TimingCurveView.swift

struct TimingCurveView: View {
    @State var value: CGFloat = 0

    @State var cp1: RelativePoint = .zero
    @State var cp2: RelativePoint = .zero

    let timer = Timer.publish(every: 2, on: .main, in: .common).autoconnect()

    var animation: Animation {
        Animation.timingCurve(
            Double(cp1.x),
            Double(1 - cp1.y),
            Double(cp2.x),
            Double(1 - cp2.y),
            duration: 2
        )
    }

    var body: some View {
        VStack {
            CurveEditorView(controlPoint1: $cp1, controlPoint2: $cp2)
                .aspectRatio(contentMode: .fill)
            Spacer()
            GeometryReader { reader in
                Circle()
                    .position(x: 0, y: 6)
                    .offset(x: self.value * reader.size.width, y: 0)
                    .frame(height: 12)
            }.frame(height: 40)
        }
        .onReceive(timer) { _ in
            self.value = 0
            withAnimation(self.animation) {
                self.value = 1
            }
        }
    }
}