CurveEditorView.swift

struct CurveEditorView: View {

    // 1
    private let initialPoint0: CGSize = .init(width: 0.4, height: 0.3)
    private let initialPoint1: CGSize = .init(width: 0.6, height: 0.6)

    // 2
    @State private var offsetPoint0: CGSize = .zero
    @State private var offsetPoint1: CGSize = .zero

    private var curvePoint0: RelativePoint {
        (initialPoint0 + offsetPoint0).toPoint
    }

    private var curvePoint1: RelativePoint {
        (initialPoint1 + offsetPoint1).toPoint
    }

    // 3
    @Binding var controlPoint1: RelativePoint
    @Binding var controlPoint2: RelativePoint

    var body: some View {

        let primaryColor   = Color.blue
        let secondaryColor = primaryColor.opacity(0.7)

        return GeometryReader { reader in
            Color.white

            // 4
            CurveShape(cp0: self.curvePoint0, cp1: self.curvePoint1)
                .stroke(primaryColor, lineWidth: 4)

            // 5
            Path { p in
                p.move(to: CGPoint(x: 0, y: 1 * reader.size.height))
                p.addLine(to: self.curvePoint0 * reader.size)
            }.stroke(secondaryColor, lineWidth: 2)

            Path { p in
                p.move(to: CGPoint(x: 1 * reader.size.width, y: 0))
                p.addLine(to: self.curvePoint1 * reader.size)
            }.stroke(secondaryColor, lineWidth: 2)

            // 6
            ControlPointHandle()
                .offset(self.initialPoint0 * reader.size)
                .foregroundColor(primaryColor)
                .draggable(onChanged: { (size) in
                    self.offsetPoint0 = size / reader.size
                    self.controlPoint1 = self.curvePoint0
                })

            ControlPointHandle()
                .offset(self.initialPoint1 * reader.size)
                .foregroundColor(primaryColor)
                .draggable(onChanged: { (size) in
                    self.offsetPoint1 = size / reader.size
                    self.controlPoint2 = self.curvePoint1
                })
        }
        .aspectRatio(contentMode: .fit)
        .onAppear {
            self.controlPoint1 = self.curvePoint0
            self.controlPoint2 = self.curvePoint1
        }
    }
}