ArgbEvaluator使用

• 上次提到IntEvaluator和FloatEvaluator，除了这两种Evaluator外，android.animation包下还有另外一个Evaluator, 名字为ArgbEvaluator，它是用来实现颜色值过度转换的。

private void doAnimator(){    
    final ValueAnimator valueAnimator = ValueAnimator.ofInt(0xffffff00, 
0xff0000ff);    
    valueAnimator.setEvaluator(new ArgbEvaluator());    
    valueAnimator.setDuration(1000);    
    valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() { 
        @Override        
        public void onAnimationUpdate(ValueAnimator animation) {            
            int curValue = (int) valueAnimator.getAnimatedValue();            
            textView.setBackgroundColor(curValue);        
        }    
    });    
    valueAnimator.start();
}

• 注意：必须使用ValueEvaluator.ofInt()函数来定义颜色的取值范围，并且颜色必须包括A，R，G，B 4个值
• 写完了一个例子我们看一下原理吧：
  *

public Object evaluate(float fraction, Object startValue, Object endValue) {

//第一部分求出A, R, G, B色彩的初始值
    int startInt = (Integer) startValue;
    float startA = ((startInt >> 24) & 0xff) / 255.0f;
    float startR = ((startInt >> 16) & 0xff) / 255.0f;
    float startG = ((startInt >>  8) & 0xff) / 255.0f;
    float startB = ( startInt        & 0xff) / 255.0f;


//第二部分求出A，R，G，B的结束值
    int endInt = (Integer) endValue;
    float endA = ((endInt >> 24) & 0xff) / 255.0f;
    float endR = ((endInt >> 16) & 0xff) / 255.0f;
    float endG = ((endInt >>  8) & 0xff) / 255.0f;
    float endB = ( endInt        & 0xff) / 255.0f;



    // convert from sRGB to linear
    startR = (float) Math.pow(startR, 2.2);
    startG = (float) Math.pow(startG, 2.2);
    startB = (float) Math.pow(startB, 2.2);


    endR = (float) Math.pow(endR, 2.2);
    endG = (float) Math.pow(endG, 2.2);
    endB = (float) Math.pow(endB, 2.2);


//根据当前的进度求出对应的数值
    // compute the interpolated color in linear space
    float a = startA + fraction * (endA - startA);
    float r = startR + fraction * (endR - startR);
    float g = startG + fraction * (endG - startG);
    float b = startB + fraction * (endB - startB);


    // convert back to sRGB in the [0..255] range
    a = a * 255.0f;
    r = (float) Math.pow(r, 1.0 / 2.2) * 255.0f;
    g = (float) Math.pow(g, 1.0 / 2.2) * 255.0f;
    b = (float) Math.pow(b, 1.0 / 2.2) * 255.0f;


    return Math.round(a) << 24 | Math.round(r) << 16 | Math.round(g) << 8 | Math.round(b);
}

• ValueAnimator进阶—ofObject

• 该方法可以传入任何类型的变量，函数定义如下：

public static ValueAnimator ofObject(TypeEvaluator evaluator, Object...values)
//第一个参数是自定义的Evaluator; 第二个参数是可变长参数，属于Object类型

• 示例：

public Character evaluate(float fraction, Character startValue, Character endValue) {
    int startInt = (int)startValue;
    int endInt = (int)endValue;
    int curValue = (int) (startInt+fraction*(endInt-startInt));
    char result = (char)curValue;
    return result;
}


private void doChangeLetter() {
    final ValueAnimator valueAnimator = ValueAnimator.ofObject(new CharEvaluator(), new Character('A'), new Character('Z'));
    valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
        @Override
        public void onAnimationUpdate(ValueAnimator animation) {
            char text = (Character)valueAnimator.getAnimatedValue();
            textView.setText(String.valueOf(text));
        }
    });
    valueAnimator.setDuration(1000);
    valueAnimator.setInterpolator(new AccelerateInterpolator());
    valueAnimator.start();
}

• 实现一个简单的抛物运动：

class FallingBallEvaluator implements TypeEvaluator<Point> {
    private Point point = new Point();

    //pao'wu
    @Override
    public Point evaluate(float fraction, Point startValue, Point endValue) {
        point.x = (int)(startValue.x + fraction*(endValue.x-startValue.x));
        if(fraction*2<=1) {
            point.y = (int) (startValue.y + fraction *2* (endValue.y - startValue.y));
        }else{
            point.y = endValue.y;
        }
        return point;


    }
}


private void doAnimStart() {
    final ValueAnimator valueAnimator = ValueAnimator.ofObject(new FallingBallEvaluator(), new Point(0, 0), new Point(500, 500));
    valueAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
        @Override
        public void onAnimationUpdate(ValueAnimator animation) {
            Point curValue = (Point) valueAnimator.getAnimatedValue();
            imageView.layout(curValue.x, curValue.y, curValue.x+imageView.getWidth(), curValue.y+imageView.getHeight());
        }
    });
    valueAnimator.setDuration(1000);
    valueAnimator.start();
}