Android 动画可作用于 View/ViewGroup，Actvity，Fragment 实现炫酷的交互成果。通过几天的探索，搞清楚了各类动画的应用和动画的实现原理，在此记录以下。
只管 Android 动画有好几种类别，然而各种动画的实现外围都是 TimeInterpolator->Interpolator-> 各种 Interpolator。大抵过程是通过 Interpolator 计算出工夫相干的 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（工夫相干），将这些参数应用到动画成果上，Android 会通过肯定的机制一直反复这个过程 (16ms 为周期) 就形成了咱们看到的动画。

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 能够通过 xml 来实现，也可利用代码来实现：
xml 实现时根结点必须是 <animation-list>，根结点内蕴含多个 <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，就是一直的更换显示的 Drawable 来实现动态效果。

2、TweenAnimation

能够对 View 进行一系列的变换，如平移，翻转，缩放，淡入淡出，也能够将他们组合起来造成混合的动画成果。TweenAnimation 的实现形式也有两种，别离能够用代码和 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 动画能够用 Android SDK 提供的工具类：

AnimationUtils.loadAnimations();

至于代码实现形式：

// 提供了以下几种 Animation
AlphaAnimation TranslateAnimation ScaleAnimation RotateAnimation
AnimationSet.addAnimation(Animation)
// 应用办法根本与定义 xml 统一

Animation 的运作依赖两个办法：

 /**
     * 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);

失去一个通过 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，能够对它的 alpha 或者 matrix 进行运算，而后 Android 会读取通过 Animation 运算的 outTransformation 里的这两个变量而后作用于要实现动画成果的组件 View/ViewGroup，Actvity，Fragment 上实现动画成果。至于如何实现这个过程，上面会有剖析。
通过下面的剖析得悉，Animation 的运行依赖 Android 自身的机制回调（每帧都得回调 getTransformation 和 applyTransformation）无奈本身进行运算计算 fraction，并且可参加运算的只有 Transformation 对象里的 alpha 和 matrix，所以 Animation 只能实现简略的 Alpha，Scale，Translate，Rotate 变换成果。

3、PropertyAnimator

Android 提供了 3 种，别离是：

ObjectAnimator
TimeAnimator
ValueAnimator

下面剖析的 Animation 受限于 Android 自身的回调，只能实现 Alpha，Scale，Translate，Rotate 的变换。而 Animator 没有此限度，它不依赖于 Android 自身的机制回调，然而它意依赖于 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 开始剖析，从代码可知只能在 Looper Thread 上开启 Animator，一看到 Looper 咱们就霎时豁然开朗，原来 Animator 的实现也离不卡 Handler 机制。

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

Animator 取得一个 AnimationHandler 实例，并把本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);
    }
}

然而咱们发现。。。特么的这个 AnimationHandler 基本就不是一个 Handler。
从新回到 Animator 的执行流程上。。。
在 start 函数里咱们看到有一个：

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.
     * <p>
     * The callback runs once then is automatically removed.
     * </p>
     *
     * @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。。。终于现出原形了呈现了一个 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，从命名上看仿佛是一个与 Frame（帧）相干的 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;
            }
        }
    }

然而还有一点：

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);
            }
        }
    };

这又是一系列简单的 callback，剖析明确了也写不明确，但 FrameHandler 的 doFrame 最终会调用咱们的 mFrameCallback，也就是会调用到 doAnimationFrame，最终会调用到 Animator 的 animateValue 办法。再之后大家都晓得了。。。会调用 AnimatorUpdateListener 的 onAnimationUpdate。一顿剖析，大抵过程明确了，然而这个机制也太简单了，牵扯了太多的方面，太多的类。什么时候本人能有这样的设计能力啊。。
对于 Choreographer 这个类：
*https://www.cnblogs.com/kross/p/4087780.html
这篇文件有比拟具体的剖析，临时还不能了解那么多。

其它的

对于 Chroeographer 源码正文是这样写的：

/**
 * Coordinates the timing of animations, input and drawing.
 * <p>
 * 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.
 * </p><p>
 * 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.
 * </p>
 * <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>
 * <p>
 * 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.
 * </p>
 * <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>
 * <p>
 * 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.
 * </p>
 */

有一句话引起了我的留神：
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，通常咱们的 View 只能在创立它的线程内进行事件处理，动画，或者绘制的起因在于，这些框架和机制的实现都依赖于这个类，这个类是线程相干。若要在其余线程内渲染能够间接应用 Choreographer#postFrameCallback？？？
  有待探索。。

文章转自 https://www.jianshu.com/p/435…，如有侵权，请分割删除。

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