Simple flow layout in SwiftUI

FlowStack.swift

import SwiftUI

/// A view that arranges its children in horizontal lines
///
///     FlowStack {
///         ForEach(1..<100) { num in
///             Text(String(num))
///                 .padding(8)
///                 .background(Circle().fill(Color.red))
///         }
///     }
///
public struct FlowStack<Content: View>: View {
    let content: () -> Content
    let spacing: CGSize

    /// Creates an instance with the given spacing and content.
    ///
    /// - Parameter spacing: A `CGSize` value indicating the space between children.
    /// - Parameter content: A view builder that creates the content of this stack.
    public init(spacing: CGSize = .zero, @ViewBuilder content: @escaping () -> Content) {
        self.content = content
        self.spacing = spacing
    }

    public var body: some View {
        ZStack(alignment: .topLeading) {
            // Setup for layout pass
            var available: CGFloat = 0
            var x: CGFloat = 0
            var y: CGFloat = 0
            Color.clear
                .frame(height: 0)
                .alignmentGuide(.top) { item in
                    available = item.width
                    x = 0
                    y = 0
                    return 0
                }

            content()
                .alignmentGuide(.leading) { item in
                    if x + item.width > available {
                        x = 0
                        y += item.height + spacing.height
                    }
                    let result = x
                    x += item.width + spacing.width
                    return -result
                }
                .alignmentGuide(.top) { _ in
                    -y
                }
        }
    }
}

struct FlowStack_Previews: PreviewProvider {
    static var previews: some View {
        FlowStack {
            ForEach(1..<100) { num in
                Text(String(num))
                    .frame(minWidth: 30, minHeight: 30)
                    .background(Circle().fill(Color.red))
            }
        }
    }
}

This implementation does not use GeometryReader, instead relying on Color to expand to the available width. While this still fills the available width, it self-sizes height just fine, and doesn't require asynchronously setting @State or PreferenceKey complexity.