Android Choreographer机制
一 概述
在分析 Choreographer 机制之前，同学们需要先了解一些关于 Android 显示系统的一些基础概念和背景。可以参考 Android图形显示系统1 概述
我们知道 Google 在 Android 4.1 系统中对 Android Display 系统进行了优化：在收到 VSync 信号后，将马上开始下一帧的渲染。即一旦收到 VSync 通知，CPU 和 GPU 就立刻开始计算，然后把数据写入 buffer。本篇文章重点分析 Choreographer，以及 Choreographer 与 Vsync 信号之间的交互。
- Choreographer，意为舞蹈编导、编舞者。在这里就是指对 CPU/GPU 绘制的指导 —— 收到 VSync 信号才开始绘制，保证绘制拥有完整的 16.6ms，避免绘制的随机性
- Choreographer，是一个 Java 类，包路径 android.view.Choreographer。类注释是 “协调动画、输入和绘图的计时”
- 通常应用层不会直接使用 Choreographer，而是使用更高级的 API，例如动画和 View 绘制相关的 ValueAnimator.start()、View.invalidate() 等。
- 业界一般通过 Choreographer 来监控应用的帧率
二 Choreographer启动流程
在 Activity 启动过程中，执行完 onResume 后，会调用 Activity.makeVisible()，然后再调用到 addView()， 层层调用会进入 ViewRootImpl 的构造方法：
public ViewRootImpl(Context context, Display display) {
......
//这里获取Choreographer实例
mChoreographer = Choreographer.getInstance();
......
}
2.1 getInstance
Choreographer.java
public static Choreographer getInstance() {
return sThreadInstance.get();
}
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;
}
};
可知在 Choreographer 中使用到了 ThreadLocal。当前所在线程为 UI 线程，也就是常说的主线程。
2.2 创建Choreographer
private Choreographer(Looper looper, int vsyncSource) {
mLooper = looper;
//创建Handler对象,这个handler用于向主线程发送消息
mHandler = new FrameHandler(looper);
//创建用于接收VSync信号的对象
mDisplayEventReceiver = USE_VSYNC ?
new FrameDisplayEventReceiver(looper, vsyncSource) : null;
mLastFrameTimeNanos = Long.MIN_VALUE;
mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
//创建回调对象,其中CALLBACK_LAST＝４
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));
}
- mLastFrameTimeNanos：指上一次帧绘制时间点
- mFrameIntervalNanos：帧间时长，一般等于16.7ms
2.3 创建FrameHandler
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:
// 执行doFrame,即绘制过程
doFrame(System.nanoTime(), 0);
break;
case MSG_DO_SCHEDULE_VSYNC:
//申请VSYNC信号，例如当前需要绘制任务时
doScheduleVsync();
break;
case MSG_DO_SCHEDULE_CALLBACK:
//需要延迟的任务，最终还是执行上述两个事件
doScheduleCallback(msg.arg1);
break;
}
}
}
2.4 创建FrameDisplayEventReceiver
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
implements Runnable {
private boolean mHavePendingVsync;
private long mTimestampNanos;
private int mFrame;
public FrameDisplayEventReceiver(Looper looper, int vsyncSource) {
super(looper, vsyncSource, CONFIG_CHANGED_EVENT_SUPPRESS);
}
//关键点：收到 Vsync 信号后的回调
@Override
public void onVsync(long timestampNanos, long physicalDisplayId, int frame) {
// Post the vsync event to the Handler.
// The idea is to prevent incoming vsync events from completely starving
// the message queue. If there are no messages in the queue with timestamps
// earlier than the frame time, then the vsync event will be processed immediately.
// Otherwise, messages that predate the vsync event will be handled first.
long now = System.nanoTime();
if (timestampNanos > now) {
Log.w(TAG, "Frame time is " + ((timestampNanos - now) * 0.000001f)
+ " ms in the future! Check that graphics HAL is generating vsync "
+ "timestamps using the correct timebase.");
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;
//将本身作为 runnable 传入 msg， 发消息后会走 run()，即 doFrame()，也是异步消息
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
}
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame);
}
}
2.4.1 DisplayEventReceiver
DisplayEventReceiver.java
public DisplayEventReceiver(Looper looper) {
this(looper, VSYNC_SOURCE_APP, CONFIG_CHANGED_EVENT_SUPPRESS);
}
public DisplayEventReceiver(Looper looper, int vsyncSource, int configChanged) {
if (looper == null) {
throw new IllegalArgumentException("looper must not be null");
}
mMessageQueue = looper.getQueue();
mReceiverPtr = nativeInit(new WeakReference<DisplayEventReceiver>(this), mMessageQueue,
vsyncSource, configChanged);
mCloseGuard.open("dispose");
}
经过 JNI 调用进入如下 Native 方法。
2.4.2 nativeInit
android_view_DisplayEventReceiver.cpp
static jlong nativeInit(JNIEnv* env, jclass clazz, jobject receiverWeak,
jobject messageQueueObj, jint vsyncSource, jint configChanged) {
sp<MessageQueue> messageQueue =
android_os_MessageQueue_getMessageQueue(env, messageQueueObj);
if (messageQueue == NULL) {
jniThrowRuntimeException(env, "MessageQueue is not initialized.");
return 0;
}
sp<NativeDisplayEventReceiver> receiver = new NativeDisplayEventReceiver(env,
receiverWeak, messageQueue, vsyncSource, configChanged);
//会调用到其父类 DisplayEventDispatcher 的 initialize 方法
status_t status = receiver->initialize();
if (status) {
String8 message;
message.appendFormat("Failed to initialize display event receiver. status=%d", status);
jniThrowRuntimeException(env, message.string());
return 0;
}
// retain a reference for the object
// 获取 DisplayEventReceiver 对象的引用
receiver->incStrong(gDisplayEventReceiverClassInfo.clazz);
return reinterpret_cast<jlong>(receiver.get());
}
2.4.3 创建NativeDisplayEventReceiver
NativeDisplayEventReceiver::NativeDisplayEventReceiver(JNIEnv* env,
jobject receiverWeak, const sp<MessageQueue>& messageQueue, jint vsyncSource,
jint configChanged) :
DisplayEventDispatcher(messageQueue->getLooper(),
static_cast<ISurfaceComposer::VsyncSource>(vsyncSource),
static_cast<ISurfaceComposer::ConfigChanged>(configChanged)),
mReceiverWeakGlobal(env->NewGlobalRef(receiverWeak)),
mMessageQueue(messageQueue) {
ALOGV("receiver %p ~ Initializing display event receiver.", this);
}
NativeDisplayEventReceiver 继承于 DisplayEventDispatcher，而 DisplayEventDispatcher 又继承于 LooperCallback 对象，此处 mReceiverWeakGlobal 记录的是 Java 层 DisplayEventReceiver 对象的全局引用。
2.4.4 DisplayEventDispatcher.initialize
framework/base/libs/androidfw/DisplayEventDispatcher.cpp
status_t DisplayEventDispatcher::initialize() {
status_t result = mReceiver.initCheck();
if (result) {
ALOGW("Failed to initialize display event receiver, status=%d", result);
return result;
}
int rc = mLooper->addFd(mReceiver.getFd(), 0, Looper::EVENT_INPUT,
this, NULL);
if (rc < 0) {
return UNKNOWN_ERROR;
}
return OK;
}
从代码中可以看到，是把 mReceiver 的文件句柄添加到 Looper 中了，用来监听，一旦有数据到来，则回调 this (此处为 DisplayEventDispatcher) 中所复写 LooperCallback 对象的 handleEvent。
三 Vysnc回调流程
接上分析，当 Vysnc 信号由 SurfaceFlinger 中创建 HWC 触发，唤醒 DispSyncThread 线程，再到 EventThread 线程，然后再通过 BitTube 直接传递到目标进程所对应的目标线程，（这一部分内容，请参考 SurfaceFlinger 流程）执行 handleEvent 方法。
3.1 DisplayEventDispatcher.handleEvent
framework/base/libs/androidfw/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;
//清除所有的pending事件，只保留最后一次vsync
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;
//关键点：回调执行 dispatchVsync，用来分发 Vysnc
dispatchVsync(vsyncTimestamp, vsyncDisplayId, vsyncCount);
}
return 1; // keep the callback
}
3.2 NativeDisplayEventReceiver.dispatchVsync
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);
//关键点：通过 JNI 调用 Java 层的 dispatchVsync 函数
env->CallVoidMethod(receiverObj.get(),
gDisplayEventReceiverClassInfo.dispatchVsync, timestamp, displayId, count);
ALOGV("receiver %p ~ Returned from vsync handler.", this);
}
mMessageQueue->raiseAndClearException(env, "dispatchVsync");
}
DisplayEventReceiver.java
private void dispatchVsync(long timestampNanos, long physicalDisplayId, int frame) {
onVsync(timestampNanos, physicalDisplayId, frame);
}
最后回调到 Java 层 DisplayEventReceiver 的 onVsync，进而走到 FrameDisplayEventReceiver 的 onVsync。
3.3 onVsync
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) {
super(looper, vsyncSource, CONFIG_CHANGED_EVENT_SUPPRESS);
}
@Override
public void onVsync(long timestampNanos, long physicalDisplayId, int frame) {
long now = System.nanoTime();
if (timestampNanos > now) {
timestampNanos = now;
}
if (mHavePendingVsync) {
} else {
mHavePendingVsync = true;
}
mTimestampNanos = timestampNanos;
mFrame = frame;
//该消息的callback为当前对象FrameDisplayEventReceiver
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
//此处mHandler为FrameHandler，发送到主线程
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
}
@Override
public void run() {
mHavePendingVsync = false;
// 在 run 方法中执行 doFrame
doFrame(mTimestampNanos, mFrame);
}
}
可见 onVsync() 过程是通 过FrameHandler 向主线程 Looper 发送了一个自带 callback 的消息，此处 callback 为 FrameDisplayEventReceiver。 当主线程 Looper 执行到该消息时，则调用 FrameDisplayEventReceiver.run() 方法，从上面代码可以看出，在 run 方法中执行的是 doFrame 函数。
3.4 Choreographer.doFrame
Choreographer.java
void doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
if (!mFrameScheduled) {
return; // no work to do
}
if (DEBUG_JANK && mDebugPrintNextFrameTimeDelta) {
mDebugPrintNextFrameTimeDelta = false;
Log.d(TAG, "Frame time delta: "
+ ((frameTimeNanos - mLastFrameTimeNanos) * 0.000001f) + " ms");
}
long intendedFrameTimeNanos = frameTimeNanos;//原本计划的绘帧时间点
startNanos = System.nanoTime();
final long jitterNanos = startNanos - frameTimeNanos;
if (jitterNanos >= mFrameIntervalNanos) {
final long skippedFrames = jitterNanos / mFrameIntervalNanos;
//当掉帧个数超过30，则输出相应log
if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main thread.");
}
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
if (DEBUG_JANK) {
Log.d(TAG, "Missed vsync by " + (jitterNanos * 0.000001f) + " ms "
+ "which is more than the frame interval of "
+ (mFrameIntervalNanos * 0.000001f) + " ms! "
+ "Skipping " + skippedFrames + " frames and setting frame "
+ "time to " + (lastFrameOffset * 0.000001f) + " ms in the past.");
}
frameTimeNanos = startNanos - lastFrameOffset;//对齐帧的时间间隔
}
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.");
}
scheduleVsyncLocked();
return;
}
if (mFPSDivisor > 1) {
long timeSinceVsync = frameTimeNanos - mLastFrameTimeNanos;
if (timeSinceVsync < (mFrameIntervalNanos * mFPSDivisor) && timeSinceVsync > 0) {
scheduleVsyncLocked();
return;
}
}
mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
mFrameScheduled = false;
mLastFrameTimeNanos = frameTimeNanos;
}
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
AnimationUtils.lockAnimationClock(frameTimeNanos / TimeUtils.NANOS_PER_MS);
mFrameInfo.markInputHandlingStart();
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
mFrameInfo.markAnimationsStart();//标记动画开始时间
//执行回调方法
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_INSETS_ANIMATION, frameTimeNanos);
mFrameInfo.markPerformTraversalsStart();
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.");
}
}
此处 frameTimeNanos 是底层 Vsync 信号到达的时间戳。
- 每调用一次 scheduleFrameLocked()，则 mFrameScheduled=true，能执行一次 doFrame() 操作，执行完 doFrame() 并设置 mFrameScheduled=false
- 最终有如下4个回调类别
- INPUT：输入事件
- ANIMATION：动画
- TRAVERSAL：窗口刷新，执行 measure/layout/draw 操作
- COMMIT：遍历完成的提交操作，用来修正动画启动时间
3.5 Choreographer.doCallbacks
void doCallbacks(int callbackType, long frameTimeNanos) {
CallbackRecord callbacks;
synchronized (mLock) {
final long now = System.nanoTime();
// 从队列查找相应类型的CallbackRecord对象
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
now / TimeUtils.NANOS_PER_MS);
if (callbacks == null) {
return;
}
mCallbacksRunning = true;
if (callbackType == Choreographer.CALLBACK_COMMIT) {
final long jitterNanos = now - frameTimeNanos;
Trace.traceCounter(Trace.TRACE_TAG_VIEW, "jitterNanos", (int) jitterNanos);
//当commit类型回调执行的时间点超过2帧，则更新mLastFrameTimeNanos
if (jitterNanos >= 2 * mFrameIntervalNanos) {
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos
+ mFrameIntervalNanos;
if (DEBUG_JANK) {
Log.d(TAG, "Commit callback delayed by " + (jitterNanos * 0.000001f)
+ " ms which is more than twice the frame interval of "
+ (mFrameIntervalNanos * 0.000001f) + " ms! "
+ "Setting frame time to " + (lastFrameOffset * 0.000001f)
+ " ms in the past.");
mDebugPrintNextFrameTimeDelta = true;
}
frameTimeNanos = now - lastFrameOffset;
mLastFrameTimeNanos = frameTimeNanos;
}
}
}
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));
}
//关键点：执行 callback 的回调
c.run(frameTimeNanos);
}
} finally {
synchronized (mLock) {
mCallbacksRunning = false;
do {
final CallbackRecord next = callbacks.next;
//回收callbacks，加入对象池 mCallbackPool
recycleCallbackLocked(callbacks);
callbacks = next;
} while (callbacks != null);
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
该方法主要功能：
- 从队列头 mHead 查找 CallbackRecord 对象，当队列头部的 callbacks 对象为空或者执行时间还没到达，则直接返回
- 开始执行相应回调的 run() 方法
- 回收 callbacks，加入对象池 mCallbackPool，就是说 callback 一旦执行完成，则会被回收
3.6 CallbackRecord.run
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 {
((Runnable)action).run();
}
}
}
这里的回调方法 run() 有两种执行情况：
- 当 token 的数据类型为 FRAME_CALLBACK_TOKEN，则执行该对象的 doFrame() 方法
- 当 token 为其他类型，则执行该对象的 run() 方法
那么需要的场景便是由 WMS 调用 scheduleAnimationLocked() 方法来设置 mFrameScheduled=true 来触发动画， 接下来说说动画控制的过程。
四 动画显示过程
调用栈：
WMS.scheduleAnimationLocked
postFrameCallback
postFrameCallbackDelayed
postCallbackDelayedInternal
scheduleFrameLocked
4.1 WMS.scheduleAnimationLocked
WindowManagerService.java
void scheduleAnimationLocked() {
if (mAnimator != null) {
mAnimator.scheduleAnimation();
}
}
这里的 mAnimator 指的是 WindowAnimator。
4.1.1 WindowAnimator.scheduleAnimation
void scheduleAnimation() {
if (!mAnimationFrameCallbackScheduled) {
mAnimationFrameCallbackScheduled = true;
mChoreographer.postFrameCallback(mAnimationFrameCallback);
}
}
4.1.2 WindowAnimator的创建
private WindowManagerService(
......
mAnimator = new WindowAnimator(this);
......
}
WindowAnimator(final WindowManagerService service) {
mService = service;
mContext = service.mContext;
mPolicy = service.mPolicy;
AnimationThread.getHandler().runWithScissors(
() -> mChoreographer = Choreographer.getSfInstance(), 0 /* timeout */);
mAnimationFrameCallback = frameTimeNs -> {
synchronized (mService.mGlobalLock) {
mAnimationFrameCallbackScheduled = false;
}
animate(frameTimeNs);
};
}
mAnimationFrameCallback 的数据类型为 Choreographer.FrameCallback。从以上代码可以看出 Choreographer 构造在 AnimationThread 线程中。
4.2 postFrameCallback
Choreographer.java
public void postFrameCallback(FrameCallback callback) {
postFrameCallbackDelayed(callback, 0);
}
public void postFrameCallbackDelayed(FrameCallback callback, long delayMillis) {
if (callback == null) {
throw new IllegalArgumentException("callback must not be null");
}
postCallbackDelayedInternal(CALLBACK_ANIMATION,
callback, FRAME_CALLBACK_TOKEN, delayMillis);
}
4.3 postCallbackDelayedInternal
// callbackType为动画，action为mAnimationFrameCallback
// token为FRAME_CALLBACK_TOKEN，delayMillis=0
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;
//添加到mCallbackQueues队列
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
//执行 scheduleFrameLocked
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
因为 dueTime == now，所以调用 scheduleFrameLocked。
4.4 scheduleFrameLocked
private void scheduleFrameLocked(long now) {
if (!mFrameScheduled) {
mFrameScheduled = true;
if (USE_VSYNC) {
if (DEBUG_FRAMES) {
Log.d(TAG, "Scheduling next frame on vsync.");
}
// If running on the Looper thread, then schedule the vsync immediately,
// otherwise post a message to schedule the vsync from the UI thread
// as soon as possible.
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);
}
}
}
private boolean isRunningOnLooperThreadLocked() {
return Looper.myLooper() == mLooper;
}
这里的 Choreographer 运行在 AnimationThread 线程中，所以 isRunningOnLooperThreadLocked 为 false，则发送 MSG_DO_SCHEDULE_VSYNC，从之前的分析可知，最终调用到 doScheduleVsync。
4.5 doScheduleVsync
void doScheduleVsync() {
synchronized (mLock) {
if (mFrameScheduled) {
scheduleVsyncLocked();
}
}
}
4.6 scheduleVsyncLocked
private void scheduleVsyncLocked() {
mDisplayEventReceiver.scheduleVsync();
}
mDisplayEventReceiver 对象是在 Choreographer 的实例化过程所创建的。
4.7 DisplayEventReceiver.scheduleVsync
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);
}
}
4.8 nativeScheduleVsync
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.string());
}
}
4.9 DisplayEventDispatcher.scheduleVsync
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;
if (processPendingEvents(&vsyncTimestamp, &vsyncDisplayId, &vsyncCount)) {
ALOGE("dispatcher %p ~ last event processed while scheduling was for %" PRId64 "",
this, ns2ms(static_cast<nsecs_t>(vsyncTimestamp)));
}
//关键点：请求requestNextVsync
status_t status = mReceiver.requestNextVsync();
if (status) {
ALOGW("Failed to request next vsync, status=%d", status);
return status;
}
mWaitingForVsync = true;
}
return OK;
}
我们看到最终调用的是 mReceiver 的 requestNextVsync 方法，这个 mReceiver 是什么呢？我们来看 DisplayEventDispatcher.h 文件
DisplayEventDispatcher.h
#include 4.10 requestNextVsync
frameworks/native/libs/gui/DisplayEventReceiver.cpp
status_t DisplayEventReceiver::requestNextVsync() {
if (mEventConnection != nullptr) {
mEventConnection->requestNextVsync();
return NO_ERROR;
}
return NO_INIT;
}
该方法的作用请求下一次 Vsync 信息处理。
五 总结
- 尽量避免在执行动画渲染的前后在主线程放入耗时操作，否则会造成卡顿感，影响用户体验
- 可通过 Choreographer.getInstance().postFrameCallback() 来监听帧率情况
- 每调用一次 scheduleFrameLocked()，则 mFrameScheduled = true，可进入 doFrame() 方法体内部，执行完 doFrame() 并设置mFrameScheduled=false
- doCallbacks 回调方法有4个类别：INPUT（输入事件），ANIMATION（动画），TRAVERSAL（窗口刷新），COMMIT（完成后的提交操作）