前言
大家在日常開發(fā)中離不開動畫，屬性動畫更為強大，我們不僅要知道如何使用，更要知道他的原理。這樣，才能得心應手。那么，今天，就從最簡單的來說，了解下屬性動畫的原理。
ObjectAnimator .ofInt(mView,"width",100,500) .setDuration(1000) .start();
ObjectAnimator#ofInt
以這個為例，代碼如下。
public static ObjectAnimator ofInt(Object target, String propertyName, int... values) { ObjectAnimator anim = new ObjectAnimator(target, propertyName); anim.setIntValues(values); return anim;}在這個方法中，首先會new一個ObjectAnimator對象，然后通過setIntValues方法將值設置進去，然后返回。在ObjectAnimator的構(gòu)造方法中，會通過setTarget方法設置當前動畫的對象，通過setPropertyName設置當前的屬性名。我們重點說下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); }}首先會判斷，mValues是不是null，我們這里是null，并且mProperty也是null，所以會調(diào)用setValues(PropertyValuesHolder.ofInt(mPropertyName, values));方法。先看PropertyValuesHolder.ofInt方法，PropertyValuesHolder這個類是holds屬性和值的，在這個方法會構(gòu)造一個IntPropertyValuesHolder對象并返回。
public static PropertyValuesHolder ofInt(String propertyName, int... values) { return new IntPropertyValuesHolder(propertyName, values);}IntPropertyValuesHolder的構(gòu)造方法如下：
public IntPropertyValuesHolder(String propertyName, int... values) { super(propertyName); setIntValues(values);}在這里，首先會調(diào)用他的分類的構(gòu)造方法，然后調(diào)用setIntValues方法，在他父類的構(gòu)造方法中，只是設置了下propertyName。setIntValues內(nèi)容如下：
public void setIntValues(int... values) { super.setIntValues(values); mIntKeyframes = (Keyframes.IntKeyframes) mKeyframes;}在父類的setIntValues方法中，初始化了mValueType為int.class，mKeyframes為KeyframeSet.ofInt(values)。其中KeyframeSet為關(guān)鍵幀集合。然后將mKeyframes賦值給mIntKeyframes。
KeyframeSet
這個類是記錄關(guān)鍵幀的。我們看下他的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);}在這里呢？根據(jù)傳入的values來計算關(guān)鍵幀，最后返回IntKeyframeSet。
回到ObjectAnimator里面，這里的setValues用的是父類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;}這里的操作就簡單了，就是把PropertyValuesHolder放入到mValuesMap中。
ObjectAnimator#start
這個方法就是動畫開始的地方。
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，會獲取AnimationHandler對象，如果不為空的話，就會判斷是mAnimations、mPendingAnimations、mDelayedAnims中的動畫，并且取消。最后調(diào)用父類的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，會調(diào)用scheduleAnimation方法，在這個種，會用mChoreographerpost一個callback，最終會執(zhí)行mAnimate的run方法。mChoreographerpost涉及到VSYNC，這里不多介紹。
mAnimate#run
doAnimationFrame(mChoreographer.getFrameTime());
在這里會用過doAnimationFrame設置動畫幀，我們看下這個方法的代碼。
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(); }}方法較長，邏輯如下：
從上面我們能看出，執(zhí)行動畫的關(guān)鍵是doAnimationFrame方法。在這個方法中，會調(diào)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; }根據(jù)虛擬機執(zhí)行引擎動態(tài)分派原則，這里會調(diào)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); }}這里主要干了兩件事，
其父類的方法如下：
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); } }}在這個方法中，會通過Interpolator得到出當前的fraction，并通過calculateValue來計算當前應該的值，這里會調(diào)用IntPropertyValuesHolder的calculateValue
void calculateValue(float fraction) { mIntAnimatedValue = mIntKeyframes.getIntValue(fraction);}我們知道，mIntKeyframes對應的是IntKeyframeSet。在這個類的getIntValue中，會通過TypeEvaluator來計算當前對應的值。不多說了。
最后，回到animateValue。計算了值之后，會調(diào)用setAnimatedValue來設置值。我們看看他的實現(xiàn)。
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，到這里就能看到修改屬性值得痕跡了，有以下四種情況
首先，我們通過String propertyName, int… values參數(shù)構(gòu)造的對象，mIntProperty為null，并且mProperty也為null。那其他兩個是怎么來的呢？似乎漏了什么？
還節(jié)的，在doAnimationFrame中，直接調(diào)用startAnimation么?沒錯，就是這里。
startAnimation
在這個方法中調(diào)用了initAnimation方法。還是根據(jù)動態(tài)分派規(guī)則，這里調(diào)用ObjectAnimator的initAnimation方法。在這里調(diào)用PropertyValuesHolder的setupSetterAndGetter方法，在這里對mSetter等進行了初始化，這里就不多說了，大家自己看代碼吧。
好了，以上就是關(guān)于Android中屬性動畫對的全部內(nèi)容，希望本文的內(nèi)容對各位Android開發(fā)者們能帶來一定的幫助，如果有疑問大家可以留言交流，謝謝大家對武林網(wǎng)的支持。
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