Android源码中，有两个Interpolator。一个是frameworks/base/graphics/java/android/graphics/Interpolator.java。另一个是
frameworks/base/core/java/android/view/animation/Interpolator.java。今天想要聊的是后者。
frameworks/base/core/java/android/view/animation/Interpolator.java的源码如下：
/**
* An interpolator defines the rate of change of an animation. This allows
* the basic animation effects (alpha, scale, translate, rotate) to be
* accelerated, decelerated, repeated, etc.
*/
public interface Interpolator extends TimeInterpolator {
// A new interface, TimeInterpolator, was introduced for the new android.animation
// package. This older Interpolator interface extends TimeInterpolator so that users of
// the new Animator-based animations can use either the old Interpolator implementations or
// new classes that implement TimeInterpolator directly.
}Android在3.0引入属性动画时，加入了TimeInterpolator接口。Interpolator变成与之(TimeInterpolator)对应的旧接口。Interpolator继承TimeInterpolator，可以兼容旧代码。建议开发者在开发新代码的时候，直接实现TimeInterpolator接口。所以，我们需要再看一下TimeInterpolator的源码。
frameworks/base/core/java/android/animation/TimeInterpolator.java的源码如下：
/**
* A time interpolator defines the rate of change of an animation. This allows animations
* to have non-linear motion, such as acceleration and deceleration.
*/
public interface TimeInterpolator {
/**
* Maps a value representing the elapsed fraction of an animation to a value that represents
* the interpolated fraction. This interpolated value is then multiplied by the change in
* value of an animation to derive the animated value at the current elapsed animation time.
*
* @param input A value between 0 and 1.0 indicating our current point
* in the animation where 0 represents the start and 1.0 represents
* the end
* @return The interpolation value. This value can be more than 1.0 for
* interpolators which overshoot their targets, or less than 0 for
* interpolators that undershoot their targets.
*/
float getInterpolation(float input);
}TimeInterpolator定义了动画变化的接口。float getInterpolation(float input)的输入是[0, 1.0]，是动画插值器的归一化时间，0表示开始，1.0表示结束。返回值是动画插值器的归一化结果[0, 1.0]，但是，返回值在超越目标(overshoot their targets)的时候，也可以<0或者>1.0。