A SwiftUI view that gives subviews access to a UIKit view controller.

ViewControllerReader.swift

//
//  ViewControllerReader.swift
//
//  Created by John Scott on 28/05/2026.
//

import SwiftUI

/// A view that gives subviews access to a UIKit view controller.
///
/// ⚠️ Do use this unless you absolutely have to; no good can come from treating
/// this a normal thing to do.
///
/// Note that due to the order of execution the nil UIController used in
/// `makeUIViewController(context:)` is never actually used. Below is the order of
/// execution:
///
/// `ViewControllerReader.updateUIViewController(_:context:)` >
/// `ViewControllerReader.updateUIViewController(_:context:)` <
/// `ViewControllerReader.sizeThatFits(_:uiViewController:context:)` >
/// `ContentView.body` >
/// `ContentView.body` <
/// `ViewControllerReader.sizeThatFits(_:uiViewController:context:)` <
///
@available(iOS, introduced: 13.0)
@available(tvOS, introduced: 13.0)
@available(macOS, unavailable)
@available(watchOS, unavailable)
struct ViewControllerReader<Content>: UIViewControllerRepresentable where Content: View {

    @ViewBuilder var content: (UIViewController) -> Content

    struct ContentView: View {
        let uiViewController: UIViewController?
        let content: (UIViewController) -> Content

        var body: some View {
            if let uiViewController {
                content(uiViewController)
            }
        }
    }

    func makeUIViewController(context: Context) -> UIHostingController<ContentView> {
        UIHostingController(rootView: ContentView(uiViewController: nil, content: content))
    }

    func updateUIViewController(_ uiViewController: UIHostingController<ContentView>, context: Context) {
        uiViewController.rootView = ContentView(uiViewController: uiViewController, content: content)
        uiViewController.view.isUserInteractionEnabled = context.environment.isEnabled
    }

    func sizeThatFits(_ proposal: ProposedViewSize, uiViewController: UIHostingController<ContentView>, context: Context) -> CGSize? {
        uiViewController.sizeThatFits(in: proposal.replacingUnspecifiedDimensions())
    }
}

#Preview {
    ViewControllerReader { viewController in
        Button("Hello, world!") {
            let alert = UIAlertController(title: "Hello", message: "World", preferredStyle: .alert)
            alert.addAction(
                UIAlertAction(title: "OK", style: .default) { _ in }
            )
            viewController.present(alert, animated: true)
        }
    }
}