Drive UIView frame layout with SwiftUI Layout protocol

README.md

https://twitter.com/auramagi/status/1535600662922158080?s=21&t=3PdsJRuqGzznCk3_ahrblQ

LayoutAdaptor.swift

// The main layout logic 

import SwiftUI

final class LayoutAdaptor: UIView {
    var layout: (any Layout)? {
        didSet { setNeedsLayout() }
    }
        
    override func layoutSubviews() {
        super.layoutSubviews()
        guard let layout = layout else { return }
        let content = AnyLayout(layout) {
            ForEach(subviews, id: \.objectIdentifier) { view in
                UIViewProxyLayout(view: view)
            }
        }
            .frame(width: bounds.width, height: bounds.height)
            .ignoresSafeArea()
        
        _ = ImageRenderer(content: content).uiImage // force render without showing on-screen
    }
}

struct UIViewProxyLayout: Layout, View {
    let view: UIView
    
    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        view.sizeThatFits(proposal: proposal)
    }
    
    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        view.frame = bounds
    }
    
    var body: some View {
        self { Path() }
    }
}

// This is mostly an untested shot in the dark, probably will fail to properly size more advanced views
extension UIView {
    func sizeThatFits(proposal: ProposedViewSize) -> CGSize {
        let widthTarget = target(for: proposal.width)
        let heightTarget = target(for: proposal.height)
        return systemLayoutSizeFitting(
            CGSize(width: widthTarget.0, height: heightTarget.0),
            withHorizontalFittingPriority: widthTarget.1,
            verticalFittingPriority: heightTarget.1
        )
    }
    
    func target(for proposal: CGFloat?) -> (CGFloat, UILayoutPriority) {
        switch proposal {
        case .none: return (UIView.layoutFittingCompressedSize.width, .fittingSizeLevel)
        case .some(.zero): return (UIView.layoutFittingCompressedSize.width, .defaultHigh)
        case .some(.infinity): return (UIView.layoutFittingExpandedSize.width, .defaultLow)
        case let .some(value): return (value, .defaultHigh)
        }
    }
}

extension UIView {
    var objectIdentifier: ObjectIdentifier {
        .init(self)
    }
}

LayoutAdaptorApp.swift

// App for testing. Has a sample VC, a button to switches between 4 layouts.

import SwiftUI

@main
struct LayoutAdaptorApp: App {
    @State var i = 0
    
    var layoutType: LayoutType {
        LayoutType.allCases[i % LayoutType.allCases.count]
    }
    
    var body: some Scene {
        WindowGroup {
            VStack {
                TestVCRepresentable(layout: layoutType.layout)
                    .frame(width: 150, height: 150)
                    .border(.red)
                    .frame(maxWidth: .infinity, maxHeight: .infinity)
                
                Button(
                    action: { i += 1 },
                    label: { Text("Change UIView layout") }
                )
            }
        }
    }
}

enum LayoutType: CaseIterable {
    case hstack
    case vstack
    case zstack
    case circle50
    
    var layout: any Layout {
        switch self {
        case .vstack: return VStack()
        case .hstack: return HStack()
        case .zstack: return _ZStackLayout()
        case .circle50: return _CircleLayout(radius: 50)
        }
    }
}

final class TestVC: UIViewController {
    private lazy var adaptor = LayoutAdaptor()
    
    private func makeLabel(text: String) -> UILabel {
        let view = UILabel()
        view.text = text
        return view
    }
    
    private func makeImageView(systemImage: String) -> UIImageView {
        UIImageView(image: .init(systemName: systemImage))
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.addSubview(makeLabel(text: "Hello"))
        view.addSubview(makeLabel(text: "World"))
        view.addSubview(makeImageView(systemImage: "globe"))
        setLayout(HStack())
    }
    
    override func loadView() {
        view = adaptor
    }
    
    func setLayout(_ layout: any Layout, animated: Bool = false) {
        func _setLayout(_ layout: any Layout) {
            adaptor.layout = layout
            adaptor.layoutIfNeeded()
        }
        
        if animated {
            UIView.animate(withDuration: 0.3) { _setLayout(layout) }
        } else {
            _setLayout(layout)
        }
    }
}

struct TestVCRepresentable: UIViewControllerRepresentable {
    var layout: any Layout
    
    func makeUIViewController(context: Context) -> TestVC {
        .init()
    }
    
    func updateUIViewController(_ uiViewController: TestVC, context: Context) {
        uiViewController.setLayout(layout, animated: true)
    }
}