AnchorLayout.swift

import SwiftUI

/// Implementation of `Layout` that aligns subviews relative it their `anchor` value
///
/// Considerations before using:
/// * Sizing is not taken into account
/// * No stacking is provided if multiple subviews are anchored to the same position
/// * Subviews will be able to overlap if anchored closely together and they're both large enough
public struct AnchorLayout: Layout {
    public init() { }

    public func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        proposal.replacingUnspecifiedDimensions()
    }

    public func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        for subview in subviews {
            let anchor = subview[AnchorValue.self]
            subview.place(
                at: .init(
                    x: bounds.minX + (bounds.size.width * anchor.x),
                    y: bounds.minY + (bounds.size.height * anchor.y)
                ),
                anchor: anchor,
                proposal: proposal
            )
        }
    }
}

private struct AnchorValue: LayoutValueKey {
    static let defaultValue = UnitPoint.center
}

extension View {
    public func anchor(_ anchor: UnitPoint) -> some View {
        self.layoutValue(key: AnchorValue.self, value: anchor)
    }
}

struct AnchorLayout_Previews: PreviewProvider {
    static var previews: some View {
        ZStack {
            Color.yellow.frame(width: 5.0)
            Color.yellow.frame(height: 5.0)
            AnchorLayout {
                Group {
                    Color.red.anchor(.topLeading)
                    Color.green.anchor(.top)
                    Color.blue.anchor(.topTrailing)

                    Color.pink.anchor(.leading)
                    Color.mint.anchor(.center)
                    Color.indigo.anchor(.trailing)

                    Color.orange.anchor(.bottomLeading)
                    Color.brown.anchor(.bottom)
                    Color.purple.anchor(.bottomTrailing)

                    Color.gray.anchor(.init(x: 0.25, y: 0.25))
                }.frame(width: 50.0, height: 50.0).cornerRadius(20.0)
            }.padding(5.0)
        }
        .frame(width: 300.0, height: 500.0)
        .previewLayout(.sizeThatFits)
    }
}

example.png