Generic structures to host previews of UIView and UIViewController subclasses.

Previewing_ViewControllers_DifferentDevices.swift

#if canImport(SwiftUI) && DEBUG
import SwiftUI

let deviceNames: [String] = [
    "iPhone SE",
    "iPad 11 Pro Max",
    "iPad Pro (11-inch)"
]

@available(iOS 13.0, *)
struct ViewController_Preview: PreviewProvider {
  static var previews: some View {
    ForEach(deviceNames, id: \.self) { deviceName in
      UIViewControllerPreview {
        UIStoryboard(name: "Main", bundle: nil)
            .instantiateInitialViewController { coder in
            ViewController(coder: coder)
        }!
      }.previewDevice(PreviewDevice(rawValue: deviceName))
        .previewDisplayName(deviceName)
    }
  }
}
#endif

PreviewingMultipleStates.swift

Group {
  UIViewPreview {
    let button = FavoriteButton(frame: .zero)
    return button
  }
  UIViewPreview {
    let button = FavoriteButton(frame: .zero)
    button.isFavorited = true
    return button
  }
}.previewLayout(.sizeThatFits)
 .padding(10)

UIViewControllerPreview.swift

import UIKit

#if canImport(SwiftUI) && DEBUG
import SwiftUI

@available(iOS 13.0, *)
struct UIViewControllerPreview<ViewController: UIViewController>: UIViewControllerRepresentable {
    let viewController: ViewController

    init(_ builder: @escaping () -> ViewController) {
        viewController = builder()
    }

    // MARK: - UIViewControllerRepresentable

    func makeUIViewController(context: Context) -> ViewController {
        viewController
    }

    func updateUIViewController(_ uiViewController: ViewController, context: UIViewControllerRepresentableContext<UIViewControllerPreview<ViewController>>) {
        return
    }
}
#endif

UIViewPreview.swift

import UIKit

#if canImport(SwiftUI) && DEBUG
import SwiftUI

@available(iOS 13.0, *)
struct UIViewPreview<View: UIView>: UIViewRepresentable {
    let view: View

    init(_ builder: @escaping () -> View) {
        view = builder()
    }

    // MARK: - UIViewRepresentable

    func makeUIView(context: Context) -> UIView {
        return view
    }

    func updateUIView(_ view: UIView, context: Context) {
        view.setContentHuggingPriority(.defaultHigh, for: .horizontal)
        view.setContentHuggingPriority(.defaultHigh, for: .vertical)
    }
}
#endif