AnimatedValue.kt

class AnimatedValue<T>(
    initial: T,
    private val spec: AnimationSpec,
    private val updateCallback: (value: T) -> Unit,
    private val evaluation: (fraction: Float, startValue: T, endValue: T) -> T
) {

    private var animator = ValueAnimator.ofFloat(0f, 1f).apply {
        duration = spec.duration
        interpolator = spec.interpolator
        addUpdateListener { currentValue = evaluation(it.animatedFraction, startValue, value) }
    }

    init {
        updateCallback(initial)
    }

    var value: T = initial
        set(value) {
            if (field != value) {
                field = value
                animate()
            }
        }

    private var startValue: T = initial

    private var currentValue: T = initial
        set(value) {
            field = value
            updateCallback(value)
        }

    private fun animate() {
        startValue = currentValue
        animator.cancel()
        animator.start()
    }

    fun dispose() {
        animator.cancel()
        animator.removeAllListeners()
    }
}