はじめに: 現実的であることによってのみ、私たちは多くのことを積み上げることができます。未来は、夢に向かって努力する人だけが持っています。
前回の記事では ValueAnimator のほとんどの機能の使い方を紹介しましたが、それでも簡単な使い方を今回はアクセラレータ、アニメーター、キーフレームについての知識を見ていきます。
ValueAnimator animator = ValueAnimator.ofInt(0,600);animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override public void onAnimationUpdate(ValueAnimator animation) { int curValue = (int)animation.getAnimatedValue(); tv.layout(tv.getLeft(),curValue,tv.getRight(),curValue+tv.getHeight()); }});animator.setDuration(1000);animator.setInterpolator(new BounceInterpolator());animator.start();public class LinearInterpolator implements Interpolator { public LinearInterpolator() { } public LinearInterpolator(Context context, AttributeSet attrs) { } public float getInterpolation(float input) { return input; }}public interface Interpolator extends TimeInterpolator {}/** * A time interpolator defines the rate of change of an animation. This allows animations * to have non-linear motion, such as acceleration and deceleration. */public interface TimeInterpolator { /** * Maps a value representing the elapsed fraction of an animation to a value that represents * the interpolated fraction. This interpolated value is then multiplied by the change in * value of an animation to derive the animated value at the current elapsed animation time. * * @param input A value between 0 and 1.0 indicating our current point * in the animation where 0 represents the start and 1.0 represents * the end * @return The interpolation value. This value can be more than 1.0 for * interpolators which overshoot their targets, or less than 0 for * interpolators that undershoot their targets. */ float getInterpolation(float input);}ValueAnimator anim = ValueAnimator.ofInt(100, 400); anim.setDuration(1000); anim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override public void onAnimationUpdate(ValueAnimator animation) { float currentValue = (float) animation.getAnimatedValue(); Log.d("TAG", "cuurent value is " + currentValue); } }); anim.start(); public class LinearInterpolator implements Interpolator { ………… public float getInterpolation(float input) { return input; }}ValueAnimator animator = ValueAnimator.ofInt(0,600);animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override public void onAnimationUpdate(ValueAnimator animation) { int curValue = (int)animation.getAnimatedValue(); tv.layout(tv.getLeft(),curValue,tv.getRight(),curValue+tv.getHeight()); }});animator.setDuration(1000);animator.setInterpolator(new MyInterploator());animator.start();从效果图中可见，我们将数值进度倒序返回——即随着动画进度的推进，动画的数值进度从结束位置进行到起始位置；
到这里，想必大家应该已经理解了getInterpolation(float input)函数中input参数与返回值的关系，在重写插值器时，需要强有力的数学知识做基础，一般而言，都是通过数学公式来计算插值器的变化趋势的，大家可以再分析分析其它几个插值器的写法；可以把它他们总结成公式，放到公式画图软件里，看看对应的数学图在(0,1)之间的走向，这个走向就是插值器在数值变化时的样子。
源码在文章底部给出
这幅图讲述了从定义动画的数字区间到通过AnimatorUpdateListener中得到当前动画所对应数值的整个过程。下面我们对这四个步骤具体讲解一下：
(1)、ofInt(0,400)表示指定动画的数字区间，是从0运动到400；
(2)、加速器：上面我们讲了，在动画开始后，通过加速器会返回当前动画进度所对应的数字进度，但这个数字进度是百分制的，以小数表示，如0.2
(3)、Evaluator:我们知道我们通过监听器拿到的是当前动画所对应的具体数值，而不是百分制的进度。那么就必须有一个地方会根据当前的数字进度，将其转化为对应的数值，这个地方就是Evaluator；Evaluator就是将从加速器返回的数字进度转成对应的数字值。所以上部分中，我们讲到的公式：
当前的值 = 100 + （400 - 100）* 显示进度
ValueAnimator animator = ValueAnimator.ofInt(0,600);animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override public void onAnimationUpdate(ValueAnimator animation) { int curValue = (int)animation.getAnimatedValue(); tv.layout(tv.getLeft(),curValue,tv.getRight(),curValue+tv.getHeight()); }});animator.setDuration(1000);animator.setEvaluator(new IntEvaluator());animator.setInterpolator(new BounceInterpolator());animator.start();/** * This evaluator can be used to perform type interpolation between <code>int</code> values. */public class IntEvaluator implements TypeEvaluator<Integer> { /** * This function returns the result of linearly interpolating the start and end values, with * <code>fraction</code> representing the proportion between the start and end values. The * calculation is a simple parametric calculation: <code>result = x0 + t * (v1 - v0)</code>, * where <code>x0</code> is <code>startValue</code>, <code>x1</code> is <code>endValue</code>, * and <code>t</code> is <code>fraction</code>. * * @param fraction The fraction from the starting to the ending values * @param startValue The start value; should be of type <code>int</code> or * <code>Integer</code> * @param endValue The end value; should be of type <code>int</code> or <code>Integer</code> * @return A linear interpolation between the start and end values, given the * <code>fraction</code> parameter. */ public Integer evaluate(float fraction, Integer startValue, Integer endValue) { int startInt = startValue; return (int)(startInt + fraction * (endValue - startInt)); }}return (int)(startInt + fraction * (endValue - startInt));
当前的值 = 100 + （400 - 100）* 显示进度
public class MyEvaluator implements TypeEvaluator<Integer> { @Override public Integer evaluate(float fraction, Integer startValue, Integer endValue) { return null; }}public class MyEvaluator implements TypeEvaluator<Integer> { @Override public Integer evaluate(float fraction, Integer startValue, Integer endValue) { int startInt = startValue; return (int)(200+startInt + fraction * (endValue - startInt)); }}ValueAnimator animator = ValueAnimator.ofInt(0,400);animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override public void onAnimationUpdate(ValueAnimator animation) { int curValue = (int)animation.getAnimatedValue(); tv.layout(tv.getLeft(),curValue,tv.getRight(),curValue+tv.getHeight()); }});animator.setDuration(1000);animator.setEvaluator(new MyEvaluator());animator.start();然后再看看我们设置了MyEvaluator以后的效果：
很明显，textview的动画位置都向下移动了200个点；
再重新看一下下面的这个流程图：
在加速器中，我们可以通过自定义加速器的返回的数值进度来改变返回数值的位置。比如上面我们实现的倒序动画
在Evaluator中，我们又可以通过改变进度值所对应的具体数字来改变数值的位置。
所以，结论来了：
我们可以通过重写加速器改变数值进度来改变数值位置，也可以通过改变Evaluator中进度所对应的数值来改变数值位置。
源码在文章底部给出
下面我们就只通过重写Evaluator来实现数值的倒序输出；public class ReverseEvaluator implements TypeEvaluator<Integer> { @Override public Integer evaluate(float fraction, Integer startValue, Integer endValue) { int startInt = startValue; return (int) (endValue - fraction * (endValue - startInt)); }}ValueAnimator animator = ValueAnimator.ofInt(0,400);animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override public void onAnimationUpdate(ValueAnimator animation) { int curValue = (int)animation.getAnimatedValue(); tv.layout(tv.getLeft(),curValue,tv.getRight(),curValue+tv.getHeight()); }});animator.setDuration(1000);animator.setEvaluator(new ReverseEvaluator());animator.start();源码在文章底部给出
public class ArgbEvaluator implements TypeEvaluator { public Object evaluate(float fraction, Object startValue, Object endValue) { int startInt = (Integer) startValue; int startA = (startInt >> 24); int startR = (startInt >> 16) & 0xff; int startG = (startInt >> 8) & 0xff; int startB = startInt & 0xff; int endInt = (Integer) endValue; int endA = (endInt >> 24); int endR = (endInt >> 16) & 0xff; int endG = (endInt >> 8) & 0xff; int endB = endInt & 0xff; return (int)((startA + (int)(fraction * (endA - startA))) << 24) | (int)((startR + (int)(fraction * (endR - startR))) << 16) | (int)((startG + (int)(fraction * (endG - startG))) << 8) | (int)((startB + (int)(fraction * (endB - startB)))); }}ValueAnimator animator = ValueAnimator.ofInt(0xffffff00,0xff0000ff);animator.setEvaluator(new ArgbEvaluator());animator.setDuration(3000);animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { @Override public void onAnimationUpdate(ValueAnimator animation) { int curValue = (int)animation.getAnimatedValue(); tv.setBackgroundColor(curValue); }});animator.start();源码在文章底部给出
到这里，我们就已经知道ArgbEvalutor的使用方法和效果了，下面我们再来回头看看ArgbEvalutor的实现方法/** * This evaluator can be used to perform type interpolation between integer * values that represent ARGB colors. */public class ArgbEvaluator implements TypeEvaluator { /** * This function returns the calculated in-between value for a color * given integers that represent the start and end values in the four * bytes of the 32-bit int. Each channel is separately linearly interpolated * and the resulting calculated values are recombined into the return value. * * @param fraction The fraction from the starting to the ending values * @param startValue A 32-bit int value representing colors in the * separate bytes of the parameter * @param endValue A 32-bit int value representing colors in the * separate bytes of the parameter * @return A value that is calculated to be the linearly interpolated * result, derived by separating the start and end values into separate * color channels and interpolating each one separately, recombining the * resulting values in the same way. */ public Object evaluate(float fraction, Object startValue, Object endValue) { int startInt = (Integer) startValue; int startA = (startInt >> 24); int startR = (startInt >> 16) & 0xff; int startG = (startInt >> 8) & 0xff; int startB = startInt & 0xff; int endInt = (Integer) endValue; int endA = (endInt >> 24); int endR = (endInt >> 16) & 0xff; int endG = (endInt >> 8) & 0xff; int endB = endInt & 0xff; return (int)((startA + (int)(fraction * (endA - startA))) << 24) | (int)((startR + (int)(fraction * (endR - startR))) << 16) | (int)((startG + (int)(fraction * (endG - startG))) << 8) | (int)((startB + (int)(fraction * (endB - startB)))); }}int startInt = (Integer) startValue;int startA = (startInt >> 24);int startR = (startInt >> 16) & 0xff;int startG = (startInt >> 8) & 0xff;int startB = startInt & 0xff;
所以我们的初始值是0xffffff00,那么求出来的startA = 0xff,startR = oxff,startG = 0xff,startB = 0x00;
关于通过位移和与运算如何得到指定位的值的问题，我就不再讲了，大家如果不理解，可以搜一下相关运算符使用方法的文章。
同样，我们看看第二部分根据endValue求出其中A,R,G,B中各个色彩的结束值：
int endInt = (Integer) endValue;int endA = (endInt >> 24);int endR = (endInt >> 16) & 0xff;int endG = (endInt >> 8) & 0xff;int endB = endInt & 0xff;
startA + (int)(fraction * (endA - startA)))
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