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 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.
* <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.
* <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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