前言
本文为Android动画系列的最后一篇文章,通过对源码的分析,能够让大家更深刻地理解属性动画的工作原理,这有助于我们更好地使用属性动画。但是,由于动画的底层实现已经深入到jni层,并且涉及到显示子系统,因此,深入地分析动画的底层实现不仅比较困难而且意义不大,因此,本文的分析到jni层为止。
Android动画系列:
android动画简介
Android动画进阶—使用开源动画库nineoldandroids
Android属性动画深入分析:让你成为动画牛人
Android源码分析—属性动画的工作原理
属性动画的原理
属性动画要求动画作用的对象提供该属性的set方法,属性动画根据你传递的该熟悉的初始值和最终值,以动画的效果多次去调用set方法,每次传递给set方法的值都不一样,确切来说是随着时间的推移,所传递的值越来越接近最终值。如果动画的时候没有传递初始值,那么还要提供get方法,因为系统要去拿属性的初始值。对于属性动画来说,其动画过程中所做的就是这么多,下面看源码分析。
源码分析
首先我们要找一个入口,就从ObjectAnimator.ofInt(mButton, width, 500).setDuration(5000).start()开始吧,其他动画都是类似的。
看ObjectAnimator的start方法
说明:上面的代码别看那么长,其实做的事情很简单,首先会判断一下,如果当前动画、等待的动画(Pending)和延迟的动画(Delay)中有和当前动画相同的动画,那么就把相同的动画给取消掉,接下来那一段是log,再接着就调用了父类的super.start()方法,因为ObjectAnimator继承了ValueAnimator,所以接下来我们看一下ValueAnimator的Start方法@Override public void start() {
// See if any of the current active/pending animators need to be canceled AnimationHandler handler = sAnimationHandler.get();
if (handler != null) {
int numAnims = handler.mAnimations.size();
for (int i = numAnims - 1;
i >= 0;
i--) {
if (handler.mAnimations.get(i) instanceof ObjectAnimator) {
ObjectAnimator anim = (ObjectAnimator) handler.mAnimations.get(i);
if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
anim.cancel();
}
}
}
numAnims = handler.mPendingAnimations.size();
for (int i = numAnims - 1;
i >= 0;
i--) {
if (handler.mPendingAnimations.get(i) instanceof ObjectAnimator) {
ObjectAnimator anim = (ObjectAnimator) handler.mPendingAnimations.get(i);
if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
anim.cancel();
}
}
}
numAnims = handler.mDelayedAnims.size();
for (int i = numAnims - 1;
i >= 0;
i--) {
if (handler.mDelayedAnims.get(i) instanceof ObjectAnimator) {
ObjectAnimator anim = (ObjectAnimator) handler.mDelayedAnims.get(i);
if (anim.mAutoCancel && hasSameTargetAndProperties(anim)) {
anim.cancel();
}
}
}
}
if (DBG) {
Log.d(ObjectAnimator, Anim target, duration: + mTarget + , + getDuration());
for (int i = 0;
i < mValues.length;
++i) {
PropertyValuesHolder pvh = mValues[i];
ArrayListkeyframes = pvh.mKeyframeSet.mKeyframes;
Log.d(ObjectAnimator, Values[ + i + ]: + pvh.getPropertyName() + , + keyframes.get(0).getValue() + , + keyframes.get(pvh.mKeyframeSet.mNumKeyframes - 1).getValue());
}
}
super.start();
}
说明:上述代码最终会调用AnimationHandler的start方法,这个AnimationHandler并不是Handler,它是个Runnable。看下它的代码,通过代码我们发现,很快就调到了jni层,不过jni层最终还是要调回来的。它的run方法会被调用,这个Runnable涉及到和底层的交互,我们就忽略这部分,直接看重点:ValueAnimator中的doAnimationFrame方法private void start(boolean playBackwards) {
if (Looper.myLooper() == null) {
throw new AndroidRuntimeException(Animators may only be run on Looper threads);
}
mPlayingBackwards = playBackwards;
mCurrentIteration = 0;
mPlayingState = STOPPED;
mStarted = true;
mStartedDelay = false;
mPaused = false;
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();
}
注意到上述代码末尾调用了animationFrame方法,而animationFrame内部调用了animateValue,下面看animateValue的代码final boolean doAnimationFrame(long frameTime) {
if (mPlayingState == STOPPED) {
mPlayingState = RUNNING;
if (mSeekTime < 0) {
mStartTime = frameTime;
}
else {
mStartTime = frameTime - mSeekTime;
// Now that we'
re playing, reset the seek time mSeekTime = -1;
}
}
if (mPaused) {
if (mPauseTime < 0) {
mPauseTime = frameTime;
}
return false;
}
else if (mResumed) {
mResumed = false;
if (mPauseTime > 0) {
// Offset by the duration that the animation was paused mStartTime += (frameTime - mPauseTime);
}
}
// The frame time might be before the start time during the first frame of // an animation. The current time must always be on or after the start // time to avoid animating frames at negative time intervals. In practice, this // is very rare and only happens when seeking backwards. final long currentTime = Math.max(frameTime, mStartTime);
return animationFrame(currentTime);
}
上述代码中的calculateValue方法就是计算每帧动画所对应的属性的值,下面着重看一下到底是在哪里调用属性的get和set方法的,毕竟这个才是我们最关心的。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);
}
}
}
get方法:在初始化的时候,如果属性的初始值没有提供,则get方法将会被调用。
private void setupValue(Object target, Keyframe kf) {
if (mProperty != null) {
kf.setValue(mProperty.get(target));
}
try {
if (mGetter == null) {
Class targetClass = target.getClass();
setupGetter(targetClass);
if (mGetter == null) {
// Already logged the error - just return to avoid NPE return;
}
}
kf.setValue(mGetter.invoke(target));
}
catch (InvocationTargetException e) {
Log.e(PropertyValuesHolder, e.toString());
}
catch (IllegalAccessException e) {
Log.e(PropertyValuesHolder, e.toString());
}
}
set方法:当动画的下一帧到来的时候,PropertyValuesHolder中的setAnimatedValue方法会将新的属性值设置给对象,调用其set方法
void setAnimatedValue(Object target) {
if (mProperty != null) {
mProperty.set(target, getAnimatedValue());
}
if (mSetter != null) {
try {
mTmpValueArray[0] = getAnimatedValue();
mSetter.invoke(target, mTmpValueArray);
}
catch (InvocationTargetException e) {
Log.e(PropertyValuesHolder, e.toString());
}
catch (IllegalAccessException e) {
Log.e(PropertyValuesHolder, e.toString());
}
}
}
总结
我觉得这篇源码分析写的逻辑有点混乱,希望不要给大家带来误导。从源码上来说,属性动画的源码逻辑层次有点跳跃,不过没关系,大家只要了解属性动画的工作原理就好,源码的作用在于让我们发现其工作原理的确如此。到此为止,Android动画系列已经全部完成,十分感谢大家阅读,希望能给大家带来一点帮助!