For Medium article "Android Dynamic Custom View is Easy"

IconProgress.kt

private var iconProgress = 0f
  set(value) {
      if (field != value) {
          field = value
        
          val iconOffset = lerp(0f, iconRadius - iconCollapsedWidth / 2, value)
          iconRect.left = width - switcherCornerRadius - iconCollapsedWidth / 2 - iconOffset
          iconRect.right = width - switcherCornerRadius + iconCollapsedWidth / 2 + iconOffset

          postInvalidateOnAnimation()
      }
  }