ADSRSliderView.swift

struct ADSRSliderView: View {

  fileprivate struct DragGestureState {
    var previousTranslationY: CGFloat?
    var deltaPercentY: CGFloat = 0.0
  }

  let color: Color

  @State var percent: CGFloat = 1.0

  @GestureState fileprivate var dragGestureState: DragGestureState = DragGestureState()

  var body: some View {
    GeometryReader { geo in
      ZStack(alignment: .bottomLeading) {
        Rectangle()
          .frame(minWidth: 0.0, maxWidth: .infinity, minHeight: 0.0, maxHeight: .infinity, alignment: .bottomLeading)
          .background(Color.clear)
          .foregroundColor(Color.clear)
        Rectangle()
          .foregroundColor(self.color)
          .frame(height: geo.size.height * self.percent, alignment: .bottomTrailing)
      }
      .frame(minWidth: 0.0, maxWidth: .infinity, minHeight: 0.0, maxHeight: .infinity, alignment: .leading)
      .gesture(
        DragGesture(minimumDistance: 0.0)
          .updating(self.$dragGestureState) { (dragGesture, dragGestureState, _) in
            let deltaY: CGFloat
            if dragGestureState.previousTranslationY == nil {
              dragGestureState.previousTranslationY = dragGesture.translation.height
              deltaY = 0.0
            } else {
              deltaY = -(dragGesture.translation.height - dragGestureState.previousTranslationY!)
              dragGestureState.previousTranslationY = dragGesture.translation.height
            }

            dragGestureState.deltaPercentY = deltaY / geo.size.height
          }
          .onChanged { gesture in
            self.percent = (self.percent + self.dragGestureState.deltaPercentY).clamped(to: 0.0...1.0)
          }
      )
    }
  }

}