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UIKit 接口

SwiftUI 可与所有 Apple 平台上的现有 UI 框架无缝协作。例如我们可以在 SwiftUI view 中放置 UIKit view 和 view controllers，反之亦然。

本文将展现如何把地标从 home screen 中转换到包装 UIPageViewController 和 UIPageControl 的实例。我们将使用 UIPageViewController 显示 SwiftUI view 的轮播，并使用状态变量和绑定来协调整个 UI 中的数据升级。

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1. 创立表示 UIPageViewController 的 View

要在 SwiftUI 中表示 UIKit view 和 view controllers，我们需要创立遵循 UIViewRepresentable 和 UIViewControllerRepresentable 协议的类型。我们的自己设置类型创立和配置它们所代表的 UIKit 类型，而 SwiftUI 管理它们的生命周期并在需要时升级它们。

1.1 创立一个新的 SwiftUI view，命名为 PageViewController.swift ，公告遵循 UIViewControllerRepresentable 协议的 PageViewController 类型。

页面的 view controller 存储了 UIViewController 实例的数组。这些是在地标之间滚动的页面。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]}

接下增加 UIViewControllerRepresentable 协议的两个需求。

1.2 增加一个 makeUIViewController(context:) 方法，创立一个满足需求的 UIPageViewController 。

当 SwiftUI 准备好显示 view 时，它会调用此方法一次，而后管理 view controller 的生命周期。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        return pageViewController    }}

1.3 增加一个 updateUIViewController(_:context:) 方法，在其中调用 setViewControllers(_:direction:animated:) 来显示数组中的第一个 view controller。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        return pageViewController    }    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {        pageViewController.setViewControllers(            [controllers[0]], direction: .forward, animated: true)    }}

创立另一个 SwiftUI view 来显示我们的 UIViewControllerRepresentable view。

1.4 创立一个新的 SwiftUI view，命名为 PageView.swift，公告一个 PageViewController 作为子 view。

需要注意的是，泛型初始化方法接收一个 view 数组，并将每个 view 嵌套在 UIHostingController 中。 UIHostingController 是一个 UIViewController 的子类，表示 UIKit 上下文中的 SwiftUI view。

PageView.swift

import SwiftUIstruct PageView<Page: View>: View {    var viewControllers: [UIHostingController<Page>]    init(_ views: [Page]) {        self.viewControllers = views.map { UIHostingController(rootView: $0) }    }    var body: some View {        PageViewController(controllers: viewControllers)    }}struct PageView_Preview: PreviewProvider {    static var previews: some View {        PageView()    }}

1.5 升级 preview provider ，传入必要的 view 数组，之后预览就会开始工作。

PageView.swift

import SwiftUIstruct PageView<Page: View>: View {    var viewControllers: [UIHostingController<Page>]    init(_ views: [Page]) {        self.viewControllers = views.map { UIHostingController(rootView: $0) }    }    var body: some View {        PageViewController(controllers: viewControllers)    }}struct PageView_Preview: PreviewProvider {    static var previews: some View {        PageView(features.map { FeatureCard(landmark: $0) })            .aspectRatio(3/2, contentMode: .fit)    }}

1.6 在进行下一步之前，在 canvas 中固定 PageView 的预览，所有的操作都将发生在这个 view 上。

2. 创立 View Controller 的数据源

在几个简短的步骤中，我们已经做了很多工作：PageViewController 使用 UIPageViewController 从 SwiftUI view 中显示内容。现在启用滑动交互来从一个页面移动到另一个页面。

一个表示 UIKit view controller 的 SwiftUI view 可以定义 SwiftUI 管理的 Coordinator 类型，并将其作为表示 view 上下文的一部分提供。

2.1 在 PageViewController 中创立一个嵌套的 Coordinator 类。

SwiftUI 管理我们 UIViewControllerRepresentable 类型的 coordinator ，并在调用上面创立的方法时将其作为上下文的一部分提供。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        return pageViewController    }    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {        pageViewController.setViewControllers(            [controllers[0]], direction: .forward, animated: true)    }    class Coordinator: NSObject {        var parent: PageViewController        init(_ pageViewController: PageViewController) {            self.parent = pageViewController        }    }}

给 PageViewController 增加另外一个方法来创立 coordinator 。

SwiftUI 会在调用 makeUIViewController(context:) 方法之前调用 makeCoordinator() 方法，这样配置 view controller 时，我们可以访问 coordinator 对象。

我们可以用这个 coordinator 实现常见的 Cocoa 模式，例如代理商、数据源以及通过 target-action 响应客户事件。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    func makeCoordinator() -> Coordinator {        Coordinator(self)    }    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        return pageViewController    }    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {        pageViewController.setViewControllers(            [controllers[0]], direction: .forward, animated: true)    }    class Coordinator: NSObject {        var parent: PageViewController        init(_ pageViewController: PageViewController) {            self.parent = pageViewController        }    }}

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2.3 给 Coordinator 类型遵循 UIPageViewControllerDataSource 协议，并且实现两个必要方法。

这两个方法建立了 view controllers 之间的关系，因而我们可以在它们之间来回滑动。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    func makeCoordinator() -> Coordinator {        Coordinator(self)    }    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        return pageViewController    }    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {        pageViewController.setViewControllers(            [controllers[0]], direction: .forward, animated: true)    }    class Coordinator: NSObject, UIPageViewControllerDataSource {        var parent: PageViewController        init(_ pageViewController: PageViewController) {            self.parent = pageViewController        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerBefore viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index == 0 {                return parent.controllers.last            }            return parent.controllers[index - 1]        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerAfter viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index + 1 == parent.controllers.count {                return parent.controllers.first            }            return parent.controllers[index + 1]        }    }}

2.4 将 coordinator 作为数据源增加给 UIPageViewController 。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    func makeCoordinator() -> Coordinator {        Coordinator(self)    }    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        pageViewController.dataSource = context.coordinator        return pageViewController    }    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {        pageViewController.setViewControllers(            [controllers[0]], direction: .forward, animated: true)    }    class Coordinator: NSObject, UIPageViewControllerDataSource {        var parent: PageViewController        init(_ pageViewController: PageViewController) {            self.parent = pageViewController        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerBefore viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index == 0 {                return parent.controllers.last            }            return parent.controllers[index - 1]        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerAfter viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index + 1 == parent.controllers.count {                return parent.controllers.first            }            return parent.controllers[index + 1]        }    }}

2.5 打开实时预览并测试滑动交互。

3. 在 SwiftUI View 的状态中跟踪页面

要增加自己设置的 UIPageControl ，我们需要一种从 PageView 中跟踪当前页面的方法。

为此，我们将在 PageView 中公告一个 @State 属性，并传递一个 binding 给此属性，直到 PageViewController view。 PageViewController 升级 binding 来匹配可见页面。

3.1 给 PageViewController 增加一个 currentPage 的 binding 的属性。

除了公告 @Binding 属性外，还要升级对 setViewControllers(_:direction:animated:) 的调用，并传递 currentPage 的 binding 的值。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    @Binding var currentPage: Int    func makeCoordinator() -> Coordinator {        Coordinator(self)    }    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        pageViewController.dataSource = context.coordinator        return pageViewController    }    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {        pageViewController.setViewControllers(            [controllers[currentPage]], direction: .forward, animated: true)    }    class Coordinator: NSObject, UIPageViewControllerDataSource {        var parent: PageViewController        init(_ pageViewController: PageViewController) {            self.parent = pageViewController        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerBefore viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index == 0 {                return parent.controllers.last            }            return parent.controllers[index - 1]        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerAfter viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index + 1 == parent.controllers.count {                return parent.controllers.first            }            return parent.controllers[index + 1]        }    }}

3.2 在 PageView 中公告 @State 变量，并在创立子 PageViewController 时将 binding 传递给属性。

请记住使用 $ 语法创立用状态来存储值的 binding 。

PageView.swift

import SwiftUIstruct PageView<Page: View>: View {    var viewControllers: [UIHostingController<Page>]    @State var currentPage = 0    init(_ views: [Page]) {        self.viewControllers = views.map { UIHostingController(rootView: $0) }    }    var body: some View {        PageViewController(controllers: viewControllers, currentPage: $currentPage)    }}struct PageView_Preview: PreviewProvider {    static var previews: some View {        PageView(features.map { FeatureCard(landmark: $0) })            .aspectRatio(3/2, contentMode: .fit)    }}

3.3 通过更改 currentPage 的初始值，测试值能否通过 binding 传递给了 PageViewController 。

给 PageView 增加一个按钮，让页面 view controller 跳转到第二个 view。

PageView.swift

import SwiftUIstruct PageView<Page: View>: View {    var viewControllers: [UIHostingController<Page>]    @State var currentPage = 1    init(_ views: [Page]) {        self.viewControllers = views.map { UIHostingController(rootView: $0) }    }    var body: some View {        PageViewController(controllers: viewControllers, currentPage: $currentPage)    }}struct PageView_Preview: PreviewProvider {    static var previews: some View {        PageView(features.map { FeatureCard(landmark: $0) })            .aspectRatio(3/2, contentMode: .fit)    }}

3.4 增加带有 currentPage 属性的 text view，以便我们关注 @State 属性的值。

需要注意的是，当从一个页面滑动到另一个页面时，该值不会改变。

PageView.swift

import SwiftUIstruct PageView<Page: View>: View {    var viewControllers: [UIHostingController<Page>]    @State var currentPage = 0    init(_ views: [Page]) {        self.viewControllers = views.map { UIHostingController(rootView: $0) }    }    var body: some View {        VStack {            PageViewController(controllers: viewControllers, currentPage: $currentPage)            Text("Current Page: \(currentPage)")        }    }}struct PageView_Preview: PreviewProvider {    static var previews: some View {        PageView(features.map { FeatureCard(landmark: $0) })    }}

3.5 在 PageViewController.swift 中，让 coordinator 遵循 UIPageViewControllerDelegate 协议，而后增加 pageViewController(_:didFinishAnimating:previousViewControllers:transitionCompleted completed: Bool) 方法。

只需页面切换动画完成，SwiftUI 就会调用此方法，所以我们可以找到当前 view controller 的索引并升级 binding 。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    @Binding var currentPage: Int    func makeCoordinator() -> Coordinator {        Coordinator(self)    }    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        pageViewController.dataSource = context.coordinator        return pageViewController    }    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {        pageViewController.setViewControllers(            [controllers[currentPage]], direction: .forward, animated: true)    }    class Coordinator: NSObject, UIPageViewControllerDataSource, UIPageViewControllerDelegate {        var parent: PageViewController        init(_ pageViewController: PageViewController) {            self.parent = pageViewController        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerBefore viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index == 0 {                return parent.controllers.last            }            return parent.controllers[index - 1]        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerAfter viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index + 1 == parent.controllers.count {                return parent.controllers.first            }            return parent.controllers[index + 1]        }        func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {            if completed,                let visibleViewController = pageViewController.viewControllers?.first,                let index = parent.controllers.firstIndex(of: visibleViewController)            {                parent.currentPage = index            }        }    }}

3.6 除数据源外，还将 coordinator 指定为 UIPageViewController 的代理商。

在两个方向上连接 binding 后，text view 会在每次滑动后升级以显示正确的页码。

PageViewController.swift

import SwiftUIimport UIKitstruct PageViewController: UIViewControllerRepresentable {    var controllers: [UIViewController]    @Binding var currentPage: Int    func makeCoordinator() -> Coordinator {        Coordinator(self)    }    func makeUIViewController(context: Context) -> UIPageViewController {        let pageViewController = UIPageViewController(            transitionStyle: .scroll,            navigationOrientation: .horizontal)        pageViewController.dataSource = context.coordinator        pageViewController.delegate = context.coordinator        return pageViewController    }    func updateUIViewController(_ pageViewController: UIPageViewController, context: Context) {        pageViewController.setViewControllers(            [controllers[currentPage]], direction: .forward, animated: true)    }    class Coordinator: NSObject, UIPageViewControllerDataSource, UIPageViewControllerDelegate {        var parent: PageViewController        init(_ pageViewController: PageViewController) {            self.parent = pageViewController        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerBefore viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index == 0 {                return parent.controllers.last            }            return parent.controllers[index - 1]        }        func pageViewController(            _ pageViewController: UIPageViewController,            viewControllerAfter viewController: UIViewController) -> UIViewController?        {            guard let index = parent.controllers.firstIndex(of: viewController) else {                return nil            }            if index + 1 == parent.controllers.count {                return parent.controllers.first            }            return parent.controllers[index + 1]        }        func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {            if completed,                let visibleViewController = pageViewController.viewControllers?.first,                let index = parent.controllers.firstIndex(of: visibleViewController)            {                parent.currentPage = index            }        }    }}

4. 增加自己设置的 Page Control

现在我们已经准备好给 view 增加自己设置的包装在 SwiftUI UIViewRepresentable 中的 UIPageControl 了。

4.1 创立一个新的 SwiftUI view 文件，命名为 PageControl.swift 。让 PageControl 遵循 UIViewRepresentable 协议。

UIViewRepresentable 和 UIViewControllerRepresentable 类型拥有相同的生命周期，其方法与其基础 UIKit 类型相对应。

PageControl.swift

import SwiftUIimport UIKitstruct PageControl: UIViewRepresentable {    var numberOfPages: Int    @Binding var currentPage: Int    func makeUIView(context: Context) -> UIPageControl {        let control = UIPageControl()        control.numberOfPages = numberOfPages        return control    }    func updateUIView(_ uiView: UIPageControl, context: Context) {        uiView.currentPage = currentPage    }}

4.2 将 text box 换成 page control，把布局从 VStack 换成 ZStack 。

由于我们正在将页面计数和 binding 传递给当前页面，所以 page control 已显示正确的值。

PageView.swift

import SwiftUIstruct PageView<Page: View>: View {    var viewControllers: [UIHostingController<Page>]    @State var currentPage = 0    init(_ views: [Page]) {        self.viewControllers = views.map { UIHostingController(rootView: $0) }    }    var body: some View {        ZStack(alignment: .bottomTrailing) {            PageViewController(controllers: viewControllers, currentPage: $currentPage)            PageControl(numberOfPages: viewControllers.count, currentPage: $currentPage)                .padding(.trailing)        }    }}struct PageView_Preview: PreviewProvider {    static var previews: some View {        PageView(features.map { FeatureCard(landmark: $0) })    }}

接下来让 page control 可以交互，以便客户可以点击一侧或者另一侧在页面之间移动。

4.3 在 PageControl 中创立嵌套的 Coordinator 类型，而后增加一个 Coordinator() 方法来创立并返回一个新的 coordinator 。

因为 UIPageControl 这样的 UIControl 子类使用 arget-action 模式而不是代理商，所以此 Coordinator 实现了 @objc 方法来升级当前页面的 binding 。

PageControl.swift

import SwiftUIimport UIKitstruct PageControl: UIViewRepresentable {    var numberOfPages: Int    @Binding var currentPage: Int    func makeCoordinator() -> Coordinator {        Coordinator(self)    }    func makeUIView(context: Context) -> UIPageControl {        let control = UIPageControl()        control.numberOfPages = numberOfPages        return control    }    func updateUIView(_ uiView: UIPageControl, context: Context) {        uiView.currentPage = currentPage    }    class Coordinator: NSObject {        var control: PageControl        init(_ control: PageControl) {            self.control = control        }        @objc func updateCurrentPage(sender: UIPageControl) {            control.currentPage = sender.currentPage        }    }}

4.4 增加 coordinator 作为 .valueChanged 事件的目标，将 updateCurrentPage(sender:) 方法指定为要执行的操作。

PageControl.swift

import SwiftUIimport UIKitstruct PageControl: UIViewRepresentable {    var numberOfPages: Int    @Binding var currentPage: Int    func makeCoordinator() -> Coordinator {        Coordinator(self)    }    func makeUIView(context: Context) -> UIPageControl {        let control = UIPageControl()        control.numberOfPages = numberOfPages        control.addTarget(            context.coordinator,            action: #selector(Coordinator.updateCurrentPage(sender:)),            for: .valueChanged)        return control    }    func updateUIView(_ uiView: UIPageControl, context: Context) {        uiView.currentPage = currentPage    }    class Coordinator: NSObject {        var control: PageControl        init(_ control: PageControl) {            self.control = control        }        @objc func updateCurrentPage(sender: UIPageControl) {            control.currentPage = sender.currentPage        }    }}

4.5 现在来尝试所有不同的交互， PageView 展现了 UIKit 和 SwiftUI view 和 controllers 是如何协同工作的。