如何为 Android 动画 (Kotlin) 制作自定义插值器？

Question

Loosely, this is my code, boiled down to the relevant parts:

fun animate(tv: TextView) {
    var animationSlide = TranslateAnimation(0.0f, 0.0f, 100.0f, 0f)
    var animationScale = ScaleAnimation(1.0f, 1.0f, 0.0f, 1.0f)
    animationSlide.interpolator = AccelerateInterpolator() // want custom interpolator
    animationScale.interpolator = DecelerateInterpolator() // want custom interpolator
    animationSlide.duration = 1000
    animationScale.duration = 1000
    var animationSet = AnimationSet(false)
    animationSet.addAnimation(animationSlide)
    animationSet.addAnimation(animationScale)
    tv.startAnimation(animationSet)
}

Is it possible to make a custom interpolator which takes an array of points, or a lambda function which takes a time input and returns a value for that time point? The Android documentation only shows a small list of interpolators, none of which can be customised to a general function curve.

Thanks for any help!

Answer 1

This is done by making a class that implements Interpolator. Or you can use a SAM-converted lambda. For example, an interpolation based on the smoothstep function:

animationSlide.setInterpolator { amount ->
    amount * amount * (3f - 2f * amount)
}

If you want to define a set of points to define your curve, you can do that with the Path class and instantiate a PathInterpolator with that. From looking at the source code, it looks like PathInterpolator approximates the Path by breaking it down into straight line segments, so if there are arcs and Beziers, it could potentially become kind of a heavy class instance that you might want to cache a reference to if you use it repeatedly.