??最近由于項目需要，寫了一個動畫。歷經坎坷，花了好幾天的時間，被 UX 批評了好幾次，事后回想，發現這個動畫其實也比較簡單，無非就是對一系列動作的拆解工作，形成一系列的 animator ， 然后按照時間順序執行就可以了，動畫效果圖如下：
??雖然寫的這個動畫比較簡單，但是在寫動畫過程中了解了一些 Animator 和 Animation 的原理，在此記錄一下，畢竟溫故知新，可以為師矣。本篇涉及到的Android 源碼 7.0 進行分析。
Animator
??以 ValueAnimator 為例，從 start() 方法開始分析，代碼如下：
private void start(boolean playBackwards) {
.....
mReversing = playBackwards;
...
// 變量重置初始化
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;
// 重點關注此處 AnimationHandler
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.
// 看看該方法具體做了什么.
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);
}
}
}
在 startAnimation() 中，做了一些變量的初始化工作，重點關注該方法中的 AnimationHandler 和 startAnimation() 具體做了什么，首先看startAnimation() 代碼如下：
/**
* Called internally to start an animation by adding it to the active animations list. Must be
* called on the UI thread.
*/
private void startAnimation() {
if (Trace.isTagEnabled(Trace.TRACE_TAG_VIEW)) {
Trace.asyncTraceBegin(Trace.TRACE_TAG_VIEW, getNameForTrace(),
System.identityHashCode(this));
}
mAnimationEndRequested = false;
initAnimation();
mRunning = true;
if (mSeekFraction >= 0) {
mOverallFraction = mSeekFraction;
} else {
mOverallFraction = 0f;
}
if (mListeners != null) {
notifyStartListeners();
}
}
??可以看到，在startAnimation() 中，僅僅只是進行了動畫的初始化工作，并且通知 listener 去回調 onAnimationStart() 方法，那么真正的動畫執行時機是什么時候呢，回到剛才的 AnimationHandler ,其addAnimationFrameCallback() 代碼如下：
/**
* Register to get a callback on the next frame after the delay.
*/
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));
}
}
?? 上述方法比較簡單，首先看下getProvider()實例如下代碼：
/**
* Default provider of timing pulse that uses Choreographer for frame callbacks.
*/
private class MyFrameCallbackProvider implements AnimationFrameCallbackProvider {
final Choreographer mChoreographer = Choreographer.getInstance();
@Override
public void postFrameCallback(Choreographer.FrameCallback callback) {
mChoreographer.postFrameCallback(callback);
}
@Override
public void postCommitCallback(Runnable runnable) {
mChoreographer.postCallback(Choreographer.CALLBACK_COMMIT, runnable, null);
}
@Override
public long getFrameTime() {
return mChoreographer.getFrameTime();
}
@Override
public long getFrameDelay() {
return Choreographer.getFrameDelay();
}
@Override
public void setFrameDelay(long delay) {
Choreographer.setFrameDelay(delay);
}
}
?? 在上述類中，有個成員變量為 Choreographer（編舞者） 實例，它負責提供垂直同步信號（每次間隔16.67ms），用來進行屏幕刷新，如果對 Choreographer 有興趣可以參考Android Choreographer 源碼分析這篇文章。postFrameCallback() 方法向 Choreographer 注冊了一份 callback , 在 Choreographer 接收到垂直同步信號后，便會對該 callback 實例進行回調。下來看看這個 callback 是什么，代碼如下：
private final Choreographer.FrameCallback mFrameCallback = new Choreographer.FrameCallback() {
@Override
public void doFrame(long frameTimeNanos) {
doAnimationFrame(getProvider().getFrameTime());
if (mAnimationCallbacks.size() > 0) {
getProvider().postFrameCallback(this);
}
}
};
?? callback最終會回調到 doFrame() 方法，在該方法中，通過 doAnimationFrame() 真正執行更新邏輯，如果動畫沒有結束，那么將該callBack 重新進行注冊，接收后續的垂直同步信息。doAnimationFrame() 代碼如下：
/**
* Processes a frame of the animation, adjusting the start time if needed.
*
* @param frameTime The frame time.
* @return true if the animation has ended.
* @hide
*/
public final void doAnimationFrame(long frameTime) {
AnimationHandler handler = AnimationHandler.getInstance();
....
//
boolean finished = animateBasedOnTime(currentTime);
if (finished) {
endAnimation();
}
}
?? 可以看到，動畫的主要邏輯都在 animateBasedOnTime() 里面， 并且根據返回值來判斷是否要進行結束的回調，下面看一下該方法具體做了什么，代碼如下：
/**
* This internal function processes a single animation frame for a given animation. The
* currentTime parameter is the timing pulse sent by the handler, used to calculate the
* elapsed duration, and therefore
* the elapsed fraction, of the animation. The return value indicates whether the animation
* should be ended (which happens when the elapsed time of the animation exceeds the
* animation's duration, including the repeatCount).
*
* @param currentTime The current time, as tracked by the static timing handler
* @return true if the animation's duration, including any repetitions due to
* <code>repeatCount</code> has been exceeded and the animation should be ended.
*/
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;
}
??方法的前半部分主要進行當前fraction進度的修正，以及是否要執行onAnimationRepeat() 的回調，真正刷新繼續往下找，animateValue()方法如下：
void animateValue(float fraction) {
// 根據插值器類型的不同，計算當前fraction 對應的值..
fraction = mInterpolator.getInterpolation(fraction);
mCurrentFraction = fraction;
int numValues = mValues.length;
for (int i = 0; i < numValues; ++i) {
// 將 fraction 轉換成真正的 animatedValue ..
mValues[i].calculateValue(fraction);
}
if (mUpdateListeners != null) {
int numListeners = mUpdateListeners.size();
for (int i = 0; i < numListeners; ++i) {
// 最熟悉的 onAnimationUpdate() 回調，可以在該回調做
// 任何自己想要做的操作， 根據上面算出的 animatedValue
// 去更新自己想要的屬性。
mUpdateListeners.get(i).onAnimationUpdate(this);
}
}
}
??從代碼可以看出，首先通過插值器去修正 fraction 的值，插值器類型可以通過 setInterpolator() 來指定，得到 fraction 以后，再將當前 fraction 轉換成真正的 animatedValue ，最后通過 listener完成onAnimationUpdate() 回調即可。
??自此， Animator 大體運作原理分析完畢，Animator 的核心邏輯還是依靠 Choreographer 來完成更新，并且需要注意一點的是， 因為 AnimationHandler 和 Choregrapher 是 ThreadLocal 線程私有的，每一個線程如果需要都會有各自的 Choregrapher 實例，所以 ValueAnimator 是完全可以運行在子線程中的，只要在 ValueAnimator 運行中不涉及到 UI 的更新即可。這一點 ValueAnimator 注釋也已經很清楚了，說明如下：
/**
* Start the animation playing. This version of start() takes a boolean flag that indicates
* whether the animation should play in reverse. The flag is usually false, but may be set
* to true if called from the reverse() method.
*
* <p>The animation started by calling this method will be run on the thread that called
* this method. This thread should have a Looper on it (a runtime exception will be thrown if
* this is not the case). Also, if the animation will animate
* properties of objects in the view hierarchy, then the calling thread should be the UI
* thread for that view hierarchy.</p>
*
* @param playBackwards Whether the ValueAnimator should start playing in reverse.
*/
??依靠著 Choreographer ，在理想情況下，ValueAnimator 每間隔 16.67ms 會重新計算 animatorValue , 以此來更新界面。在實際情況中，可以用 Choreographer 做很多事情，比如應用的幀率檢測等等，在此推薦一個優秀的幀率檢測開源項目。其核心也是通過 choreographer 來實現。
??戳我 >> TinyDancer
?? 參考文章：
??View 動畫 Animation 運行原理解析
??Android Choreographer 源碼分析
??Android Choreographer 源碼分析
