Transitioning a GradientDrawable

I have a function that creates a gradient across the background of my MainActivity. The gradient is created programmatically as a GradientDrawable. Certain interactions in the App change the color of the gradient.

The gradient and colors are properly shown, and they do change accordingly, however, I would like a transition effect on the color changes. Right now, the colors transition instantly with no crossfade.

fun createGradient() {
    gd.shape = GradientDrawable.RECTANGLE
    mainActivityID.background = gd
    gd.gradientType = GradientDrawable.LINEAR_GRADIENT
    gd.orientation = (GradientDrawable.Orientation.BL_TR)
}

Based on interactions in the App, I use a when statement to change the colors of the GradientDrawable

fun changeGradient(){

    //val td = gd as TransitionDrawable
    //td.isCrossFadeEnabled = true
    //td.startTransition(1500)

    when(calories){
        in 0..50 -> gd.colors = intArrayOf(
            ContextCompat.getColor(this, R.color.dBlue),
            ContextCompat.getColor(this, R.color.lBlue)
        )
        in 51..100 -> gd.colors = intArrayOf(
            ContextCompat.getColor(this, R.color.dRed),
            ContextCompat.getColor(this, R.color.lRed)
        )
        // etc...
    }
}

I tried casting the GradientDrawable as a TransitionDrawable (as seen in the commented code above), but that crashes with:

java.lang.ClassCastException: android.graphics.drawable.GradientDrawable cannot be cast to android.graphics.drawable.TransitionDrawable

How do I add a 1500 millisecond transition between the GradientDrawable color changes?

Progress crashes with java.lang.NegativeArraySizeException: -16777216

when(calories){
    in 0..50 -> animateGradient(gd, IntArray(Color.BLACK), IntArray(Color.CYAN))
    in 51..100 -> animateGradient(gd, IntArray(Color.RED), IntArray(Color.BLUE))
    in 101..150 -> animateGradient(gd, IntArray(Color.GREEN), IntArray(Color.YELLOW))
    // etc... about 15 more statements.
}

I don't think gradient drawable has built in transition methods, but You can run your own value animator to perform color transition.

var anim : Animator? = null

fun animateGradient(gd: GradientDrawable, from : IntArray, to: IntArray){
    require(from.size == to.size)
    anim?.cancel()
    val arraySize = from.size
    val props = Array<PropertyValuesHolder>(arraySize){
        PropertyValuesHolder.ofObject(it.toString(), ArgbEvaluator(), from[it], to[it])
    }
    val anim = ValueAnimator.ofPropertyValuesHolder(*props)
    anim.addUpdateListener {
        gd.colors = IntArray(arraySize){i ->
            it.getAnimatedValue(i.toString()) as Int
        }
    }
    anim.duration = 1500
    anim.start()
    this.anim = anim
}

Edit: modified method so it that tracks current color itself, reduced number of arguments.

var anim : Animator? = null
// variable that tracks current color - must be initialized with default gradient colors
var currentGradient = intArrayOf(
        ContextCompat.getColor(this, R.color.dBlue),
        ContextCompat.getColor(this, R.color.lBlue)
    )

fun animateGradient(targetColors: IntArray){
    val from = currentGradient
    require(from.size == targetColors.size)
    anim?.cancel()
    val arraySize = from.size
    val props = Array<PropertyValuesHolder>(arraySize){
        PropertyValuesHolder.ofObject(it.toString(), ArgbEvaluator(), from[it], targetColors[it])
    }
    val anim = ValueAnimator.ofPropertyValuesHolder(*props)
    anim.addUpdateListener {valueAnim ->
        IntArray(arraySize){i ->
            valueAnim.getAnimatedValue(i.toString()) as Int
        }.let{
            currentGradient = it
            gd.colors = it
        }
    }
    anim.duration = 1500
    anim.start()
    this.anim = anim
}

Call site:

when(calories){
    in 0..50 -> animateGradient(intArrayOf(
        ContextCompat.getColor(this, R.color.dBlue),
        ContextCompat.getColor(this, R.color.lBlue)
    ))
    in 51..100 -> animateGradient(intArrayOf(
        ContextCompat.getColor(this, R.color.dRed),
        ContextCompat.getColor(this, R.color.lRed)
    ))
    // etc...
}