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`<animation-list>`，根结点内包含多个`<item>`元素，例如:

```
<animation-list xmlns:android="http://schemas.android.com/apk/res/android"
 android:oneshot=["true" | "false"] >
 <item
 android:drawable="@drawable/frame1"
 android:duration="250" />
 <item
 android:drawable="@drawable/frame2"
 android:duration="250" />
 <item
 android:drawable="@drawable/frame3"
 android:duration="250" />
 <item
 android:drawable="@drawable/frame4"
 android:duration="250" />
</animation-list>
```

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:

```
<?xml version="1.0" encoding="utf-8"?>
<set xmlns:android="http://schemas.android.com/apk/res/android"
 android:interpolator="@[package:]anim/interpolator_resource"
 android:shareInterpolator=["true" | "false"] >
 <alpha
 android:fromAlpha="float"
 android:toAlpha="float" />
 <scale
 android:fromXScale="float"
 android:toXScale="float"
 android:fromYScale="float"
 android:toYScale="float"
 android:pivotX="float"
 android:pivotY="float" />
 <translate
 android:fromXDelta="float"
 android:toXDelta="float"
 android:fromYDelta="float"
 android:toYDelta="float" />
 <rotate
 android:fromDegrees="float"
 android:toDegrees="float"
 android:pivotX="float"
 android:pivotY="float" />
 <set>
 ...
 </set>
</set>
```

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](https://link.jianshu.com?t=https%3A%2F%2Fwww.cnblogs.com%2Fkross%2Fp%2F4087780.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.
 * 
 * <ul>
 * <li>To post an animation to be processed on a regular time basis synchronized with
 * display frame rendering, use {@link android.animation.ValueAnimator#start}.</li>
 * <li>To post a {@link Runnable} to be invoked once at the beginning of the next display
 * frame, use {@link View#postOnAnimation}.</li>
 * <li>To post a {@link Runnable} to be invoked once at the beginning of the next display
 * frame after a delay, use {@link View#postOnAnimationDelayed}.</li>
 * <li>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)}.</li>
 * <li>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.</li>
 * </ul>
 * 
 * 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.
 * 
 * <ul>
 * <li>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}.</li>
 * <li>... and that's about it.</li>
 * </ul>
 * 
 * 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/435e9645bc4c ，如有侵权，请联系删除。

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