Android的应用启动时，或者切换Activity时都会以动画的方式显示前后两屏的切换过程。动画的原理很简单，把一帧一帧的图像按一定时间间隔显示出来就完成了。
动画的绘制需要定时驱动，通常的做法是启动一个定时消息，每隔一定时间发一个消息，收到消息后输出一帧画面。Android支持VSync信号后，动画的驱动就有VSync信号承担了。
窗口动画的基本元素是窗口Surface中保存的图像，通过对窗口的Surface设置不同的变换矩阵和透明度，然后强制Surface刷新，就能在屏幕上显示出窗口的变化过程。
Choreographer对象初始化
我们先来看WMS中的mChoreographer 变量
final Choreographer mChoreographer = Choreographer.getInstance();
该变量是一个线程局部存储变量，在它的initialValue中创建了Choreographer对象并返回。这里使用线程局部存储的目录就是保证在线程中只有一个Choreographer对象。
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!");
}
return new Choreographer(looper);
}
};
再来看下Choreographer的构造函数，这里主要是创建了FrameDisplayEventReceiver用来接受VSync信号的对象。
private Choreographer(Looper looper) {
mLooper = looper;
mHandler = new FrameHandler(looper);
mDisplayEventReceiver = USE_VSYNC ? new FrameDisplayEventReceiver(looper) : null;//接受VSync信号对象
mLastFrameTimeNanos = Long.MIN_VALUE;
mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());//计算刷新的时间间隔
mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
for (int i = 0; i <= CALLBACK_LAST; i++) {
mCallbackQueues[i] = new CallbackQueue();
}
}
FrameDisplayEventReceiver接受VSync信号
我们在http://blog.csdn.net/kc58236582/article/details/52892384（ Android6.0 VSync信号如何到用户进程 ）这篇博客已经分析过FrameDisplayEventReceiver的原理了，当VSync信号过来时，最后会调用到FrameDisplayEventReceiver类的onVsync函数：
@Override
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
if (builtInDisplayId != SurfaceControl.BUILT_IN_DISPLAY_ID_MAIN) {
scheduleVsync();
return;
}
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;
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
}
这主要是发送了一个信号，而是是Runnable的那种消息。
因此我们主要看下这个类的run函数，这里就是调用了Choreographer的doFrame函数。
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame);
}
doFrame函数
doFrame函数主要有一些VSync时间逻辑处理如果抛弃该VSync信号的话会调用scheduleVsyncLocked函数让SurfaceFlinger发送一个VSync信号，如果正常会调用4个doCallBacks函数。
void doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
......
long intendedFrameTimeNanos = frameTimeNanos;
startNanos = System.nanoTime();
final long jitterNanos = startNanos - frameTimeNanos;
if (jitterNanos >= mFrameIntervalNanos) {
final long skippedFrames = jitterNanos / mFrameIntervalNanos;
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
frameTimeNanos = startNanos - lastFrameOffset;
}
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();//让SurfaceFlinger立马发送一个VSync信号
return;
}
mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
mFrameScheduled = false;
mLastFrameTimeNanos = frameTimeNanos;
}
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
mFrameInfo.markInputHandlingStart();
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);//按键相关
mFrameInfo.markAnimationsStart();
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);//动画相关
mFrameInfo.markPerformTraversalsStart();
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);//power相关
doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
doCallbacks函数，我们首先会检查当前这个CallBackType是否有对应的CallBack回调，如果没有直接return，如果有的话会调用其回调的run函数。
void doCallbacks(int callbackType, long frameTimeNanos) {
CallbackRecord callbacks;
synchronized (mLock) {
final long now = System.nanoTime();
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
now / TimeUtils.NANOS_PER_MS);
if (callbacks == null) {//没有对应CallBack回调
return;
}
mCallbacksRunning = true;
// safe by ensuring the commit time is always at least one frame behind.
if (callbackType == Choreographer.CALLBACK_COMMIT) {
final long jitterNanos = now - frameTimeNanos;
Trace.traceCounter(Trace.TRACE_TAG_VIEW, "jitterNanos", (int) jitterNanos);
if (jitterNanos >= 2 * mFrameIntervalNanos) {
final long lastFrameOffset = jitterNanos % mFrameIntervalNanos
+ mFrameIntervalNanos;
if (DEBUG_JANK) {
mDebugPrintNextFrameTimeDelta = true;
}
frameTimeNanos = now - lastFrameOffset;
mLastFrameTimeNanos = frameTimeNanos;
}
}
}
try {
for (CallbackRecord c = callbacks; c != null; c = c.next) {
c.run(frameTimeNanos);//调用回调run函数
}
}
......
}
这也就意味着当你没有CallBackType对应的回调，每次VSync信号过来到doFrame函数再到doCallBacks函数都是没有意义的。
WMS启动动画
那我们下面看在哪里把CallBackType对应的回调加入了，这里我们只关注动画相关的。
上面我们说到VSync会不断的发送，每秒60多次，但是动画不会不停的播放，就是这个CallBackType对应的回调没有。哪动画的启动和结束也就是受这个影响，而就是在WMS中调用scheduleAnimationLocked函数发起的动画启动。
void scheduleAnimationLocked() {
if (!mAnimationScheduled) {
mAnimationScheduled = true;
mChoreographer.postFrameCallback(mAnimator.mAnimationFrameCallback);
}
}
这里就是调用Choreographer设置CallBackType，相关的回调。这里我们的callbackType是CALLBACK_ANIMATION
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);
}
我们最后看postCallbackDelayedInternal函数，就是在mCallBackQueues对应的CallBackType中增加相应的回调。这里也就是前面在WMS的scheduleAnimationLocked的参数mAnimator.mAnimationFrameCallback就是回调。
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
我们来看下scheduleFrameLocked函数，我们注意mFrameScheduled这个变量，这个时候赋值为true，后面就是用这个变量来控制每次VSync信号过来调用doFrame函数的时候是否要播放动画
private void scheduleFrameLocked(long now) {
if (!mFrameScheduled) {
mFrameScheduled = true;//注意这个变量
if (USE_VSYNC) {
if (isRunningOnLooperThreadLocked()) {
scheduleVsyncLocked();//尽快让SurfaceFlinger中的EventThread发送一个VSync信号
} 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);
}
}
}
我们再回过头来看doFrame函数，当mFrameScheduled为false时，VSync信号过来该函数直接return不会播放动画。
void doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
if (!mFrameScheduled) {
return; // no work to do
}
继续看postCallbackDelayedInternal函数中增加的回调，这个回调在WindowAnimator的构造函数中就新建了Choreographer.FrameCallback回调
WindowAnimator(final WindowManagerService service) {
mService = service;
mContext = service.mContext;
mPolicy = service.mPolicy;
mAnimationFrameCallback = new Choreographer.FrameCallback() {
public void doFrame(long frameTimeNs) {
synchronized (mService.mWindowMap) {
mService.mAnimationScheduled = false;
animateLocked(frameTimeNs);
}
}
};
}
我们最后看回调的run函数，如果是FRAME_CALLBACK_TOKEN，就是调用回调的doFrame函数。
private static final class CallbackRecord {
public CallbackRecord next;
public long dueTime;
public Object action; // Runnable or FrameCallback
public Object token;
public void run(long frameTimeNanos) {
if (token == FRAME_CALLBACK_TOKEN) {
((FrameCallback)action).doFrame(frameTimeNanos);
} else {
((Runnable)action).run();
}
}
}
播放动画
在上面doFrame函数启动动画，而动画的播放主要在WindowAnimator的animateLocked函数。
private void animateLocked(long frameTimeNs) {
......
boolean wasAnimating = mAnimating;
mAnimating = false;//设置mAnimating为false
mAppWindowAnimating = false;
SurfaceControl.openTransaction();
SurfaceControl.setAnimationTransaction();
try {
final int numDisplays = mDisplayContentsAnimators.size();
for (int i = 0; i < numDisplays; i++) {
final int displayId = mDisplayContentsAnimators.keyAt(i);
updateAppWindowsLocked(displayId);
......
// Update animations of all applications, including those
// associated with exiting/removed apps
updateWindowsLocked(displayId);
updateWallpaperLocked(displayId);
final WindowList windows = mService.getWindowListLocked(displayId);
final int N = windows.size();
for (int j = 0; j < N; j++) {
windows.get(j).mWinAnimator.prepareSurfaceLocked(true);//输出动画帧
}
}
for (int i = 0; i < numDisplays; i++) {
final int displayId = mDisplayContentsAnimators.keyAt(i);
testTokenMayBeDrawnLocked(displayId);
final ScreenRotationAnimation screenRotationAnimation =
mDisplayContentsAnimators.valueAt(i).mScreenRotationAnimation;
if (screenRotationAnimation != null) {
screenRotationAnimation.updateSurfacesInTransaction();
}
mAnimating |= mService.getDisplayContentLocked(displayId).animateDimLayers();
if (mService.mAccessibilityController != null
&& displayId == Display.DEFAULT_DISPLAY) {
mService.mAccessibilityController.drawMagnifiedRegionBorderIfNeededLocked();
}
}
if (mAnimating) {//为true，继续调用WMS的scheduleAnimationLocked播放下一帧
mService.scheduleAnimationLocked();
}
......
finally {
SurfaceControl.closeTransaction();
}
......
boolean doRequest = false;
if (mBulkUpdateParams != 0) {
doRequest = mService.copyAnimToLayoutParamsLocked();
}
if (hasPendingLayoutChanges || doRequest) {
mService.requestTraversalLocked();//重新刷新UI
}
if (!mAnimating && wasAnimating) {
mService.requestTraversalLocked();
}
}
animateLocked方法先将mAnimating 设置为false，然后调用updateWindowsLocked函数和updateWallpaperLocked函数，updateWindowsLocked这个函数会调用WindowStateAnimator类的stepAnimationLocker方法，如果动画已经显示完最后一帧，stepAnimationLocker方法将会WindowStateAnimator类的mAnimating设置为false，表示该窗口的动画已经结束。而在updateWallpaperLocked函数中会判断所有窗口的动画是否已经结束，只要有一个动画没结束，就会将winAnimator的mAnimating设置为true。
for (int i = windows.size() - 1; i >= 0; i--) {
final WindowState win = windows.get(i);
WindowStateAnimator winAnimator = win.mWinAnimator;
if (winAnimator.mSurfaceControl == null) {
continue;
}
final int flags = win.mAttrs.flags;
if (winAnimator.mAnimating) {
......
mAnimating = true;
}
......
再回到animatelocked方法，当mAnimating为true是会调用WMS的scheduleAnimationLocked方法继续显示动画，否则动画显示就结束了。
下面我们总结下动画的播放过程：需要播放动画时，先会调用WMS的scheduleAnimationLocked方法。调用这个方法后，才会接受并处理一次VSync信号，对VSync信号的处理，就是所有需要绘制的窗口根据各自动画的谁知重新调整窗口Surface的变化矩阵和透明度；如果还有窗口动画需要显示，继续调用scheduleAnimationLocked方法准备下一帧。
准备一帧动画的时间可以跨越多个VSync信号周期，但是只有收到VSync信号才能更新窗口的Surface的属性和内容，对应用而言收到VSync信号意味着SurfaceFlinger中已经把上次设置的动画数据取走了，可以安全地设置下一帧动画的属性和内容了。
窗口动画对象WindowStateAnimator
窗口对象WindowState中定义了一个类型为WindowStateAnimator的成员变量mWinAnimator，用来表示窗口的动画对象。
下面是一些成员变量
boolean mAnimating;//表示是否正在显示动画
boolean mLocalAnimating;//表示窗口动画是否已经初始化
Animation mAnimation;//表示窗口动画对象
boolean mAnimationIsEntrance;//
boolean mHasTransformation;//表示当前动画的mTransformation是否可用
boolean mHasLocalTransformation;//表示当前动画时一个窗口动画还是Activity动画
final Transformation mTransformation = new Transformation();//变换矩阵对象
当前正在显示的动画有两种类型，一种的窗口切换动画，一种是Activity切换动画，这里使用了mLocalAnimating和mHasLocalTransformation分别表示窗口动画的状态。
stepAnimationLocked是WindowStateAnimator类中显示动画首先调用的方法，它会初始化WindowStateAnimator对象的一些成员变量
boolean stepAnimationLocked(long currentTime) {
final DisplayContent displayContent = mWin.getDisplayContent();
if (displayContent != null && mService.okToDisplay()) {
if (mWin.isDrawnLw() && mAnimation != null) {//窗口准备好绘制了，窗口动画对象不为空
mHasTransformation = true;
mHasLocalTransformation = true;
if (!mLocalAnimating) {//还没有初始化窗口对象
final DisplayInfo displayInfo = displayContent.getDisplayInfo();
if (mAnimateMove) {
mAnimateMove = false;
mAnimation.initialize(mWin.mFrame.width(), mWin.mFrame.height(),//初始化窗口对象
mAnimDw, mAnimDh);
} else {
mAnimation.initialize(mWin.mFrame.width(), mWin.mFrame.height(),
displayInfo.appWidth, displayInfo.appHeight);
}
mAnimDw = displayInfo.appWidth;
mAnimDh = displayInfo.appHeight;
mAnimation.setStartTime(mAnimationStartTime != -1
? mAnimationStartTime
: currentTime);
mLocalAnimating = true;// 设置为true代表已经初始化窗口对象
mAnimating = true;
}
if ((mAnimation != null) && mLocalAnimating) {
mLastAnimationTime = currentTime;
if (stepAnimation(currentTime)) {//通过时间判断动画是否显示完毕
return true;
}
}
}
mHasLocalTransformation = false;
if ((!mLocalAnimating || mAnimationIsEntrance) && mAppAnimator != null//没有设置窗口动画或者窗口动画结束了
&& mAppAnimator.animation != null) {
// 如果有Activity动画，将mAnimating设为true
mAnimating = true;
mHasTransformation = true;
mTransformation.clear();
return false;
} else if (mHasTransformation) {
// Little trick to get through the path below to act like
// we have finished an animation.
mAnimating = true;
} else if (isAnimating()) {
mAnimating = true;
}
} else if (mAnimation != null) {
mAnimating = true;
}
if (!mAnimating && !mLocalAnimating) {
return false;
}
mAnimating = false;
mKeyguardGoingAwayAnimation = false;
mAnimatingMove = false;
mLocalAnimating = false;
......
mHasLocalTransformation = false;
......
mTransformation.clear();
......
return false;
}
该方法的工作就是设置WindowStateAnimator对象的几个成员变量，首先调用WindowState对象的isDrawnLw来判断窗口系统的状态，只有准备好了才能显示，接着判断mAnimation是否为空，不为空代表已经设置好了动画对象。
接下来判断mLocalAnimating变量的值，为false则调用mAnimation的intialize方法来完成动画对象的初始化（主要设置动画的高度和宽度），然后将mLocalAnimating和mAnimating设为true。完成初始化后，接着调用stepAnimation方法来判断动画是否已经显示完成，没有完成返回true。
如果没有设置动画或者动画已经结束了，则还有判断窗口所在的Activity是否还存在动画，如果有，将mAnimating设置true（表示还要继续播放动画），如果同时mHasTransformation的值仍然为true，或者isAnimating方法返回true，也将mAnimating设置为true。
isAnimating会根据当前动画对象mAnimation是否为空，它的附加窗口的动画对象是否为空，以及窗口所在的Activity的动画对象是否为空等条件来判断，这表示只要有可能mAnimating就会设置为true。这样的目的尽量让动画完成显示，即使没有可显示的动画，多刷新几次不会有副作用，但如果少画了一次，屏幕上就可能留下不正确画面了。
boolean isAnimating() {
return mAnimation != null
|| (mAttachedWinAnimator != null && mAttachedWinAnimator.mAnimation != null)
|| (mAppAnimator != null && mAppAnimator.isAnimating());
}
我们再看看动画的生成过程，WindowStateAnimator的prepareSurfaceLocked方法来完成计算一帧动画并显示工作：
public void prepareSurfaceLocked(final boolean recoveringMemory) {
......
computeShownFrameLocked();//计算要显示的动画帧
setSurfaceBoundariesLocked(recoveringMemory);
if (mIsWallpaper && !mWin.mWallpaperVisible) {
hide();//如果是壁纸窗口，隐藏
} else if (w.mAttachedHidden || !w.isOnScreen()) {
hide();//如果窗口不可见，隐藏
......
} else if (mLastLayer != mAnimLayer
|| mLastAlpha != mShownAlpha
|| mLastDsDx != mDsDx
|| mLastDtDx != mDtDx
|| mLastDsDy != mDsDy
|| mLastDtDy != mDtDy
|| w.mLastHScale != w.mHScale
|| w.mLastVScale != w.mVScale
|| mLastHidden) {//每个值是否有变化
displayed = true;
mLastAlpha = mShownAlpha;
mLastLayer = mAnimLayer;
mLastDsDx = mDsDx;
mLastDtDx = mDtDx;
mLastDsDy = mDsDy;
mLastDtDy = mDtDy;
w.mLastHScale = w.mHScale;
w.mLastVScale = w.mVScale;
if (mSurfaceControl != null) {
try {
mSurfaceAlpha = mShownAlpha;
mSurfaceControl.setAlpha(mShownAlpha);
mSurfaceLayer = mAnimLayer;
mSurfaceControl.setLayer(mAnimLayer);
mSurfaceControl.setMatrix(
mDsDx * w.mHScale, mDtDx * w.mVScale,
mDsDy * w.mHScale, mDtDy * w.mVScale);
if (mLastHidden && mDrawState == HAS_DRAWN) {
if (showSurfaceRobustlyLocked()) {//输出动画帧
mLastHidden = false;
if (mIsWallpaper) {
mService.dispatchWallpaperVisibility(w, true);
}
mAnimator.setPendingLayoutChanges(w.getDisplayId(),
WindowManagerPolicy.FINISH_LAYOUT_REDO_ANIM);
} else {
w.mOrientationChanging = false;
}
}
if (mSurfaceControl != null) {
w.mToken.hasVisible = true;
}
} catch (RuntimeException e) {
Slog.w(TAG, "Error updating surface in " + w, e);
if (!recoveringMemory) {
mService.reclaimSomeSurfaceMemoryLocked(this, "update", true);
}
}
}
}
......
}
该函数先调用了computeShownFrameLocked函数计算当前需要显示的动画帧数据，mAnimLayer表示窗口的Z轴、mShownAlpha窗口透明度；mDsDx、mDtDx、mDsDy和mDtDy表示二维变换矩阵；w.mHScale w.mVScale表示窗口的缩放比例
只有计算出的数据和上一次数据不一样才会调用showSurfaceRobustlyLocked输出动画帧。