通过 SwiftUI App 设置时，Shared AppDelegate 为 nil

Question

I need to get access to a global object. I used to rely on the UIApplication.shared.delegate to reference any global variables I stored there. Now, the new SwiftUI way has taken that from me. In leu of a better global object I would like to still use the UIApplicationDelegate . I found a few tutorials on the web that said to set it up this way.

I make the app delegate like this.

class AppDelegate: NSObject, UIApplicationDelegate {
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
        print("Your code here")
        return true
    }
}

And I set it like this.

@main
struct MyApp: App {
    
    @UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate
    
    var body: some Scene {
        WindowGroup {
            RootView()
        }
    }
}

However when I access the app delegate like this I hit the fatalError

guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
    fatalError("No App Delegate")
}

How can I access the AppDelegate globally now? Is there an alternative for a truly static global variable in the App now?

Answer 1

In terms of using a static global variable, you could define a singleton instance yourself:

class SharedObject : ObservableObject{
    static var shared = SharedObject() //static instance
    @Published var string = "Test"
}

struct ContentView : View {
    var body: some View {
        Text("Hello, world")
        Text("From shared: \(SharedObject.shared.string)")
    }
}

If you just need it within the view hierarchy, you could make use of something like .environmentObject :

class SharedObject : ObservableObject{
    @Published var string = "Test"
}

struct ContentView : View {
    @StateObject var sharedObj = SharedObject()
    
    var body: some View {
        ChildView().environmentObject(SharedObject())
    }
}

struct ChildView : View {
    @EnvironmentObject private var sharedObj : SharedObject
    
    var body: some View {
        Text(sharedObj.string)
    }
}

You may be aware of this already, but it's worth pointing out that there's a lot of skepticism about using global static shared objects. It makes testing harder, it may make things complicated for having multiple window support in iOS, etc. Just something to think about when deciding whether this is the right architectural choice for your app.