A small demonstration of my problems with sizing a wrapped UIButton in a UIViewRepresentable properly.

ButtonRepresentable.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        ButtonRepresentable(title: "Test") {
            print("press")
        }
//        .frame(maxWidth: .infinity) // Comment in or out for the two cases I want to support.
    }
}

struct ButtonRepresentable: UIViewRepresentable {
    let title: String
    let action: () -> Void
    
    func makeUIView(context: Context) -> UIButton {
        let button = UIButton(primaryAction: UIAction(handler: { _ in action() }))
        // Setting high content hugging prio makes button use its intrinsic content size when using the default
        // sizing algo or not implementing sizeThatFits or returning nil in sizeThatFits.
        button.setContentHuggingPriority(.defaultHigh, for: .horizontal)
        button.setContentHuggingPriority(.defaultHigh, for: .vertical)
        button.backgroundColor = .red
        button.setTitle(title, for: .normal)
        return button
    }
    
    func updateUIView(_ uiView: UIButton, context: Context) {
        uiView.setTitle(title, for: .normal)
    }
    
    func sizeThatFits(_ proposal: ProposedViewSize, uiView: UIButton, context: Context) -> CGSize? {
        switch proposal {
        case .infinity:
            // This case never gets called, even when using .frame(maxWidth: .infinity)
            guard let width = proposal.width, let height = proposal.height else {
                return uiView.systemLayoutSizeFitting(UIView.layoutFittingExpandedSize)
            }
            return CGSize(width: width, height: height)
        default:
            // This case always gets called.
            // The algorithm passes a proposed size when not explicitly specifying a frame but also when explicitly specifying one. That makes it impossible to distinguish.
            // I would like a way to determine:
            //
            // 1. If frame(maxWidth: .infinity) was used -> make the view expand to full size
            // 2. If an explicit size e.g. frame(width: 200) was used -> use that size
            // 3. If nothing was used -> use default sizing algo and make the wrapped UIButton use its intrinsicContentSize
            guard let width = proposal.width, let height = proposal.height else {
                return nil
            }
            return CGSize(width: width, height: height)
        }
    }
}

#Preview {
    ContentView()
}