属性动画估值器

补间动画只能对View进行操作，属性动画不受限制，那么值的变化就用到了估值器

evaluator[ɪ'væljʊeɪtə(r)] 估值器

1、TypeEvaluator

　　告诉系统动画如何从初始值过渡到结束值，大多情况使用系统提供的估值器。ValueAnimator.ofFloat就使用了如下估值器：

public class FloatEvaluator implements TypeEvaluator {
    public Object evaluate(float fraction, Object startValue, Object endValue) {
        float startFloat = ((Number) startValue).floatValue();
        return startFloat + fraction * (((Number) endValue).floatValue() - startFloat);
    }
}

　　对于系统无法提供的，那么就要自己实现TypeEvaludator实现估值器

2、估值器的应用

　　ObjectAnimator本质对属性的更改，实际使用了ValueAnimator.ofObject操作对象，下面需要刷新页面使用了ofObject

　　两个自定义的估值器

public class PointEvaluator implements TypeEvaluator{
    @Override
    public Object evaluate(float fraction, Object startValue, Object endValue) {
        PointF startPoint = (PointF) startValue;
        PointF endPoint = (PointF) endValue;
        float x = startPoint.x + fraction * (endPoint.x - startPoint.x);
        float y = startPoint.y + fraction * (endPoint.y - startPoint.y);
        PointF point = new PointF(x, y);
        return point;
    }
}

　　

public class ColorEvaluator implements TypeEvaluator {

    private int mCurrentRed = -1;

    private int mCurrentGreen = -1;

    private int mCurrentBlue = -1;

    @Override
    public Object evaluate(float fraction, Object startValue, Object endValue) {
        String startColor = (String) startValue;
        String endColor = (String) endValue;
        int startRed = Integer.parseInt(startColor.substring(1, 3), 16);
        int startGreen = Integer.parseInt(startColor.substring(3, 5), 16);
        int startBlue = Integer.parseInt(startColor.substring(5, 7), 16);
        int endRed = Integer.parseInt(endColor.substring(1, 3), 16);
        int endGreen = Integer.parseInt(endColor.substring(3, 5), 16);
        int endBlue = Integer.parseInt(endColor.substring(5, 7), 16);
        // 初始化颜色的值
        if (mCurrentRed == -1) {
            mCurrentRed = startRed;
        }
        if (mCurrentGreen == -1) {
            mCurrentGreen = startGreen;
        }
        if (mCurrentBlue == -1) {
            mCurrentBlue = startBlue;
        }
        // 计算初始颜色和结束颜色之间的差值
        int redDiff = Math.abs(startRed - endRed);
        int greenDiff = Math.abs(startGreen - endGreen);
        int blueDiff = Math.abs(startBlue - endBlue);
        int colorDiff = redDiff + greenDiff + blueDiff;
        if (mCurrentRed != endRed) {
            mCurrentRed = getCurrentColor(startRed, endRed, colorDiff, 0,
                    fraction);
        } else if (mCurrentGreen != endGreen) {
            mCurrentGreen = getCurrentColor(startGreen, endGreen, colorDiff,
                    redDiff, fraction);
        } else if (mCurrentBlue != endBlue) {
            mCurrentBlue = getCurrentColor(startBlue, endBlue, colorDiff,
                    redDiff + greenDiff, fraction);
        }
        // 将计算出的当前颜色的值组装返回
        String currentColor = "#" + getHexString(mCurrentRed)
                + getHexString(mCurrentGreen) + getHexString(mCurrentBlue);
        return currentColor;
    }

    /**
     * 根据fraction值来计算当前的颜色。
     */
    private int getCurrentColor(int startColor, int endColor, int colorDiff,
                                int offset, float fraction) {
        int currentColor;
        if (startColor > endColor) {
            currentColor = (int) (startColor - (fraction * colorDiff - offset));
            if (currentColor < endColor) {
                currentColor = endColor;
            }
        } else {
            currentColor = (int) (startColor + (fraction * colorDiff - offset));
            if (currentColor > endColor) {
                currentColor = endColor;
            }
        }
        return currentColor;
    }

    /**
     * 将10进制颜色值转换成16进制。
     */
    private String getHexString(int value) {
        String hexString = Integer.toHexString(value);
        if (hexString.length() == 1) {
            hexString = "0" + hexString;
        }
        return hexString;
    }

}

　　View中绘制并使用动画

public class AnimView extends View {

    public static final float radius = 50f;
    private PointF currentPoint;
    private Paint mPaint;

    private String color;

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    public AnimView(Context context, AttributeSet attrs) {
        super(context, attrs);
        mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        mPaint.setColor(Color.BLUE);
    }


    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if(currentPoint == null){
            currentPoint = new PointF(radius, radius);
            drawCircle(canvas);
            startAnimator();
        }else{
            drawCircle(canvas);
        }
    }

    private void drawCircle(Canvas canvas){
        float x = currentPoint.x;
        float y = currentPoint.y;
        canvas.drawCircle(x, y, radius, mPaint);
    }


    private void startAnimator(){
        PointF startPoint = new PointF(radius, radius);
        PointF endPoint = new PointF(getWidth() - radius, getHeight() - radius);
        ValueAnimator anim = ValueAnimator.ofObject(new PointEvaluator(), startPoint, endPoint);
        anim.addUpdateListener(new ValueAnimator.AnimatorUpdateListener(){

            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                currentPoint = (PointF) animation.getAnimatedValue();
                invalidate();
            }
        });
        ObjectAnimator anim2 = ObjectAnimator.ofObject(this, "color", new ColorEvaluator(),
                "#0000FF", "#FF0000");
        AnimatorSet animSet = new AnimatorSet();
        animSet.play(anim).with(anim2);
        animSet.setDuration(5000);
        animSet.start();
    }


}

　　XML中直接使用

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <me.zcy.animatortest.AnimView
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>