浅析Android中的Choreographer工作原理

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。

1. 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。

// 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。
    }

2. 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。

/**
 * 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;
                // ...
            }
// ...
}
}

3. 主线程调用TransactionExecutor#execute方法处理消息，并在executeCallbacks方法中执行了ResumeActivityItem

/**
 * 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);
            }
        }
    }

// ...
}

4. 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。

/**
 * 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());
    }

5. 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。

/**
 * 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;
            }
        }
    }

// ...
}

6. 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。

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。

    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));
    }

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。

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。

/**
 * 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。

// 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。

// 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。

// 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。

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，

// 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。

// 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");
}

// 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。

// 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。

原文地址：https://blog.csdn.net/boker_han/article/details/142755317

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