How to present a view full-screen in SwiftUI?

I do not want the user to have the possibility to return to the login view

In that case you shouldn't be presenting away from the login view but replacing it entirely.

You could do this by conditionally building the login view or the "app view".

Something like this...

// create the full screen login view
struct LoginView: View {
    // ...
}

//create the full screen app veiw
struct AppView: View {
    // ...
}

// create the view that swaps between them
struct StartView: View {
    @EnvironmentObject var isLoggedIn: Bool // you might not want to use this specifically.

    var body: some View {
        isLoggedIn ? AppView() : LoginView()
    }
}

By using a pattern like this you are not presenting or navigating away from the login view but you are replacing it entirely so it is no longer in the view hierarchy at all.

This makes sure that the user cannot navigate back to the login screen.

Equally... by using an @EnvironmentObject like this you can edit it later (to sign out) and your app will automatically be taken back to the login screen.

struct ContentView: View {
    @EnvironmentObject var userAuth: UserAuth 
    var body: some View {
        if !userAuth.isLoggedin {
            return AnyView(LoginView())
        } else {
            return AnyView(HomeView())
        }
    }
}

Encapsulate the body in a Group to eliminate compiler errors:

struct StartView: View {

@EnvironmentObject var userAuth: UserAuth

var body: some View {
    Group {
        if userAuth.isLoggedin {
            AppView()
        } else {
            LoginView()
        }

    }
}

I made this extension for myself. Any feedback/ideas are welcome. :)

https://github.com/klemenkosir/SwiftUI-FullModal

Usage

struct ContentView: View {

    @State var isPresented: Bool = false

    var body: some View {
        NavigationView {
            Button(action: {
                self.isPresented.toggle()
            }) {
                Text("Present")
            }
            .navigationBarTitle("Some title")
        }
        .present($isPresented, view: ModalView(isPresented: $isPresented))
    }
}

struct ModalView: View {

    @Binding var isPresented: Bool

    var body: some View {
        Button(action: {
            self.isPresented.toggle()
        }) {
            Text("Dismiss")
        }
    }
}