Change toolbar alpha by ScrollView

transparent_toolbar

// Notify scroll
class ObservableScrollView : ScrollView {
    interface OnScrollChangeListener {
        fun OnScrollChangeListener(scrollView: ObservableScrollView, x: Int, y: Int, oldX: Int, oldY: Int)
    }
    var onScrollChangeListener: OnScrollChangeListener? = null

    constructor(context: Context) : this(context, null)
    constructor(context: Context, attrs: AttributeSet?) : this(context, attrs, 0)
    constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0) : super(context, attrs, defStyleAttr)

    override fun onScrollChanged(x: Int, y: Int, oldX: Int, oldY: Int) {
        super.onScrollChanged(x, y, oldX, oldY)
        onScrollChangeListener?.OnScrollChangeListener(this, x, y, oldX, oldY)
    }
}

// CustomView
override fun OnScrollChangeListener(scrollView: ObservableScrollView, x: Int, y: Int, oldX: Int, oldY: Int) {
  changeToolbarAlpha(y)
}

private fun changeToolbarAlpha(scrollY: Int) {
    val offset = DisplayMetricsUtil.dpToPx(context, 140)
    val alpha = scrollY.toFloat() / offset
    if (alpha in 0..1) {
        toolbarBackgroundView.alpha = alpha
    }
}