Example of how to use `alignmentGuide` to align components along two dimensions

AlignmentGuides.swift

//
//  ContentView.swift
//  AlignDemo
//
//  Created by Marc Prud'hommeaux on 2/28/20.
//  Copyright © 2020 Glimpse I/O. All rights reserved.
//

import SwiftUI

/*
 * Example of how to use `alignmentGuide` to align components along two dimensions.
 * This code will make the following layout centered around
 * the "XXX" Text view:
 *
 *         O
 * OOO -- XXX -------- OOOO
 *         O
 *         O
 *         O
 */
struct ContentView: View {
    var body: some View {
        HStack {
            Text("OOO")
            VStack() {
                Text("O")
                HStack() {
                    Text("--")
                    Text("XXX")
                        .alignmentGuide(.topViewCenter) { d in d[VerticalAlignment.center] }
                        .alignmentGuide(.leadingViewCenter) { d in d[HorizontalAlignment.center] }
                    Text("--------")
                }
                .alignmentGuide(HorizontalAlignment.center) { d in d[.leadingViewCenter] }

                Text("O")
                Text("O")
                Text("O")
            }
            .alignmentGuide(VerticalAlignment.center) { d in d[.topViewCenter] }
            Text("OOOO")
        }
    }
}

extension VerticalAlignment {
    /// The center of the uppermost view in a stack; handy for aligning inspectors sets so the label
    /// lines up with the center of the topmost view.
    static let topViewCenter = VerticalAlignment(TopViewCenter.self)
    private enum TopViewCenter: AlignmentID {
        static func defaultValue(in d: ViewDimensions) -> CGFloat { d[VerticalAlignment.center] }
    }
}


extension HorizontalAlignment {
    /// The horizontal center of the leadoing view in a stack; handy for aligning the primary inspector control
    /// with a label beneath it.
    static let leadingViewCenter = HorizontalAlignment(LeadingViewCenter.self)
    private enum LeadingViewCenter: AlignmentID {
        static func defaultValue(in d: ViewDimensions) -> CGFloat { d[HorizontalAlignment.center] }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}