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大家在日常开发中离不开动画，属性动画更为强大，我们不仅要知道如何使用，更要知道他的原理。这样，才能得心应手。那么，今天，就从最简单的来说，了解下属性动画的原理。
ObjectAnimator .ofInt(mView,"width",100,500) .setDuration(1000) .start();
ObjectAnimator#ofInt
以这个为例，代码如下。
public static ObjectAnimator ofInt(Object target, String propertyName, int... values) {
ObjectAnimator anim = new ObjectAnimator(target, propertyName);
anim.setIntValues(values);
return anim;
}在这个方法中，首先会new一个ObjectAnimator对象，然后通过setIntValues方法将值设置进去，然后返回。在ObjectAnimator的构造方法中，会通过setTarget方法设置当前动画的对象，通过setPropertyName设置当前的属性名。我们重点说下setIntValues方法。
public void setIntValues(int... values) {
if (mValues == null || mValues.length == 0) {
// No values yet - this animator is being constructed piecemeal. Init the values with
// whatever the current propertyName is
if (mProperty != null) {
setValues(PropertyValuesHolder.ofInt(mProperty, values));
} else {
setValues(PropertyValuesHolder.ofInt(mPropertyName, values));
}
} else {
super.setIntValues(values);
}
}首先会判断，mValues是不是null，我们这里是null，并且mProperty也是null，所以会调用setValues(PropertyValuesHolder.ofInt(mPropertyName, values));方法。先看PropertyValuesHolder.ofInt方法，PropertyValuesHolder这个类是holds属性和值的，在这个方法会构造一个IntPropertyValuesHolder对象并返回。
public static PropertyValuesHolder ofInt(String propertyName, int... values) {
return new IntPropertyValuesHolder(propertyName, values);
}IntPropertyValuesHolder的构造方法如下：
public IntPropertyValuesHolder(String propertyName, int... values) {
super(propertyName);
setIntValues(values);
}在这里，首先会调用他的分类的构造方法，然后调用setIntValues方法，在他父类的构造方法中，只是设置了下propertyName。setIntValues内容如下：
public void setIntValues(int... values) {
super.setIntValues(values);
mIntKeyframes = (Keyframes.IntKeyframes) mKeyframes;
}在父类的setIntValues方法中，初始化了mValueType为int.class，mKeyframes为KeyframeSet.ofInt(values)。其中KeyframeSet为关键帧集合。然后将mKeyframes赋值给mIntKeyframes。
KeyframeSet
这个类是记录关键帧的。我们看下他的ofInt方法。
public static KeyframeSet ofInt(int... values) {
int numKeyframes = values.length;
IntKeyframe keyframes[] = new IntKeyframe[Math.max(numKeyframes,2)];
if (numKeyframes == 1) {
keyframes[0] = (IntKeyframe) Keyframe.ofInt(0f);
keyframes[1] = (IntKeyframe) Keyframe.ofInt(1f, values[0]);
} else {
keyframes[0] = (IntKeyframe) Keyframe.ofInt(0f, values[0]);
for (int i = 1; i < numKeyframes; ++i) {
keyframes[i] =
(IntKeyframe) Keyframe.ofInt((float) i / (numKeyframes - 1), values[i]);
}
}
return new IntKeyframeSet(keyframes);
}在这里呢？根据传入的values来计算关键帧，最后返回IntKeyframeSet。
回到ObjectAnimator里面，这里的setValues用的是父类ValueAnimator的
ValueAnimator#setValues
public void setValues(PropertyValuesHolder... values) {
int numValues = values.length;
mValues = values;
mValuesMap = new HashMap(numValues);
for (int i = 0; i < numValues; ++i) {
PropertyValuesHolder valuesHolder = values[i];
mValuesMap.put(valuesHolder.getPropertyName(), valuesHolder);
}
// New property/values/target should cause re-initialization prior to starting
mInitialized = false;
} 这里的操作就简单了，就是把PropertyValuesHolder放入到mValuesMap中。
ObjectAnimator#start
这个方法就是动画开始的地方。
public void start() {
// See if any of the current active/pending animators need to be canceled
AnimationHandler handler = sAnimationHandler.get();
if (handler != null) {
int numAnims = handler.mAnimations.size();
for (int i = numAnims - 1; i >= 0; i--) {
if (handler.mAnimations.get(i) instanceof ObjectAnimator) {
ObjectAnimator anim = (ObjectAnimator) handler.mAnimations.get(i);
if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
anim.cancel();
}
}
}
numAnims = handler.mPendingAnimations.size();
for (int i = numAnims - 1; i >= 0; i--) {
if (handler.mPendingAnimations.get(i) instanceof ObjectAnimator) {
ObjectAnimator anim = (ObjectAnimator) handler.mPendingAnimations.get(i);
if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
anim.cancel();
}
}
}
numAnims = handler.mDelayedAnims.size();
for (int i = numAnims - 1; i >= 0; i--) {
if (handler.mDelayedAnims.get(i) instanceof ObjectAnimator) {
ObjectAnimator anim = (ObjectAnimator) handler.mDelayedAnims.get(i);
if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
anim.cancel();
}
}
}
}
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();
}首先呢，会获取AnimationHandler对象，如果不为空的话，就会判断是mAnimations、mPendingAnimations、mDelayedAnims中的动画，并且取消。最后调用父类的start方法。
ValueAnimator#start
private void start(boolean playBackwards) {
if (Looper.myLooper() == null) {
throw new AndroidRuntimeException("Animators may only be run on Looper threads");
}
mReversing = playBackwards;
mPlayingBackwards = playBackwards;
if (playBackwards && mSeekFraction != -1) {
if (mSeekFraction == 0 && mCurrentIteration == 0) {
// special case: reversing from seek-to-0 should act as if not seeked at all
mSeekFraction = 0;
} else if (mRepeatCount == INFINITE) {
mSeekFraction = 1 - (mSeekFraction % 1);
} else {
mSeekFraction = 1 + mRepeatCount - (mCurrentIteration + mSeekFraction);
}
mCurrentIteration = (int) mSeekFraction;
mSeekFraction = mSeekFraction % 1;
}
if (mCurrentIteration > 0 && mRepeatMode == REVERSE &&
(mCurrentIteration < (mRepeatCount + 1) || mRepeatCount == INFINITE)) {
// if we were seeked to some other iteration in a reversing animator,
// figure out the correct direction to start playing based on the iteration
if (playBackwards) {
mPlayingBackwards = (mCurrentIteration % 2) == 0;
} else {
mPlayingBackwards = (mCurrentIteration % 2) != 0;
}
}
int prevPlayingState = mPlayingState;
mPlayingState = STOPPED;
mStarted = true;
mStartedDelay = false;
mPaused = false;
updateScaledDuration(); // in case the scale factor has changed since creation time
AnimationHandler animationHandler = getOrCreateAnimationHandler();
animationHandler.mPendingAnimations.add(this);
if (mStartDelay == 0) {
// This sets the initial value of the animation, prior to actually starting it running
if (prevPlayingState != SEEKED) {
setCurrentPlayTime(0);
}
mPlayingState = STOPPED;
mRunning = true;
notifyStartListeners();
}
animationHandler.start();
}在animationHandler.start中，会调用scheduleAnimation方法，在这个种，会用mChoreographerpost一个callback，最终会执行mAnimate的run方法。mChoreographerpost涉及到VSYNC，这里不多介绍。
mAnimate#run
doAnimationFrame(mChoreographer.getFrameTime());
在这里会用过doAnimationFrame设置动画帧，我们看下这个方法的代码。
void doAnimationFrame(long frameTime) {
mLastFrameTime = frameTime;
// mPendingAnimations holds any animations that have requested to be started
// We're going to clear mPendingAnimations, but starting animation may
// cause more to be added to the pending list (for example, if one animation
// starting triggers another starting). So we loop until mPendingAnimations
// is empty.
while (mPendingAnimations.size() > 0) {
ArrayList pendingCopy =
(ArrayList) mPendingAnimations.clone();
mPendingAnimations.clear();
int count = pendingCopy.size();
for (int i = 0; i < count; ++i) {
ValueAnimator anim = pendingCopy.get(i);
// If the animation has a startDelay, place it on the delayed list
if (anim.mStartDelay == 0) {
anim.startAnimation(this);
} else {
mDelayedAnims.add(anim);
}
}
}
// Next, process animations currently sitting on the delayed queue, adding
// them to the active animations if they are ready
int numDelayedAnims = mDelayedAnims.size();
for (int i = 0; i < numDelayedAnims; ++i) {
ValueAnimator anim = mDelayedAnims.get(i);
if (anim.delayedAnimationFrame(frameTime)) {
mReadyAnims.add(anim);
}
}
int numReadyAnims = mReadyAnims.size();
if (numReadyAnims > 0) {
for (int i = 0; i < numReadyAnims; ++i) {
ValueAnimator anim = mReadyAnims.get(i);
anim.startAnimation(this);
anim.mRunning = true;
mDelayedAnims.remove(anim);
}
mReadyAnims.clear();
}
// Now process all active animations. The return value from animationFrame()
// tells the handler whether it should now be ended
int numAnims = mAnimations.size();
for (int i = 0; i < numAnims; ++i) {
mTmpAnimations.add(mAnimations.get(i));
}
for (int i = 0; i < numAnims; ++i) {
ValueAnimator anim = mTmpAnimations.get(i);
if (mAnimations.contains(anim) && anim.doAnimationFrame(frameTime)) {
mEndingAnims.add(anim);
}
}
mTmpAnimations.clear();
if (mEndingAnims.size() > 0) {
for (int i = 0; i < mEndingAnims.size(); ++i) {
mEndingAnims.get(i).endAnimation(this);
}
mEndingAnims.clear();
}
// Schedule final commit for the frame.
mChoreographer.postCallback(Choreographer.CALLBACK_COMMIT, mCommit, null);
// If there are still active or delayed animations, schedule a future call to
// onAnimate to process the next frame of the animations.
if (!mAnimations.isEmpty() || !mDelayedAnims.isEmpty()) {
scheduleAnimation();
}
} 方法较长，逻辑如下：
从上面我们能看出，执行动画的关键是doAnimationFrame方法。在这个方法中，会调用animationFrame方法。
ValueAniator#animationFrame
boolean animationFrame(long currentTime) {
boolean done = false;
switch (mPlayingState) {
case RUNNING:
case SEEKED:
float fraction = mDuration > 0 ? (float)(currentTime - mStartTime) / mDuration : 1f;
if (mDuration == 0 && mRepeatCount != INFINITE) {
// Skip to the end
mCurrentIteration = mRepeatCount;
if (!mReversing) {
mPlayingBackwards = false;
}
}
if (fraction >= 1f) {
if (mCurrentIteration < mRepeatCount || mRepeatCount == INFINITE) {
// Time to repeat
if (mListeners != null) {
int numListeners = mListeners.size();
for (int i = 0; i < numListeners; ++i) {
mListeners.get(i).onAnimationRepeat(this);
}
}
if (mRepeatMode == REVERSE) {
mPlayingBackwards = !mPlayingBackwards;
}
mCurrentIteration += (int) fraction;
fraction = fraction % 1f;
mStartTime += mDuration;
// Note: We do not need to update the value of mStartTimeCommitted here
// since we just added a duration offset.
} else {
done = true;
fraction = Math.min(fraction, 1.0f);
}
}
if (mPlayingBackwards) {
fraction = 1f - fraction;
}
animateValue(fraction);
break;
}
return done;
}根据虚拟机执行引擎动态分派原则，这里会调用ObjectAnimator的animateValue方法。
ObjectAnimator#animateValue
void animateValue(float fraction) {
final Object target = getTarget();
if (mTarget != null && target == null) {
// We lost the target reference, cancel and clean up.
cancel();
return;
}
super.animateValue(fraction);
int numValues = mValues.length;
for (int i = 0; i < numValues; ++i) {
mValues[i].setAnimatedValue(target);
}
}这里主要干了两件事，
其父类的方法如下：
void 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);
}
}
}在这个方法中，会通过Interpolator得到出当前的fraction，并通过calculateValue来计算当前应该的值，这里会调用IntPropertyValuesHolder的calculateValue
void calculateValue(float fraction) {
mIntAnimatedValue = mIntKeyframes.getIntValue(fraction);
}我们知道，mIntKeyframes对应的是IntKeyframeSet。在这个类的getIntValue中，会通过TypeEvaluator来计算当前对应的值。不多说了。
最后，回到animateValue。计算了值之后，会调用setAnimatedValue来设置值。我们看看他的实现。
IntPropertyValuesHolder#setAnimatedValue
void setAnimatedValue(Object target) {
if (mIntProperty != null) {
mIntProperty.setValue(target, mIntAnimatedValue);
return;
}
if (mProperty != null) {
mProperty.set(target, mIntAnimatedValue);
return;
}
if (mJniSetter != 0) {
nCallIntMethod(target, mJniSetter, mIntAnimatedValue);
return;
}
if (mSetter != null) {
try {
mTmpValueArray[0] = mIntAnimatedValue;
mSetter.invoke(target, mTmpValueArray);
} catch (InvocationTargetException e) {
Log.e("PropertyValuesHolder", e.toString());
} catch (IllegalAccessException e) {
Log.e("PropertyValuesHolder", e.toString());
}
}
}恩，到这里就能看到修改属性值得痕迹了，有以下四种情况
首先，我们通过String propertyName, int… values参数构造的对象，mIntProperty为null，并且mProperty也为null。那其他两个是怎么来的呢？似乎漏了什么？
还节的，在doAnimationFrame中，直接调用startAnimation么?没错，就是这里。
startAnimation
在这个方法中调用了initAnimation方法。还是根据动态分派规则，这里调用ObjectAnimator的initAnimation方法。在这里调用PropertyValuesHolder的setupSetterAndGetter方法，在这里对mSetter等进行了初始化，这里就不多说了，大家自己看代码吧。
好了，以上就是关于Android中属性动画对的全部内容，希望本文的内容对各位Android开发者们能带来一定的帮助，如果有疑问大家可以留言交流，谢谢大家对创新互联的支持。
