Curve.swift

struct CurveShape: Shape {
    let cp0, cp1: RelativePoint
    func path(in rect: CGRect) -> Path {
        Path { p in
            p.move(to: CGPoint(x: 0, y: rect.size.height))
            p.addCurve(to: CGPoint(x: rect.size.width, y: 0),
                       control1: cp0 * rect.size,
                       control2: cp1 * rect.size)
        }
    }
}