Inner shadows in SwiftUI

InnerShadow.swift

struct InnerShadow<S: InsettableShape>: View {
    let shape: S
    var color: Color
    var thickness: CGFloat
    var offset: CGSize
    var blur: CGFloat
    
    init(shape: S,
                  color: Color = .white.opacity(0.25),
                  thickness: CGFloat = 0.0,
                  offset: CGSize = .zero,
                  blur: CGFloat = 5.0) {
        self.shape = shape
        self.color = color
        self.thickness = thickness
        self.offset = offset
        self.blur = blur
    }
    
    public var body: some View {
        shape.fill(color).mask {
            ZStack {
                shape
                    .fill(.white)
                shape
                    .inset(by: thickness)
                    .fill(.black)
                    .offset(offset)
                    .blur(radius: blur)
            }.compositingGroup()
                .luminanceToAlpha()
        }
    }
}

public extension View {
    func innerShadow<S: InsettableShape>(shape: S,
                                         color: Color = .white.opacity(0.25),
                                         thickness: CGFloat = 0.0,
                                         offset: CGSize = .zero,
                                         blur: CGFloat = 5.0) -> some View {
        self.overlay {
            InnerShadow(shape: shape, color: color, thickness: thickness, offset: offset, blur: blur)
        }
    }
}

extension InsettableShape {
    func innerShadow<S: InsettableShape>(shape: S,
                                         color: Color = .white.opacity(0.25),
                                         thickness: CGFloat = 0.0,
                                         offset: CGSize = .zero,
                                         blur: CGFloat = 5.0) -> some View {
        InnerShadow(shape: self, color: color, thickness: thickness, offset: offset, blur: blur)
    }
}