如何在 macOS 的 SwiftUI 中使用菜单命令实现多窗口？

情况

实现一个多窗口应用程序，其中每个窗口都有自己的状态。

Example

这是一个例子（在 github 上）来展示问题：

import SwiftUI

@main
struct multi_window_menuApp: App {

  var body: some Scene {
    WindowGroup {
      ContentView()
    }.commands {
      MenuCommands()
    }
  }
}

struct ContentView: View {
  @StateObject var viewModel: ViewModel  = ViewModel()
  
  var body: some View {
    TextField("", text: $viewModel.inputText)
      .disabled(true)
      .padding()
  }
}

public class ViewModel: ObservableObject {
  
  @Published var inputText: String = "" {
    didSet {
      print("content was updated...")
    }
  }
}

Question

我们应该如何以编程方式找出当前选定的视图是什么，以便我们可以在菜单命令即将完成时更新状态并更新视图模型中的状态？

import Foundation
import SwiftUI
import Combine

struct MenuCommands: Commands {
  
  var body: some Commands {
    CommandGroup(after: CommandGroupPlacement.newItem, addition: {
      Divider()
      Button(action: {
        let dialog = NSOpenPanel();
        
        dialog.title = "Choose a file";
        dialog.showsResizeIndicator = true;
        dialog.showsHiddenFiles = false;
        dialog.allowsMultipleSelection = false;
        dialog.canChooseDirectories = false;
        
        if (dialog.runModal() ==  NSApplication.ModalResponse.OK) {
          let result = dialog.url
          if (result != nil) {
            let path: String = result!.path
            do {
              let string = try String(contentsOf: URL(fileURLWithPath: path), encoding: .utf8)
              print(string)
              // how to get access to the currently active view model to update the inputText variable?
              // viewModel.inputText = string
            }
            catch {
              print("Error \(error)")
            }
          }
        } else {
          return
        }
      }, label: {
        Text("Open File")
      })
      .keyboardShortcut("O", modifiers: .command)
    })
  }
}

可能有助于解决这个问题的链接：

• http://www.gfrigerio.com/build-a-macos-app-with-swiftui/
• https://troz.net/post/2021/swiftui_mac_menus/
• https://onmyway133.com/posts/how-to-manage-windowgroup-in-swiftui-for-macos/

我在解决类似问题时遇到了这个问题。我相信 SwiftUI 的方法是使用FocusedValue:

// create an active viewmodel key
struct ActiveViewModelKey: FocusedValueKey {
  typealias Value = ViewModel
}

extension FocusedValues {
  var activeViewModel: ViewModel? {
    get { self[ActiveViewModelKey.self] }
    set { self[ActiveViewModelKey.self] = newValue }
  }
}

struct ContentView: View {
  @StateObject var viewModel: ViewModel  = ViewModel()
  
  var body: some View {
    TextField("", text: $viewModel.inputText)
      ...
      .focusedSceneValue(\.activeViewModel, viewModel) // inject the focused value
  }
}

struct MenuCommands: Commands {
  @FocusedValue(\.activeViewModel) var activeViewModel // inject the active viewmodel
  
  var body: some Commands {
    CommandGroup(after: CommandGroupPlacement.newItem, addition: {
      Divider()
      Button(action: {
        ...
        activeViewModel?.inputText = string
      }, label: {
        Text("Open File")
      })
      .keyboardShortcut("O", modifiers: [.command])
    })
  }
}