View extension functions for animating height expansion and collapse. Use these by calling "yourView.expand()" or "yourView.collapse()".

ViewExtension.kt

import android.view.View
import android.view.ViewGroup
import android.view.animation.Animation
import android.view.animation.Transformation


/**
 * View extensions for expanding and collapsing
 */
fun View.expand() {
    val widthSpec = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED)
    val heightSpec = View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED)
    this.measure(widthSpec, heightSpec)
    val targetHeight = this.measuredHeight

    // Older versions of android (pre API 21) cancel animations for views with a height of 0.
    this.layoutParams.height = 1
    this.visibility = View.VISIBLE
    val a = object : Animation() {
        override fun applyTransformation(interpolatedTime: Float, t: Transformation) {
            this@expand.layoutParams.height = if (interpolatedTime == 1f)
                ViewGroup.LayoutParams.WRAP_CONTENT
            else
                (targetHeight * interpolatedTime).toInt() + 1 // Needed for non-zero value for ConstraintLayout
            this@expand.requestLayout()
        }

        override fun willChangeBounds(): Boolean {
            return true
        }
    }

    // 1dp/ms
    a.duration = ((targetHeight / this.context.resources.displayMetrics.density).toInt()).toLong()
    this.startAnimation(a)
}

fun View.collapse() {
    val initialHeight = this.measuredHeight

    val a = object : Animation() {
        override fun applyTransformation(interpolatedTime: Float, t: Transformation) {
            if (interpolatedTime == 1f) {
                this@collapse.visibility = View.GONE
            } else {
                this@collapse.layoutParams.height = initialHeight - (initialHeight * interpolatedTime).toInt()
                this@collapse.requestLayout()
            }
        }

        override fun willChangeBounds(): Boolean {
            return true
        }
    }

    // 1dp/ms
    a.duration = ((initialHeight / this.context.resources.displayMetrics.density).toInt()).toLong()
    this.startAnimation(a)
}