Android高级进阶之路【三】看完这篇再不会Android动画框架，我跪搓衣板 android程序员框架学习

Android动画可作用于View/ViewGroup，Actvity，Fragment实现炫酷的交互效果。经过几天的探究，搞清楚了各类动画的使用和动画的实现原理，在此记录以下。
尽管Android动画有好几种类别，但是各种动画的实现核心都是TimeInterpolator->Interpolator->各种Interpolator。大致过程是通过Interpolator计算出时间相关的input，通过这个input计算出各类fraction，利用各类Interpolator计算出的fraction计算出各种状态参数（时间相关），将这些参数使用到动画效果上，Android会通过一定的机制不断重复这个过程(16ms为周期)就构成了我们看到的动画。
从动画的分类上有以下几种类别：
1、FrameAnimation FrameAnimation顾名思义就是帧动画，通过逐帧播放来实现的动画。Frame Animation可以通过xml来实现，也可应用代码来实现：
xml实现时根结点必须是，根结点内包含多个元素，例如:

对于这种动画，就是不断的更换显示的Drawable来实现动态效果。
2、TweenAnimation 可以对View进行一系列的变换，如平移，翻转，缩放，淡入淡出，也可以将他们组合起来形成混合的动画效果。TweenAnimation的实现方式也有两种，分别可以用代码和xml实现。
xml:

...

加载xml动画可以用Android SDK提供的工具类：

AnimationUtils.loadAnimations();

至于代码实现方式：

//提供了以下几种Animation AlphaAnimation TranslateAnimation ScaleAnimation RotateAnimation AnimationSet.addAnimation(Animation) //使用方法基本与定义xml一致

Animation的运作依赖两个方法：

/** * Gets the transformation to apply at a specified point in time. Implementations of this * method should always replace the specified Transformation or document they are doing * otherwise. * * @param currentTime Where we are in the animation. This is wall clock time. * @param outTransformation A transformation object that is provided by the *caller and will be filled in by the animation. * @return True if the animation is still running */ public boolean getTransformation(long currentTime, Transformation outTransformation) { if (mStartTime == -1) { mStartTime = currentTime; }final long startOffset = getStartOffset(); final long duration = mDuration; float normalizedTime; if (duration != 0) { normalizedTime = ((float) (currentTime - (mStartTime + startOffset))) / (float) duration; } else { // time is a step-change with a zero duration normalizedTime = currentTime < mStartTime ? 0.0f : 1.0f; }final boolean expired = normalizedTime >= 1.0f || isCanceled(); mMore = !expired; if (!mFillEnabled) normalizedTime = Math.max(Math.min(normalizedTime, 1.0f), 0.0f); if ((normalizedTime >= 0.0f || mFillBefore) && (normalizedTime <= 1.0f || mFillAfter)) { if (!mStarted) { fireAnimationStart(); mStarted = true; if (NoImagePreloadHolder.USE_CLOSEGUARD) { guard.open("cancel or detach or getTransformation"); } }if (mFillEnabled) normalizedTime = Math.max(Math.min(normalizedTime, 1.0f), 0.0f); if (mCycleFlip) { normalizedTime = 1.0f - normalizedTime; }final float interpolatedTime = mInterpolator.getInterpolation(normalizedTime); applyTransformation(interpolatedTime, outTransformation); }if (expired) { if (mRepeatCount == mRepeated || isCanceled()) { if (!mEnded) { mEnded = true; guard.close(); fireAnimationEnd(); } } else { if (mRepeatCount > 0) { mRepeated++; }if (mRepeatMode == REVERSE) { mCycleFlip = !mCycleFlip; }mStartTime = -1; mMore = true; fireAnimationRepeat(); } }if (!mMore && mOneMoreTime) { mOneMoreTime = false; return true; }return mMore; }protected void applyTransformation(float interpolatedTime, Transformation t) { }

每次调用getTransformation会计算一个normalizedTime，这个normalizedTime会作为interpolator的input传到interpolator中：

final float interpolatedTime = mInterpolator.getInterpolation(normalizedTime);

得到一个经过inpterpolator计算过的fraction（interpolatedTime）。之后以这个interpolatedTime为参数回调applyTransformation：

applyTransformation(interpolatedTime, outTransformation);

applyTransformation的另一参数，outTransformation是一个Transformation对象，其中的两个成员变量如下：

protected Matrix mMatrix; protected float mAlpha;

也就是说通过这个outTransformation，可以对它的alpha或者matrix进行运算，然后Android会读取经过Animation运算的outTransformation里的这两个变量然后作用于要实现动画效果的组件View/ViewGroup，Actvity，Fragment上实现动画效果。至于如何实现这个过程，下面会有分析。
通过上面的分析得知，Animation的运行依赖Android本身的机制回调（每帧都得回调getTransformation和applyTransformation）无法自身进行运算计算fraction，并且可参与运算的只有Transformation对象里的alpha和matrix，所以Animation只能实现简单的Alpha，Scale，Translate，Rotate变换效果。
3、PropertyAnimator Android提供了3种，分别是：

ObjectAnimator TimeAnimator ValueAnimator

上面分析的Animation受限于Android本身的回调，只能实现Alpha，Scale，Translate，Rotate的变换。而Animator没有此限制，它不依赖于Android本身的机制回调，但是它意依赖于Looper的Thread，上源码：

private void start(boolean playBackwards) { if (Looper.myLooper() == null) { throw new AndroidRuntimeException("Animators may only be run on Looper threads"); } mReversing = playBackwards; // Special case: reversing from seek-to-0 should act as if not seeked at all. if (playBackwards && mSeekFraction != -1 && mSeekFraction != 0) { if (mRepeatCount == INFINITE) { // Calculate the fraction of the current iteration. float fraction = (float) (mSeekFraction - Math.floor(mSeekFraction)); mSeekFraction = 1 - fraction; } else { mSeekFraction = 1 + mRepeatCount - mSeekFraction; } } mStarted = true; mPaused = false; mRunning = false; mAnimationEndRequested = 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; 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. 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); } } }

从start开始分析，从代码可知只能在Looper Thread上开启Animator，一看到Looper我们就瞬间恍然大悟，原来Animator的实现也离不卡Handler机制。

AnimationHandler animationHandler = AnimationHandler.getInstance(); animationHandler.addAnimationFrameCallback(this, (long) (mStartDelay * sDurationScale));

Animator获得一个AnimationHandler实例，并把自身作为回调传给这个AnimationHandler：

public class ValueAnimator extends Animator implements AnimationHandler.AnimationFrameCallback { ... } public class AnimationHandler { .../** * Callbacks that receives notifications for animation timing and frame commit timing. */ interface AnimationFrameCallback { /** * Run animation based on the frame time. * @param frameTime The frame start time, in the {@link SystemClock#uptimeMillis()} time *base. */ void doAnimationFrame(long frameTime); /** * This notifies the callback of frame commit time. Frame commit time is the time after * traversals happen, as opposed to the normal animation frame time that is before * traversals. This is used to compensate expensive traversals that happen as the * animation starts. When traversals take a long time to complete, the rendering of the * initial frame will be delayed (by a long time). But since the startTime of the * animation is set before the traversal, by the time of next frame, a lot of time would * have passed since startTime was set, the animation will consequently skip a few frames * to respect the new frameTime. By having the commit time, we can adjust the start time to * when the first frame was drawn (after any expensive traversals) so that no frames * will be skipped. * * @param frameTime The frame time after traversals happen, if any, in the *{@link SystemClock#uptimeMillis()} time base. */ void commitAnimationFrame(long frameTime); } }

但是我们发现。。。特么的这个AnimationHandler根本就不是一个Handler。
重新回到Animator的执行流程上。。。
在start函数里我们看到有一个：

animationHandler.addAnimationFrameCallback(this, (long) (mStartDelay * sDurationScale));

跟进去：

/** * 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)); } }

发现有这样一句话：

getProvider().postFrameCallback(mFrameCallback);

Provider又是什么呢？

private AnimationFrameCallbackProvider getProvider() { if (mProvider == null) { mProvider = new MyFrameCallbackProvider(); } return mProvider; }

MyFrameCallbackProvider又是什么呢？

/** * 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); } }

Choreographer又是什么呢？

public final class Choreographer { ... /** * Posts a frame callback to run on the next frame. * * The callback runs once then is automatically removed. * * * @param callback The frame callback to run during the next frame. * * @see #postFrameCallbackDelayed * @see #removeFrameCallback */ public void postFrameCallback(FrameCallback callback) { postFrameCallbackDelayed(callback, 0); } public void postFrameCallbackDelayed(FrameCallback callback, long delayMillis) { if (callback == null) { throw new IllegalArgumentException("callback must not be null"); }postCallbackDelayedInternal(CALLBACK_ANIMATION, callback, FRAME_CALLBACK_TOKEN, delayMillis); } private void postCallbackDelayedInternal(int callbackType, Object action, Object token, long delayMillis) { if (DEBUG_FRAMES) { Log.d(TAG, "PostCallback: type=" + callbackType + ", action=" + action + ", token=" + token + ", delayMillis=" + delayMillis); }synchronized (mLock) { final long now = SystemClock.uptimeMillis(); final long dueTime = now + delayMillis; mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token); if (dueTime <= now) { scheduleFrameLocked(now); } else { Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action); msg.arg1 = callbackType; msg.setAsynchronous(true); mHandler.sendMessageAtTime(msg, dueTime); } } }

看到这里。。。终于现出原形了出现了一个mHandler，mHandler又是什么呢？

public final class Choreographer { private final FrameHandler mHandler; ... } 它是Choreographer的一个成员变量，从命名上看似乎是一个与Frame（帧）相关的Handler： Java private final class FrameHandler extends Handler { public FrameHandler(Looper looper) { super(looper); }@Override public void handleMessage(Message msg) { switch (msg.what) { case MSG_DO_FRAME: doFrame(System.nanoTime(), 0); break; case MSG_DO_SCHEDULE_VSYNC: doScheduleVsync(); break; case MSG_DO_SCHEDULE_CALLBACK: doScheduleCallback(msg.arg1); break; } } }

但是还有一点：

getProvider().postFrameCallback(mFrameCallback);

这个mFrameCallback对应的类是：

private final Choreographer.FrameCallback mFrameCallback = new Choreographer.FrameCallback() { @Override public void doFrame(long frameTimeNanos) { doAnimationFrame(getProvider().getFrameTime()); if (mAnimationCallbacks.size() > 0) { getProvider().postFrameCallback(this); } } };

这又是一系列复杂的callback，分析明白了也写不明白，但FrameHandler的doFrame最终会调用我们的mFrameCallback，也就是会调用到doAnimationFrame，最终会调用到Animator的animateValue方法。再之后大家都知道了。。。会调用AnimatorUpdateListener的onAnimationUpdate。一顿分析，大致过程明白了，但是这个机制也太复杂了，牵扯了太多的方面，太多的类。什么时候自己能有这样的设计能力啊。。
关于Choreographer这个类：
*https://www.cnblogs.com/kross/p/4087780.html
这篇文件有比较详细的分析，暂时还不能理解那么多。
其它的关于Chroeographer源码注释是这样写的：

/** * Coordinates the timing of animations, input and drawing. * * The choreographer receives timing pulses (such as vertical synchronization) * from the display subsystem then schedules work to occur as part of rendering * the next display frame. * * Applications typically interact with the choreographer indirectly using * higher level abstractions in the animation framework or the view hierarchy. * Here are some examples of things you can do using the higher-level APIs. * * * To post an animation to be processed on a regular time basis synchronized with * display frame rendering, use {@link android.animation.ValueAnimator#start}. * To post a {@link Runnable} to be invoked once at the beginning of the next display * frame, use {@link View#postOnAnimation}. * To post a {@link Runnable} to be invoked once at the beginning of the next display * frame after a delay, use {@link View#postOnAnimationDelayed}. * To post a call to {@link View#invalidate()} to occur once at the beginning of the * next display frame, use {@link View#postInvalidateOnAnimation()} or * {@link View#postInvalidateOnAnimation(int, int, int, int)}. * To ensure that the contents of a {@link View} scroll smoothly and are drawn in * sync with display frame rendering, do nothing.This already happens automatically. * {@link View#onDraw} will be called at the appropriate time. * * * However, there are a few cases where you might want to use the functions of the * choreographer directly in your application.Here are some examples. * * * If your application does its rendering in a different thread, possibly using GL, * or does not use the animation framework or view hierarchy at all * and you want to ensure that it is appropriately synchronized with the display, then use * {@link Choreographer#postFrameCallback}. * ... and that's about it. * * * Each {@link Looper} thread has its own choreographer.Other threads can * post callbacks to run on the choreographer but they will run on the {@link Looper} * to which the choreographer belongs. * */

有一句话引起了我的注意：
If your application does its rendering in a different thread, possibly using GL,

or does not use the animation framework or view hierarchy at all
也就是说，通常我们的View只能在创建它的线程内进行事件处理，动画，或者绘制的原因在于，这些框架和机制的实现都依赖于这个类，这个类是线程相关。若要在其他线程内渲染可以直接使用Choreographer#postFrameCallback？？？
有待探究。。

文章转自 https://www.jianshu.com/p/435... ，如有侵权，请联系删除。
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