Why using TypeEvaluator instead of Interpolator in this library？

[tankcong]

PropertyAnimation文档里似乎更推荐通过自定义Interpolator来控制所选属性的变化率。而TypeEvaluator的描述是：If you want to animate a type that is _unknown_ to the Android system, you can create your own evaluator by implementing the TypeEvaluator interface，似乎更加强调针对自定义类型或属性改变的情况。
自己在项目中几乎都使用Interpolator，这里不太明白，请教一下。
Thank you!:)

[daimajia]

@tankcong This is a really good question. And it's also the hardest part of this library.

Since easing functions need four parameters

• time
• begin value
• value change
• duration

If you try to use interpolator, you are unable to get or calculate all the value from Interpolator.

The following code is calculating these four parameters:
https://github.com/daimajia/AnimationEasingFunctions/blob/master/library/src/main/java/com/daimajia/easing/BaseEasingMethod.java#L70-L78