/**
 * Interface for use with the  ValueAnimator#setEvaluator(TypeEvaluator) function. Evaluators
 * allow developers to create animations on arbitrary任意的 property types, by allowing them to supply
 * custom evaluators for types that are not automatically understood and used by the animation system.
 */
public interface TypeEvaluator<T> {
    /**
     * This function returns the result of linearly interpolating the start and end values, with fraction representing
     * the proportion比例 between the start and end values. The calculation is a simple parametric参数
     * 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
     * @return A linear interpolation between the start and end values, given the fraction parameter.
     */
    public T evaluate(float fraction, T startValue, T endValue);
}

/** This evaluator can be used to perform type interpolation between int values. */
public class IntEvaluator implements TypeEvaluator<Integer> {
    public Integer evaluate(float fraction, Integer startValue, Integer endValue) {
        return (int)(startValue + fraction * (endValue - startValue)); 
    }
}

public Float evaluate(float fraction, Number startValue, Number endValue) {
   return startValue.floatValue() + fraction * (endValue.floatValue() - startValue.floatValue());
}

/** 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单独的 
     * 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) & 0xff;
        int startR = (startInt >> 16) & 0xff;
        int startG = (startInt >> 8) & 0xff;
        int startB = startInt & 0xff;

        int endInt = (Integer) endValue;
        int endA = (endInt >> 24) & 0xff;
        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))));
    }
}

}

}