Android源码解析之属性动画详解

前言

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<String, PropertyValuesHolder>(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();
}

• 先初始化一些值
• updateScaledDuration 缩放时间，默认为1.0f
• 获取或者创建AnimationHandler，将动画加入到mPendingAnimations列表中，
• 如果没延迟，通知监听器
• 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<ValueAnimator> pendingCopy =
 (ArrayList<ValueAnimator>) 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：

1. 从mPendingAnimations中取出动画，根据事先选择startAnimation还是加入到mDelayedAnims列表。
2. 如果mDelayedAnims列表中的动画准备好了，就加入到mReadyAnims列表中
3. 从mAnimations列表中取出要执行的动画，加入到mTmpAnimations列表
4. 通过doAnimationFrame方法执行动画帧
5. 继续执行scheduleAnimation

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;
 }

• 计算fraction
• 调用animateValue方法

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，

1. 调用父类的animateValue方法
2. 通过setAnimatedValue设置属性

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

1. mIntProperty不为null
2. mProperty不为null
3. mJniSetter不为null
4. mSetter不为null

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，谢谢大家对我们的支持。