ColorEvaluator.java

package com.cui.trypro.utils;

import android.animation.TypeEvaluator;

/**
 * 这大概是我们整个动画操作当中最复杂的一个类了。没错，属性动画的高级用法中最有技术含量的也就是如何编写出一个合
 * 适的TypeEvaluator。好在刚才我们已经编写了一个PointEvaluator，对它的基本工作原理已经有了了解，那么这里我们
 * 主要学习一下ColorEvaluator的逻辑流程吧。

 首先在evaluate()方法当中获取到颜色的初始值和结束值，并通过字符串截取的方式将颜色分为RGB三个部分，并将RGB的值
 转换成十进制数字，那么每个颜色的取值范围就是0-255。接下来计算一下初始颜色值到结束颜色值之间的差值，这个差值很
 重要，决定着颜色变化的快慢，如果初始颜色值和结束颜色值很相近，那么颜色变化就会比较缓慢，而如果颜色值相差很大，
 比如说从黑到白，那么就要经历255*3这个幅度的颜色过度，变化就会非常快。
 */
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;
    }

}