我正在尝试在适用于 iOS 的 SwiftUI 应用程序中实现撤消，但我无法让撤消手势正常工作。这是一个演示问题的示例:

class Model: ObservableObject {
    @Published var active = false

    func registerUndo(_ newValue: Bool, in undoManager: UndoManager?) {
        let oldValue = active
        undoManager?.registerUndo(withTarget: self) { target in
            target.active = oldValue
        }
        active = newValue
    }
}

struct UndoTest: View {
    @ObservedObject private var model = Model()
    @Environment(\.undoManager) var undoManager

    var body: some View {
        VStack {
            Toggle(isOn: Binding<Bool>(
                get: { self.model.active },
                set: { self.model.registerUndo($0, in: self.undoManager) }
            )) {
                Text("Toggle")
            }
            .frame(width: 120)
            Button(action: {
                self.undoManager?.undo()
            }, label: {
                Text("Undo")
                    .foregroundColor(.white)
                    .padding()
                    .background(self.undoManager?.canUndo == true ? Color.blue : Color.gray)
            })
        }
    }
}

切换开关然后点击撤消按钮可以正常工作。使用三指撤消手势或摇动撤消不会执行任何操作。你如何配合系统手势？

最佳答案

似乎编辑手势需要窗口有第一响应者，而 SwiftUI 没有设置 UIWindow 默认情况下想要选择的任何东西作为第一响应者。

如果您子类化 UIHostingController，并且在您的子类中，您覆盖 canBecomeFirstResponder 以返回 true，然后 UIWindow 默认情况下会将您的 Controller 设置为第一响应者，这似乎足以启用编辑手势。

我在运行 iPadOS 13.1 beta 2 (17A5831c) 的 iPad Pro 上测试了以下代码。它主要有效。我相信有一个 iOS 错误，可能在较新的测试版中已修复:当撤消堆栈为空时，手势有时不起作用(即使可以进行重做操作)。切换到主屏幕然后返回测试应用程序(不终止测试应用程序)似乎使编辑手势再次起作用。

import UIKit
import SwiftUI

class MyHostingController<Content: View>: UIHostingController<Content> {
    override var canBecomeFirstResponder: Bool { true }
}

class Model: ObservableObject {
    init(undoManager: UndoManager) {
        self.undoManager = undoManager
    }

    let undoManager: UndoManager

    @Published var active = false {
        willSet {
            let oldValue = active
            undoManager.registerUndo(withTarget: self) { me in
                me.active = oldValue
            }
        }
    }
}

struct ContentView: View {
    @ObservedObject var model: Model
    @Environment(\.undoManager) var undoManager

    var body: some View {
        VStack {
            Toggle("Active", isOn: $model.active)
                .frame(width: 120)
            HStack {
                Button("Undo") {
                    withAnimation {
                        self.undoManager?.undo()
                    }
                }.disabled(!(undoManager?.canUndo ?? false))
                Button("Redo") {
                    withAnimation {
                        self.undoManager?.redo()
                    }
                }.disabled(!(undoManager?.canRedo ?? false))
            }
        }
    }
}

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let scene = scene as? UIWindowScene else { return }

        let window = UIWindow(windowScene: scene)
        let model = Model(undoManager: window.undoManager!)
        let contentView = ContentView(model: model)

        window.rootViewController = MyHostingController(rootView: contentView)
        self.window = window
        window.makeKeyAndVisible()
    }
}

关于ios - SwiftUI 和三指撤消手势，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/57931860/