利用SwiftUI的预览功能来预览UIViewController.swift

import SwiftUI
import UIKit

/// 创建一个SwiftUI的容器View，将UIViewControler包含在内，即可预览
///
/// 如果要预览这个ViewController
///
/// ```swift
/// class ViewController: UIViewController {
///     override func viewDidLoad() {
///         super.viewDidLoad()
///         // Do any additional setup after loading the view.
///     }
/// }
/// ```
/// 使用的时候就可以像这样
///
/// ```swift
/// struct ViewController_Previews: PreviewProvider {
///     static var previews: some View {
///         UIViewControllerPreviewWrapper {
///             // 1
///             let storyboard = UIStoryboard(name: "Main", bundle: /// nil)
///             guard let viewController = /// storyboard.instantiateInitialViewController() as? /// ViewController
///             else {
///                 fatalError("Cannot load ViewController from Main /// storyboard.")
///             }
///
///             return viewController
///         }
///     }
/// }
/// ```
struct UIViewControllerPreviewWrapper<T: UIViewController>: UIViewControllerRepresentable {
    // 2
    let viewController: T

    // 3
    init(_ viewControllerBuilder: @escaping () -> T) {
        viewController = viewControllerBuilder()
    }

    // MARK: - UIViewControllerRepresentable

    func makeUIViewController(context: Context) -> T {
        // 4
        return viewController
    }

    func updateUIViewController(_ uiViewController: T, context: Context) {}
}

/// 预览 UIView
struct UIViewPreviewWrapper<T: UIView>: UIViewRepresentable {
    typealias UIViewType = UIView

    let view: T
    init(_ viewBuilder: @escaping () -> T) {
        view = viewBuilder()
    }

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

    func updateUIView(_ uiView: UIView, context: Context) {}
}