上一篇 Android 属性动画(一) 介绍了Property Animator 的基本用法。今天继续了解Property Animator 的Evaluator、Interpolator 和 xml 文件定义Animator 的相关知识和用法。

Evaluator 求值器

直接贴源码：

public interface TypeEvaluator {    /**         * This function returns the result of linearly interpolating the start and end values, with       * fraction representing the proportion between the start and end values. The         * calculation is a simple parametric calculation: result = x0 + t * (x1 - x0),        * where x0 is startValue, x1 is endValue,         * and t is fraction.         *     * @param fraction   The fraction from the starting to the ending values         * @param startValue The start value.         * @param endValue   The end value.         * @return A linear interpolation between the start and end values, given the         *         fraction parameter.         */    public T evaluate(float fraction, T startValue, T endValue);}

源码很简短，就一个方法，默认实现就是 根据fraction 通过 result = x0 + t * (x1 - x0) 公式计算返回一个中间值。
那么 fraction 这个参数是多少呢？通过ValueAnimator 的setEvaluator() 方法找到源码:

boolean animationFrame(long currentTime) {        boolean done = false;    switch (mPlayingState) {    case RUNNING:    case SEEKED:        float fraction = mDuration > 0 ? (float)(currentTime - mStartTime) / mDuration : 1f;        if (mDuration == 0 && mRepeatCount != INFINITE) {            // Skip to the end            mCurrentIteration = mRepeatCount;            if (!mReversing) {                mPlayingBackwards = false;            }        }        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;                // Note: We do not need to update the value of mStartTimeCommitted here                // since we just added a duration offset.            } else {                done = true;                fraction = Math.min(fraction, 1.0f);            }        }        if (mPlayingBackwards) {            fraction = 1f - fraction;        }        animateValue(fraction);        break;    }    return done;}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);        }    }}

在 ValueAnimator 类中有两个方法，可以看到在 animationFrame 方法就是 得到一个已完成时间在总时长的比例值（0~1）并赋值给 fraction，在 animateValue 方法中 调用了 mInterpolator.getInterpolation() （就是 插值器，后面会介绍） 目的就是修正 fraction 。所以我们知道 fraction 是一个0~1 的值。现在看下自定义Evaluator 的代码：

//定义了一个 PointF 类型的 Evaluator ，实现 view 的抛物线变化//默认 TypeEvaluator View 在X，Y方向都是匀速的。//自定义后，View的Y方向是变速的。ValueAnimator valueAnimator = ValueAnimator.ofObject(new TypeEvaluator(){    @Override    public PointF evaluate(float fraction, PointF startValue, PointF endValue) {        // y 方向是一个变速        PointF point = new PointF();        point.x = startValue.x + fraction * (endValue.x - startValue.x) ;        point.y = (float) (startValue.y + Math.pow(fraction , 2) * (endValue.y - startValue.y));        return point;    }},new PointF(0,0),new PointF(400,1000));valueAnimator.setDuration(3000);valueAnimator.setInterpolator(new LinearInterpolator());valueAnimator.start();valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener(){         @Override            public void onAnimationUpdate(ValueAnimator animation)    {        PointF point = (PointF) animation.getAnimatedValue();        animView.setX(point.x);        animView.setY(point.y);    }});valueAnimator.start();

Interpolator 插值器

本来想写下Interpolayor的源码分析的，但看到一篇 Interpolayor的分析文章后，感觉自己也不会写的比这个好，所以还是不写了，借鉴和学习这篇文章 李海珍大神的 android动画（一）Interpolator。膜拜！！！
一下是从 大神文章中摘抄的部分，图文并茂，讲解很详细，大家学习下！

二：简单插值器分析
** 注意下面的图，对应你脑海中的插值的大小应该是斜率。**

package android.view.animation;import android.content.Context;import android.content.res.TypedArray;import android.util.AttributeSet;/** * * 一个开始很慢然后不断加速的插值器。  * 
 * An interpolator where the rate of change starts out slowly and * and then accelerates. * */public class AccelerateInterpolator implements Interpolator {     private final float mFactor; private final double mDoubleFactor;     public AccelerateInterpolator() {        mFactor = 1.0f;        mDoubleFactor = 2.0;     }     /**            * Constructor       *       * @param factor       * 动画的快慢度。将factor设置为1.0f会产生一条y=x^2的抛物线。增加factor到1.0f之后为加大这种渐入效果（也就是说它开头更加慢，结尾更加快）       * 
Degree to which the animation should be eased. Seting       * factor to 1.0f produces a y=x^2 parabola（抛物线）. 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 AccelerateInterpolator(Context context, AttributeSet attrs) {        TypedArray a =context.obtainStyledAttributes(attrs,com.android.internal.R.styleable.AccelerateInterpolator);        mFactor = a.getFloat(com.android.internal.R.styleable.AccelerateInterpolator_factor, 1.0f);        mDoubleFactor = 2 * mFactor;        a.recycle();      }      @Override       public float getInterpolation(float input) {         if (mFactor == 1.0f) {             return input * input;            } else {              return (float)Math.pow(input, mDoubleFactor);          }       }}

加速的快慢度由参数fractor决定。
当fractor值为1.0f时，动画加速轨迹相当于一条y=x^2的抛物线。如下图：

<?xml version="1.0" encoding="utf-8"?>                

// 加载动画Animator anim = AnimatorInflater.loadAnimator(this, R.animator.animator_set);//设置 缩放的中心点animView.setPivotX(0);animView.setPivotY(0);//显示的调用invalidateanimView.invalidate();anim.setTarget(animView);anim.start();