#### 7.3.5 屬性動畫的工作原理 屬性動畫要求動畫作用的對象提供該屬性的set方法，屬性動畫根據你傳遞的該屬性的初始值和最終值，以動畫的效果多次去調用set方法。每次傳遞給set方法的值都不一樣，確切來說是隨著時間的推移，所傳遞的值越來越接近最終值。如果動畫的時候沒有傳遞初始值，那么還要提供get方法，因為系統要去獲取屬性的初始值。對于屬性動畫來說，其動畫過程中所做的就是這么多，下面看源碼分析。 首先我們要找一個入口，就從ObjectAnimator.ofInt(mButton, "width", 500).setDuration (5000).start()開始，其他動畫都是類似的。先看ObjectAnimator的start方法： 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.mPending- Animations.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.mDelayed- Anims.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(); } 上面的代碼別看那么長，其實做的事情很簡單，首先會判斷如果當前動畫、等待的動畫（Pending）和延遲的動畫（Delay）中有和當前動畫相同的動畫，那么就把相同的動畫給取消掉，接下來那一段是log，再接著就調用了父類的super.start()方法。因為ObjectAnimator繼承了ValueAnimator，所以接下來我們看一下ValueAnimator的Start方法： private void start(boolean playBackwards) { if (Looper.myLooper() == null) { throw new AndroidRuntimeException("Animators may only be run on Looper threads"); } mPlayingBackwards = playBackwards; mCurrentIteration = 0; 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 setCurrentPlayTime(0); mPlayingState = STOPPED; mRunning = true; notifyStartListeners(); } animationHandler.start(); } 可以看出屬性動畫需要運行在有Looper的線程中。上述代碼最終會調用Animation-Handler的start方法，這個AnimationHandler并不是Handler，它是一個Runnable。看一下它的代碼，通過代碼我們發現，很快就調到了JNI層，不過JNI層最終還是要調回來的。它的run方法會被調用，這個Runnable涉及和底層的交互，我們就忽略這部分，直接看重點：ValueAnimator中的doAnimationFrame方法，如下所示。 final boolean doAnimationFrame(long frameTime) { if (mPlayingState == STOPPED) { mPlayingState = RUNNING; if (mSeekTime < 0) { mStartTime = frameTime; } else { mStartTime = frameTime - mSeekTime; // Now that we're playing, reset the seek time mSeekTime = -1; } } if (mPaused) { if (mPauseTime < 0) { mPauseTime = frameTime; } return false; } else if (mResumed) { mResumed = false; if (mPauseTime > 0) { // Offset by the duration that the animation was paused mStartTime += (frameTime - mPauseTime); } } // 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); return animationFrame(currentTime); } 注意到上述代碼末尾調用了animationFrame方法，而animationFrame內部調用了animateValue，下面看animateValue的代碼： 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); } } } 上述代碼中的calculateValue方法就是計算每幀動畫所對應的屬性的值，下面著重看一下到底是在哪里調用屬性的get和set方法的，畢竟這個才是我們最關心的。 在初始化的時候，如果屬性的初始值沒有提供，則get方法將會被調用，請看Property-ValuesHolder的setupValue方法，可以發現get方法是通過反射來調用的，如下所示。 private void setupValue(Object target, Keyframe kf) { if (mProperty ! = null) { Object value = convertBack(mProperty.get(target)); kf.setValue(value); } try { if (mGetter == null) { Class targetClass = target.getClass(); setupGetter(targetClass); if (mGetter == null) { // Already logged the error - just return to avoid NPE return; } } Object value = convertBack(mGetter.invoke(target)); kf.setValue(value); } catch (InvocationTargetException e) { Log.e("PropertyValuesHolder", e.toString()); } catch (IllegalAccessException e) { Log.e("PropertyValuesHolder", e.toString()); } } 當動畫的下一幀到來的時候，PropertyValuesHolder中的setAnimatedValue方法會將新的屬性值設置給對象，調用其set方法。從下面的源碼可以看出，set方法也是通過反射來調用的： ``` void setAnimatedValue(Object target) { if (mProperty ! = null) { mProperty.set(target, getAnimatedValue()); } if (mSetter ! = null) { try { mTmpValueArray[0] = getAnimatedValue(); mSetter.invoke(target, mTmpValueArray); } catch (InvocationTargetException e) { Log.e("PropertyValuesHolder", e.toString()); } catch (IllegalAccessException e) { Log.e("PropertyValuesHolder", e.toString()); } } } ```