在 iPad 上的 SwiftUI 中呈现 ActionSheet

我已经得到了一个可以在 iPhone 设备上很好地呈现的 ActionSheet。但它在 iPad 上会崩溃。说它需要弹出窗口的位置。有人对这段代码感到幸运吗？我正在使用 iOS 13 beta 3 和 Xcode 11 beta 3。（这使用了 beta 2 中不可用的呈现 ActionSheet 的版本）

import SwiftUI

struct ContentView : View {
    @State var showSheet = false

    var body: some View {
        VStack {
            Button(action: {
                self.showSheet.toggle()
            }) {
                Text("Show")
            }
            .presentation($showSheet) { () -> ActionSheet in
                ActionSheet(title: Text("Hello"))

            }
        }
    }
}

#if DEBUG
struct ContentView_Previews : PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}
#endif

遗憾的是，这个 bug 尚未在 iOS 13 最终版本中得到修复。开发者论坛 https://forums.developer.apple.com/message/389028#389028，我已经提交了反馈（FB7397761），但暂时需要通过使用其他 UI 来解决这个问题UIDevice.current.userInterfaceIdiom == .pad.

根据记录，（无用的）异常消息是：

2019-10-21 11:26:58.205533-0400 LOOksTape[34365:1769883] *** Terminating app due to uncaught exception 'NSGenericException', reason: 'Your application has presented a UIAlertController (<UIAlertController: 0x7f826e094a00>) of style UIAlertControllerStyleActionSheet from _TtGC7SwiftUI19UIHostingController… 
The modalPresentationStyle of a UIAlertController with this style is UIModalPresentationPopover. 
You must provide location information for this popover through the alert controller's popoverPresentationController. 
You must provide either a sourceView and sourceRect or a barButtonItem.  
If this information is not known when you present the alert controller, you may provide it in the UIPopoverPresentationControllerDelegate method -prepareForPopoverPresentation.'

作为解决方法，这popSheet函数将在 iPad 上显示一个弹出框并显示ActionSheet其他地方：

public extension View {
    /// Creates an `ActionSheet` on an iPhone or the equivalent `popover` on an iPad, in order to work around `.actionSheet` crashing on iPad (`FB7397761`).
    ///
    /// - Parameters:
    ///     - isPresented: A `Binding` to whether the action sheet should be shown.
    ///     - content: A closure returning the `PopSheet` to present.
    func popSheet(isPresented: Binding<Bool>, arrowEdge: Edge = .bottom, content: @escaping () -> PopSheet) -> some View {
        Group {
            if UIDevice.current.userInterfaceIdiom == .pad {
                popover(isPresented: isPresented, attachmentAnchor: .rect(.bounds), arrowEdge: arrowEdge, content: { content().popover(isPresented: isPresented) })
            } else {
                actionSheet(isPresented: isPresented, content: { content().actionSheet() })
            }
        }
    }
}

/// A `Popover` on iPad and an `ActionSheet` on iPhone.
public struct PopSheet {
    let title: Text
    let message: Text?
    let buttons: [PopSheet.Button]

    /// Creates an action sheet with the provided buttons.
    public init(title: Text, message: Text? = nil, buttons: [PopSheet.Button] = [.cancel()]) {
        self.title = title
        self.message = message
        self.buttons = buttons
    }

    /// Creates an `ActionSheet` for use on an iPhone device
    func actionSheet() -> ActionSheet {
        ActionSheet(title: title, message: message, buttons: buttons.map({ popButton in
            // convert from PopSheet.Button to ActionSheet.Button (i.e., Alert.Button)
            switch popButton.kind {
            case .default: return .default(popButton.label, action: popButton.action)
            case .cancel: return .cancel(popButton.label, action: popButton.action)
            case .destructive: return .destructive(popButton.label, action: popButton.action)
            }
        }))
    }

    /// Creates a `.popover` for use on an iPad device
    func popover(isPresented: Binding<Bool>) -> some View {
        VStack {
            ForEach(Array(buttons.enumerated()), id: \.offset) { (offset, button) in
                Group {
                    SwiftUI.Button(action: {
                        // hide the popover whenever an action is performed
                        isPresented.wrappedValue = false
                        // another bug: if the action shows a sheet or popover, it will fail unless this one has already been dismissed
                        DispatchQueue.main.async {
                            button.action?()
                        }
                    }, label: {
                        button.label.font(.title)
                    })
                    Divider()
                }
            }
        }
    }

    /// A button representing an operation of an action sheet or popover presentation.
    ///
    /// Basically duplicates `ActionSheet.Button` (i.e., `Alert.Button`).
    public struct Button {
        let kind: Kind
        let label: Text
        let action: (() -> Void)?
        enum Kind { case `default`, cancel, destructive }

        /// Creates a `Button` with the default style.
        public static func `default`(_ label: Text, action: (() -> Void)? = {}) -> Self {
            Self(kind: .default, label: label, action: action)
        }

        /// Creates a `Button` that indicates cancellation of some operation.
        public static func cancel(_ label: Text, action: (() -> Void)? = {}) -> Self {
            Self(kind: .cancel, label: label, action: action)
        }

        /// Creates an `Alert.Button` that indicates cancellation of some operation.
        public static func cancel(_ action: (() -> Void)? = {}) -> Self {
            Self(kind: .cancel, label: Text("Cancel"), action: action)
        }

        /// Creates an `Alert.Button` with a style indicating destruction of some data.
        public static func destructive(_ label: Text, action: (() -> Void)? = {}) -> Self {
            Self(kind: .destructive, label: label, action: action)
        }
    }
}