我们从启动动画为着手点分析属性动画的执行过程
@Override
public void start() {
AnimationHandler.getInstance().autoCancelBasedOn(this);
if (DBG) {
Log.d(LOG_TAG, "Anim target, duration: " + getTarget() + ", " + getDuration());
for (int i = 0; i < mValues.length; ++i) {
PropertyValuesHolder pvh = mValues[i];
Log.d(LOG_TAG, "Values[" + i + "]: " + pvh.getPropertyName() + ", " + pvh.mKeyframes.getValue(0) + ", " + pvh.mKeyframes.getValue(1));
}
}
super.start();
}
该段代码只需看第一行和最后一行，第一行代码为查看ThreadLocal中是否有AnimationHandler实例，并为其赋初值，如果mAnimationCallbacks这个ArrayList中有保存回调，那么遍历该ArrayList并查看元素是否应该自动取消，最后一行调用了父类的start方法，那么我们就查看下ValueAnimator中的start方法
@Overridepublic void start() {
start(false);
}
private void start(boolean playBackwards) {
if (Looper.myLooper() == null) {
throw new AndroidRuntimeException("Animators may only be run on Looper threads");
}
mReversing = playBackwards;
// Special case: reversing from seek-to-0 should act as if not seeked at all.
if (playBackwards && mSeekFraction != -1 && mSeekFraction != 0) {
if (mRepeatCount == INFINITE) {
// Calculate the fraction of the current iteration.
float fraction = (float) (mSeekFraction - Math.floor(mSeekFraction));
mSeekFraction = 1 - fraction;
} else {
mSeekFraction = 1 + mRepeatCount - mSeekFraction;
}
}
mStarted = true;
mPaused = false;
mRunning = false;
// Resets mLastFrameTime when start() is called, so that if the animation was running,
// calling start() would put the animation in the
// started-but-not-yet-reached-the-first-frame phase.
mLastFrameTime = 0;
AnimationHandler animationHandler = AnimationHandler.getInstance();
animationHandler.addAnimationFrameCallback(this, (long) (mStartDelay * sDurationScale));
if (mStartDelay == 0 || mSeekFraction >= 0) {
// If there's no start delay, init the animation and notify start listeners right away
// to be consistent with the previous behavior. Otherwise, postpone this until the first
// frame after the start delay.
startAnimation();
if (mSeekFraction == -1) {
// No seek, start at play time 0. Note that the reason we are not using fraction 0
// is because for animations with 0 duration, we want to be consistent with pre-N
// behavior: skip to the final value immediately.
setCurrentPlayTime(0);
} else {
setCurrentFraction(mSeekFraction);
}
}
}
可以看出动画时运行在有Looper中的线程中的（为什么要在Looper中因为之后的代码使用到了Handler），我们看到
animationHandler.addAnimationFrameCallback(this, (long) (mStartDelay * sDurationScale));让我们走进这行代码深究一下
public void addAnimationFrameCallback(final AnimationFrameCallback callback, long delay) {
if (mAnimationCallbacks.size() == 0) {
getProvider().postFrameCallback(mFrameCallback);
}
if (!mAnimationCallbacks.contains(callback)) {
mAnimationCallbacks.add(callback);
}
if (delay > 0) {
mDelayedCallbackStartTime.put(callback, (SystemClock.uptimeMillis() + delay));
}
}
getProvider()返回的是MyFrameCallbackProvider的实例，而MyFrameCallbackProvider由实现了AnimationFrameCallbackProvider接口，所以让我们愉快的查看MyFrameCallbackProvider中的postFrameCallback方法
@Overridepublic void postFrameCallback(Choreographer.FrameCallback callback) {
mChoreographer.postFrameCallback(callback);
}
该方法内部逐层调用最终调用了
private void postCallbackDelayedInternal(int callbackType, Object action, Object token, long delayMillis) {
if (DEBUG_FRAMES) {
Log.d(TAG, "PostCallback: type=" + callbackType + ", action=" + action + ", token=" + token + ", delayMillis=" + delayMillis);
}
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
mCallbackQueues[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);
}
}
}
我们看到这里使用到了Handler，所以这就解释了为什么前面需要检查Looper是否为空了。我们可以看到逻辑是如果有延迟那么就运行else方法，没有设置延迟都是调用scheduleFrameLocked方法
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);
}
}
}
我们看出最后运行的是scheduleVsyncLocked();方法，而该方法内部调用的是native方法。因为之前调用传递的是Choreographer.FrameCallback实例，所以最终因该会调用该实例的doFrame方法，而会调用AnimationHandler的doAnimationFrame方法，而该方法又会调用ValueAnimator的doAnimationFrame方法
public final void doAnimationFrame(long frameTime) {
AnimationHandler handler = AnimationHandler.getInstance();
if (mLastFrameTime == 0) {
// First frame
handler.addOneShotCommitCallback(this);
if (mStartDelay > 0) {
startAnimation();
}
if (mSeekFraction < 0) {
mStartTime = frameTime;
} else {
long seekTime = (long) (getScaledDuration() * mSeekFraction); mStartTime = frameTime - seekTime;
mSeekFraction = -1;
}
mStartTimeCommitted = false; // allow start time to be compensated for jank
}
mLastFrameTime = frameTime;
if (mPaused) {
mPauseTime = frameTime;
handler.removeCallback(this);
return;
} else if (mResumed) {
mResumed = false;
if (mPauseTime > 0) {
// Offset by the duration that the animation was paused
mStartTime += (frameTime - mPauseTime);
mStartTimeCommitted = false; // allow start time to be compensated for jank
}
handler.addOneShotCommitCallback(this);
}
// The frame time might be before the start time during the first frame of
// an animation. The "current time" must always be on or after the start
// time to avoid animating frames at negative time intervals. In practice, this
// is very rare and only happens when seeking backwards.
final long currentTime = Math.max(frameTime, mStartTime);
boolean finished = animateBasedOnTime(currentTime);
if (finished) {
endAnimation();
}
}
我们看到从animateBasedOnTime(currentTime);方法的返回值用于判断是否结束动画
boolean animateBasedOnTime(long currentTime) {
boolean done = false;
if (mRunning) {
final long scaledDuration = getScaledDuration();
final float fraction = scaledDuration > 0 ? (float)(currentTime - mStartTime) / scaledDuration : 1f;
final float lastFraction = mOverallFraction;
final boolean newIteration = (int) fraction > (int) lastFraction;
final boolean lastIterationFinished = (fraction >= mRepeatCount + 1) && (mRepeatCount != INFINITE);
if (scaledDuration == 0) {
// 0 duration animator, ignore the repeat count and skip to the end
done = true;
} else if (newIteration && !lastIterationFinished) {
// Time to repeat
if (mListeners != null) {
int numListeners = mListeners.size();
for (int i = 0; i < numListeners; ++i) {
mListeners.get(i).onAnimationRepeat(this);
}
}
} else if (lastIterationFinished) {
done = true;
}
mOverallFraction = clampFraction(fraction);
float currentIterationFraction = getCurrentIterationFraction(mOverallFraction);
animateValue(currentIterationFraction);
}
return done;
}
@CallSupervoid animateValue(float fraction) {
fraction = mInterpolator.getInterpolation(fraction);
mCurrentFraction = fraction;
int numValues = mValues.length;
for (int i = 0; i < numValues; ++i) {
mValues[i].calculateValue(fraction);
}
if (mUpdateListeners != null) {
int numListeners = mUpdateListeners.size();
for (int i = 0; i < numListeners; ++i) {
mUpdateListeners.get(i).onAnimationUpdate(this);
}
}
}
我们看到calculateValue(fraction);方法使用PropertyValuesHolder中的calculateValue去计算值。
未完善，待续。。。