InvertedMask.swift

import Foundation
import SwiftUI

struct FocusView: View {

    let path: Path
    let backgroundColor: Color
    
    init(path: Path, backgroundColor: Color = Color.black.opacity(0.3)) {
        self.path = path
        self.backgroundColor = backgroundColor
    }
    
    var body: some View {
        GeometryReader { (geo: GeometryProxy) in
            self.dimmer(with: geo)
        }
        .edgesIgnoringSafeArea(.all)
    }

    private func dimmer(with geo: GeometryProxy) -> some View {
        Rectangle()
            .fill(backgroundColor)
            .mask(
                Path { (path) in
                    path.addPath(
                        Rectangle()
                            .path(in: geo.frame(in: .local))
                    )
                    path.addPath(
                        self.path
                    )
                }
                .fill(style: FillStyle(eoFill: true))
            )
    }
    
}