java
// android.view.Choreographer
void doFrame(long frameTimeNanos, int frame, DisplayEventReceiver.VsyncEventData vsyncEventData) {
final long startNanos;
final long frameIntervalNanos = vsyncEventData.frameInterval;
try {
FrameData frameData = new FrameData(frameTimeNanos, vsyncEventData);
synchronized (mLock) {
if (!mFrameScheduled) {
traceMessage("Frame not scheduled");
return; // no work to do
}
long intendedFrameTimeNanos = frameTimeNanos;
startNanos = System.nanoTime();
// frameTimeNanos是SurfaceFlinger传递给来的时间戳，可能会被校准为App进程接收到VSYNC信号的时间戳
// jitterNanos包含了Handler处理消息的耗时，即异步消息被处理之前，主线程还在处理其他消息所占用的时间，如果这个时间过长会导致卡顿
final long jitterNanos = startNanos - frameTimeNanos;
if (jitterNanos >= frameIntervalNanos) {
long lastFrameOffset = 0;
if (frameIntervalNanos == 0) {
Log.i(TAG, "Vsync data empty due to timeout");
} else {
lastFrameOffset = jitterNanos % frameIntervalNanos;
final long skippedFrames = jitterNanos / frameIntervalNanos;
if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main "
+ "thread.");
}
if (DEBUG_JANK) {
Log.d(TAG, "Missed vsync by " + (jitterNanos * 0.000001f) + " ms "
+ "which is more than the frame interval of "
+ (frameIntervalNanos * 0.000001f) + " ms! "
+ "Skipping " + skippedFrames + " frames and setting frame "
+ "time to " + (lastFrameOffset * 0.000001f)
+ " ms in the past.");
}
}
frameTimeNanos = startNanos - lastFrameOffset;
frameData.updateFrameData(frameTimeNanos);
}
if (frameTimeNanos < mLastFrameTimeNanos) {
if (DEBUG_JANK) {
Log.d(TAG, "Frame time appears to be going backwards. May be due to a "
+ "previously skipped frame. Waiting for next vsync.");
}
traceMessage("Frame time goes backward");
scheduleVsyncLocked();
return;
}
if (mFPSDivisor > 1) {
long timeSinceVsync = frameTimeNanos - mLastFrameTimeNanos;
if (timeSinceVsync < (frameIntervalNanos * mFPSDivisor) && timeSinceVsync > 0) {
traceMessage("Frame skipped due to FPSDivisor");
scheduleVsyncLocked();
return;
}
}
mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos,
vsyncEventData.preferredFrameTimeline().vsyncId,
vsyncEventData.preferredFrameTimeline().deadline, startNanos,
vsyncEventData.frameInterval);
mFrameScheduled = false;
mLastFrameTimeNanos = frameTimeNanos;
mLastFrameIntervalNanos = frameIntervalNanos;
mLastVsyncEventData = vsyncEventData;
}
// 开始执行各种类型的Callback
AnimationUtils.lockAnimationClock(frameTimeNanos / TimeUtils.NANOS_PER_MS);
mFrameInfo.markInputHandlingStart();
doCallbacks(Choreographer.CALLBACK_INPUT, frameData, frameIntervalNanos);
mFrameInfo.markAnimationsStart();
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameData, frameIntervalNanos);
doCallbacks(Choreographer.CALLBACK_INSETS_ANIMATION, frameData,
frameIntervalNanos);
mFrameInfo.markPerformTraversalsStart();
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameData, frameIntervalNanos);
doCallbacks(Choreographer.CALLBACK_COMMIT, frameData, frameIntervalNanos);
} finally {
AnimationUtils.unlockAnimationClock();
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}接着，就会将之前业务提交的各种类型的CallbackRecord，主要分为：
- CALLBACK_INPUT：输入类型，比如屏幕触摸事件，最先执行；
- CALLBACK_ANIMATION：动画类型，比如属性动画；
- CALLBACK_INSETS_ANIMATION：背景更新动画；
- CALLBACK_TRAVERSAL：布局绘制类型，View的测量、布局和绘制等；
- CALLBACK_COMMIT：提交绘制数据；
按照顺序执行完所有的CallbackRecord之后，App进程的绘制任务就完成了，最终数据提交到GraphicBuffer中，最终调度sf类型的VSYNC信号，最后由SurfaceFlinger完成数据合成和送显。
总结一下，VSYNC信号的分发处理流程：
总结
整个Choreographer工作机制作为App进程和SurfaceFlinger进程的协调机制，承接App进程的业务刷新UI的请求，统一调度VSYNC信号，将UI渲染任务同步到 VSYNC 信号的时间线上。同时作为中转站来分发VSYNC信号，并处理上层业务的刷新请求。按照 VSYNC 信号的周期有规律地准备每一帧数据，并通过SurfaceFlinger进程完成合成上屏。