View+ExactHitArea.swift

import SwiftUI

extension View {
    /// By reducing the hit detection, make it so that tap detection occurs within the range of the frame
    ///
    /// In SwiftUI, a View within about 10pt radius from the tap position is detected as touch,
    /// so we place a contentShape that is just as small.
    func exactHitArea() -> some View {
        modifier(ExactHitAreaModifier())
    }
}

private struct ExactHitAreaModifier: ViewModifier {
    private let hitAreaPadding: CGFloat = 10
    @State private var size: CGSize?

    func body(content: Content) -> some View {
        let scale = size.map {
            CGPoint(
                x: ($0.width - hitAreaPadding * 2) / $0.width,
                y: ($0.height - hitAreaPadding * 2) / $0.height
            )
        } ?? .zero

        content
            .contentShape(Rectangle().scale(x: scale.x, y: scale.y))
            .overlay {
                GeometryReader { proxy in
                    Color.clear
                        .onAppear {
                            self.size = proxy.size
                        }
                        .onChange(of: proxy.size) { newValue in
                            self.size = newValue
                        }
                }
            }
    }
}