SwiftUI: Read the default spacing for use elsewhere

_Example.swift

// Can also use like `ReadSpacing { spacing in ... }` if a state/binding isn't needed

struct Example: View {
    @State private var spacing: CGFloat = 0

    var body: some View {
        VStack {
            ReadSpacing(into: $spacing) {
                Color.red.frame(width: 200, height: 100)
                Color.red.frame(width: 200, height: 100)
            }
        }
        .overlay(alignment: .bottom) {
            Color.red.frame(width: 200, height: 100)
                .padding(.top, spacing)
                .alignmentGuide(VerticalAlignment.bottom) { $0[.top] }
        }
    }
}

ReadSpacing.swift

struct ReadSpacing<Content: View>: View {
    @State private var spacing: CGFloat = 0
    @Binding private var outsideSpacing: CGFloat?
    private let content: (CGFloat) -> Content

    init(@ViewBuilder content: @escaping (CGFloat) -> Content) {
        self._outsideSpacing = .constant(nil)
        self.content = content
    }
    init(into spacing: Binding<CGFloat>, @ViewBuilder content: @escaping () -> Content) {
        self._outsideSpacing = .init(spacing)
        self.content = { _ in content() }
    }

    private var activeSpacing: Binding<CGFloat> {
        .init(
            get: { outsideSpacing ?? spacing },
            set: { value in
                outsideSpacing = value
                spacing = value
            }
        )
    }

    var body: some View {
        _VariadicView.Tree(ReadSpacingLayout(spacing: activeSpacing)) {
            content(spacing)
        }
    }
}

private struct FrameKey: PreferenceKey {
    static var defaultValue: [CGRect] = []

    static func reduce(value: inout Value, nextValue: () -> Value) {
        value.append(contentsOf: nextValue())
    }
}

private struct ReadSpacingLayout: _VariadicView_MultiViewRoot {
    private struct Container: _VariadicView_UnaryViewRoot {
        func body(children: _VariadicView.Children) -> some View { children }
    }

    @Binding var spacing: CGFloat

    @ViewBuilder
    func body(children: _VariadicView.Children) -> some View {
        _VariadicView.Tree(Container()) {
            ForEach(children) { child in
                child
                    .overlay {
                        GeometryReader { proxy in
                            Color.clear
                                .preference(key: FrameKey.self, value: [proxy.frame(in: .global)])
                        }
                    }
            }
        }
        .onPreferenceChange(FrameKey.self) { values in
            self.spacing = values.indices.reduce(0.0) { spacing, current in
                if let previous = values.index(current, offsetBy: -1, limitedBy: values.startIndex) {
                    let value = values[current].minY - values[previous].maxY
                    if value > spacing { return value }
                }
                return spacing
            }
        }

    }
}