TestLayout.swift

struct TestLayout: Layout {
    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        // Assuming this layout is in a fixed size container and isn't sizing itself based on the subviews.
        return proposal.replacingUnspecifiedDimensions()
    }

    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        let availableWidth = bounds.size.width
        let offeredWidth = availableWidth / CGFloat(subviews.count)
        let subviewProposal = ProposedViewSize(width: offeredWidth, height: bounds.size.height)
        let subviewWidths = subviews.map { $0.sizeThatFits(subviewProposal).width }
        let totalSubviewWidth = subviewWidths.reduce(0) { $0 + $1 }
        let amountOfSpace = availableWidth - totalSubviewWidth
        let gapSize = amountOfSpace / CGFloat(subviews.count + 1)
        
        var pos = bounds.origin
        for i in subviews.indices {
            pos.x += gapSize
            subviews[i].place(at: pos, proposal: subviewProposal)
            pos.x += subviewWidths[i]
        }
    }
}


struct ContentView: View {
    var body: some View {
        TestLayout {
            Color.red.frame(width: 20)
            Color.green.frame(width: 50)
            Color.blue.frame(width: 30)
        }
        .frame(width: 300, height: 30)
        .border(.black)
    }
}