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Android 动画animation 深入分析

前言：本文试图通过分析动画流程，来理解android动画系统的设计与实现，学习动画的基本原则，最终希望能够指导动画的设计。

0 本文中用到的一些类图

1 view animation 

调用方法：view.startAnimation(animation);

    public void startAnimation(Animation animation) {
        animation.setStartTime(Animation.START_ON_FIRST_FRAME);
        setAnimation(animation);
        invalidateParentCaches();
        invalidate(true);
    }

在invalidate(ture);中

            if (p != null && ai != null) {
                final Rect r = ai.mTmpInvalRect;
                r.set(0, 0, mRight - mLeft, mBottom - mTop);
                // Don‘t call invalidate -- we don‘t want to internally scroll
                // our own bounds
                p.invalidateChild(this, r);
            }

即调用parent的invalidateChild，

假定父控件即为ViewRootImpl;

public final class ViewRootImpl implements ViewParent;

    @Override
    public void invalidateChild(View child, Rect dirty) {
        invalidateChildInParent(null, dirty);
    }

    public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
        //...省略一堆判断条件，最终调用
        if (!mWillDrawSoon && (intersected || mIsAnimating)) {
            scheduleTraversals();
        }

        return null;
    }

    void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().postSyncBarrier();
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
            scheduleConsumeBatchedInput();
        }
    }

其中mTraversalBarrier = mHandler.getLooper().postSyncBarrier();是设置同步障碍（syncBarrier），当looper中的消息队列执行到barrier 后，会暂停执行，只有当barrier 被释放mHandler.getLooper().removeSyncBarrier(mTraversalBarrier); 后消息队列才能继续执行。

    Choreographer mChoreographer; 是动画系统中的核心组织者， 负责统一调度。后面详细说。

    final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
    final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();
        }
    }

    void doTraversal() {
        performTraversals();
    }

perform 待补充

    final class ConsumeBatchedInputRunnable implements Runnable {
        @Override
        public void run() {
            doConsumeBatchedInput(mChoreographer.getFrameTimeNanos());
        }
    }
    final ConsumeBatchedInputRunnable mConsumedBatchedInputRunnable =
            new ConsumeBatchedInputRunnable();

doConsume 待补充

2 属性动画aninmator

valueAnimator.start();

    private void start(boolean playBackwards) {
        if (Looper.myLooper() == null) {
            throw new AndroidRuntimeException("Animators may only be run on Looper threads");
        }
        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();
    }

这里会检查调用线程必须是Looper线程，如果是view相关的属性动画，还必须是UI 线程。

得到AnimationHandle 并把自己加入到PendingAnimations  的list中.

getOrCreateAnimationHandler();

    protected static ThreadLocal<AnimationHandler> sAnimationHandler =
            new ThreadLocal<AnimationHandler>()
    protected static class AnimationHandler implements Runnable {
        // The per-thread list of all active animations
        /** @hide */
        protected final ArrayList<ValueAnimator> mAnimations = new ArrayList<ValueAnimator>();

        // Used in doAnimationFrame() to avoid concurrent modifications of mAnimations
        private final ArrayList<ValueAnimator> mTmpAnimations = new ArrayList<ValueAnimator>();

        // The per-thread set of animations to be started on the next animation frame
        /** @hide */
        protected final ArrayList<ValueAnimator> mPendingAnimations = new ArrayList<ValueAnimator>();

        /**
         * Internal per-thread collections used to avoid set collisions as animations start and end
         * while being processed.
         * @hide
         */
        protected final ArrayList<ValueAnimator> mDelayedAnims = new ArrayList<ValueAnimator>();
        private final ArrayList<ValueAnimator> mEndingAnims = new ArrayList<ValueAnimator>();
        private final ArrayList<ValueAnimator> mReadyAnims = new ArrayList<ValueAnimator>();

        private final Choreographer mChoreographer;
        private boolean mAnimationScheduled;
}

AnimationHandler 就是一个runnable， 注意成员变量中的多个animator 的list 以及重要的mChoreographer = Choreographer.getInstance();

mChoreographer 也是一个threadlocal的变量。

在animationHandler.start() 中

        public void start() {
            scheduleAnimation();
        }
        private void scheduleAnimation() {
            if (!mAnimationScheduled) {
                mChoreographer.postCallback(Choreographer.CALLBACK_ANIMATION, this, null);
                mAnimationScheduled = true;
            }
        }

this 是runnable 即把animationHandler自己添加添加到mChoreographer 的队列中。

    public void postCallback(int callbackType, Runnable action, Object token) {
        postCallbackDelayed(callbackType, action, token, 0);
    }
    public void postCallbackDelayed(int callbackType,
            Runnable action, Object token, long delayMillis) {
        postCallbackDelayedInternal(callbackType, action, token, delayMillis);
    }
    private void postCallbackDelayedInternal(int callbackType,
            Object action, Object token, long 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);
            }
        }
    }

传入的delay为0， 即调用scheduleFrameLocked(now);

    private void scheduleFrameLocked(long now) {
        if (!mFrameScheduled) {
            mFrameScheduled = true;
            if (USE_VSYNC) {
                if (DEBUG) {
                    Log.d(TAG, "Scheduling next frame on vsync.");
                }

                // If running on the Looper thread, then schedule the vsync immediately,
                // otherwise post a message to schedule the vsync from the UI thread
                // as soon as possible.
                if (isRunningOnLooperThreadLocked()) {
                    scheduleVsyncLocked();
                } else {
                    Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
                    msg.setAsynchronous(true);
                    mHandler.sendMessageAtFrontOfQueue(msg);
                }
            } else {
                final long nextFrameTime = Math.max(
                        mLastFrameTimeNanos / NANOS_PER_MS + sFrameDelay, now);
                if (DEBUG) {
                    Log.d(TAG, "Scheduling next frame in " + (nextFrameTime - now) + " ms.");
                }
                Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
                msg.setAsynchronous(true);
                mHandler.sendMessageAtTime(msg, nextFrameTime);
            }
        }
    }

    private static final boolean USE_VSYNC = SystemProperties.getBoolean(
            "debug.choreographer.vsync", true);

USE_VSYNC 默认是true；    

    private boolean isRunningOnLooperThreadLocked() {
        return Looper.myLooper() == mLooper;
    }

检查当前looper和mChoreographer的looper是否一致。一般情况是一致的。就会调用scheduleVsyncLocked();

    private void scheduleVsyncLocked() {
        mDisplayEventReceiver.scheduleVsync();
    }

    public void scheduleVsync() {
        if (mReceiverPtr == 0) {
            Log.w(TAG, "Attempted to schedule a vertical sync pulse but the display event "
                    + "receiver has already been disposed.");
        } else {
            nativeScheduleVsync(mReceiverPtr);
        }
    }

到了native 暂时先不涉及。

回头来看animationHandler 的run()。 前面提到animationHandler把自己添加到mChoreographer，当被调用时，调用run方法。

        // Called by the Choreographer.
        @Override
        public void run() {
            mAnimationScheduled = false;
            doAnimationFrame(mChoreographer.getFrameTime());
        }

    public long getFrameTime() {
        return getFrameTimeNanos() / NANOS_PER_MS;
    }
    public long getFrameTimeNanos() {
        synchronized (mLock) {
            if (!mCallbacksRunning) {
                throw new IllegalStateException("This method must only be called as "
                        + "part of a callback while a frame is in progress.");
            }
            return USE_FRAME_TIME ? mLastFrameTimeNanos : System.nanoTime();
        }
    }

doAnimationFrame()总结就是

1.遍历pending list动画，如果delay为0 则调用start，不为0，加入delay list；

2.遍历delay list， 根据frametime计算是继续delay还是ready可以播放，若是ready，则加入到ready list中；

3 遍历ready list，调用start ；

4，遍历所有animation，根据frametime计算动画是否要结束，如果可以结束，则加入到ending list中；

5，遍历ending list， 调用end；

6， 如果有列表中仍然有动画，则继续scheduleAnimation；

        private void doAnimationFrame(long frameTime) {
            // mPendingAnimations holds any animations that have requested to be started
            // We‘re going to clear mPendingAnimations, but starting animation may
            // cause more to be added to the pending list (for example, if one animation
            // starting triggers another starting). So we loop until mPendingAnimations
            // is empty.
            while (mPendingAnimations.size() > 0) {
                ArrayList<ValueAnimator> pendingCopy =
                        (ArrayList<ValueAnimator>) mPendingAnimations.clone();
                mPendingAnimations.clear();
                int count = pendingCopy.size();
                for (int i = 0; i < count; ++i) {
                    ValueAnimator anim = pendingCopy.get(i);
                    // If the animation has a startDelay, place it on the delayed list
                    if (anim.mStartDelay == 0) {
                        anim.startAnimation(this);
                    } else {
                        mDelayedAnims.add(anim);
                    }
                }
            }
            // Next, process animations currently sitting on the delayed queue, adding
            // them to the active animations if they are ready
            int numDelayedAnims = mDelayedAnims.size();
            for (int i = 0; i < numDelayedAnims; ++i) {
                ValueAnimator anim = mDelayedAnims.get(i);
                if (anim.delayedAnimationFrame(frameTime)) {
                    mReadyAnims.add(anim);
                }
            }
            int numReadyAnims = mReadyAnims.size();
            if (numReadyAnims > 0) {
                for (int i = 0; i < numReadyAnims; ++i) {
                    ValueAnimator anim = mReadyAnims.get(i);
                    anim.startAnimation(this);
                    anim.mRunning = true;
                    mDelayedAnims.remove(anim);
                }
                mReadyAnims.clear();
            }

            // Now process all active animations. The return value from animationFrame()
            // tells the handler whether it should now be ended
            int numAnims = mAnimations.size();
            for (int i = 0; i < numAnims; ++i) {
                mTmpAnimations.add(mAnimations.get(i));
            }
            for (int i = 0; i < numAnims; ++i) {
                ValueAnimator anim = mTmpAnimations.get(i);
                if (mAnimations.contains(anim) && anim.doAnimationFrame(frameTime)) {
                    mEndingAnims.add(anim);
                }
            }
            mTmpAnimations.clear();
            if (mEndingAnims.size() > 0) {
                for (int i = 0; i < mEndingAnims.size(); ++i) {
                    mEndingAnims.get(i).endAnimation(this);
                }
                mEndingAnims.clear();
            }

            // If there are still active or delayed animations, schedule a future call to
            // onAnimate to process the next frame of the animations.
            if (!mAnimations.isEmpty() || !mDelayedAnims.isEmpty()) {
                scheduleAnimation();
            }
        }

在animationFrame() 中根据当前状态，并且计算fraction，调用animateValue();

    boolean animationFrame(long currentTime) {
        boolean done = false;
        switch (mPlayingState) {
        case RUNNING:
        case SEEKED:
            //省略计算fraction的代码
            animateValue(fraction);
            break;
        }
        return done;
    }

通过mInterpolator.getInterpolation计算fraction；@Interpolator 

根据fraction计算内部所有value，如果有updateListener，调用之。

    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);
            }
        }
    }

3. 插值器

从上面的介绍可以看到，Interpolator的关键是getInterpolation();

在ValueAnimator.animationFrame()中可以看到， 传递给Interpolator 的fraction是在[0,1] 值域范围。

            float fraction = mDuration > 0 ? (float)(currentTime - mStartTime) / mDuration : 1f;
            if (fraction >= 1f) {
                if (mCurrentIteration < mRepeatCount || mRepeatCount == INFINITE) {
                    // Time to repeat
                    if (mListeners != null) {
                        int numListeners = mListeners.size();
                        for (int i = 0; i < numListeners; ++i) {
                            mListeners.get(i).onAnimationRepeat(this);
                        }
                    }
                    if (mRepeatMode == REVERSE) {
                        mPlayingBackwards = !mPlayingBackwards;
                    }
                    mCurrentIteration += (int)fraction;
                    fraction = fraction % 1f;
                    mStartTime += mDuration;
                } else {
                    done = true;
                    fraction = Math.min(fraction, 1.0f);
                }
            }
            if (mPlayingBackwards) {
                fraction = 1f - fraction;
            }

所以设计Interpolator 就是设计一个输入[0,1] 的函数。

先参观一下系统的几个Interpolator。

3.1 AccelerateDecelerateInterpolator

cos(t+1)Pi /2 +0.5f

从图可以看到，先加速后减速，病最终到达结束位置。

public class AccelerateDecelerateInterpolator implements Interpolator {
    public float getInterpolation(float input) {
        return (float)(Math.cos((input + 1) * Math.PI) / 2.0f) + 0.5f;
    }
}

3.2 AccelerateInterpolator

如果factor=1 则函数为x^2

否则函数为x^a (a 是参数)

默认函数式x^2

如图示，逐渐加速到结束位置。

public class AccelerateInterpolator implements Interpolator {
    private final float mFactor;
    private final double mDoubleFactor;

    public AccelerateInterpolator() {
        mFactor = 1.0f;
        mDoubleFactor = 2.0;
    }
    
    /**
     * Constructor
     * 
     * @param factor Degree to which the animation should be eased. Seting
     *        factor to 1.0f produces a y=x^2 parabola. Increasing factor above
     *        1.0f  exaggerates the ease-in effect (i.e., it starts even
     *        slower and ends evens faster)
     */
    public AccelerateInterpolator(float factor) {
        mFactor = factor;
        mDoubleFactor = 2 * mFactor;
    }

     public float getInterpolation(float input) {
        if (mFactor == 1.0f) {
            return input * input;
        } else {
            return (float)Math.pow(input, mDoubleFactor);
        }
    }
}

3.3 LinearInterpolator

线性的就是Y=X 没啥说的。

public class LinearInterpolator implements Interpolator {
    public float getInterpolation(float input) {
        return input;
    }
}

3.4 anticipateInterpolator  

函数是：x^2((a+1)x-a) 默认参数a=2 默认函数为x^2(3x-1)

如图示， 会先反方向执行一段，然后正向一直加速至结束位置。

public class AnticipateInterpolator implements Interpolator {
    private final float mTension;

    public AnticipateInterpolator() {
        mTension = 2.0f;
    }

    /**
     * @param tension Amount of anticipation. When tension equals 0.0f, there is
     *                no anticipation and the interpolator becomes a simple
     *                acceleration interpolator.
     */
    public AnticipateInterpolator(float tension) {
        mTension = tension;
    }

    public float getInterpolation(float t) {
        // a(t) = t * t * ((tension + 1) * t - tension)
        return t * t * ((mTension + 1) * t - mTension);
    }
}

3.5 aniticipateOvershoot

是一个分段函数，默认参数a=3

2x*x[(2x*(a+1)-a)]     0<=x<=0.5

2(x-1)(x-1)[(2x-1)(a+1)+a]    0.5<x<=1

通过下图可以看到，动画会先反方向执行，然后向正方向逐渐加速，在快结束时逐渐减速，并超过预设的值，最后回到结束位置。

2x*x[(2x*(a+1)-a)]     0<=x<=0.5 的函数图

2(x-1)(x-1)[(2x-1)(a+1)+a]    0.5<x<=1的函数图

public class AnticipateOvershootInterpolator implements Interpolator {
    private final float mTension;

    public AnticipateOvershootInterpolator() {
        mTension = 2.0f * 1.5f;
    }

    /**
     * @param tension Amount of anticipation/overshoot. When tension equals 0.0f,
     *                there is no anticipation/overshoot and the interpolator becomes
     *                a simple acceleration/deceleration interpolator.
     */
    public AnticipateOvershootInterpolator(float tension) {
        mTension = tension * 1.5f;
    }

    /**
     * @param tension Amount of anticipation/overshoot. When tension equals 0.0f,
     *                there is no anticipation/overshoot and the interpolator becomes
     *                a simple acceleration/deceleration interpolator.
     * @param extraTension Amount by which to multiply the tension. For instance,
     *                     to get the same overshoot as an OvershootInterpolator with
     *                     a tension of 2.0f, you would use an extraTension of 1.5f.
     */
    public AnticipateOvershootInterpolator(float tension, float extraTension) {
        mTension = tension * extraTension;
    }

    private static float a(float t, float s) {
        return t * t * ((s + 1) * t - s);
    }

    private static float o(float t, float s) {
        return t * t * ((s + 1) * t + s);
    }

    public float getInterpolation(float t) {
        // a(t, s) = t * t * ((s + 1) * t - s)
        // o(t, s) = t * t * ((s + 1) * t + s)
        // f(t) = 0.5 * a(t * 2, tension * extraTension), when t < 0.5
        // f(t) = 0.5 * (o(t * 2 - 2, tension * extraTension) + 2), when t <= 1.0
        if (t < 0.5f) return 0.5f * a(t * 2.0f, mTension);
        else return 0.5f * (o(t * 2.0f - 2.0f, mTension) + 2.0f);
    }
}

4. 指导设计动画。

从第3节中可以看到，想要让动画按照我们预期的行为来执行，需要做的就是找到合适的函数。