Cannot preview in this file - [App Name].app may have crashed on Xcode 11 Beta 5

Question

The Xcode preview does not work if i add a EnviromentObject property wrapper. Everytime i add one the Canvas doesn't build and i get this error:

Cannot preview in this file - [App Name].app may have crashed

If i replace the EnviromentObject property wrapper with ObservedObject and initialize it everything works fine.

Here's my code:

class NetworkManager: ObservableObject {

}

struct ContentView : View {
    @EnvironmentObject var networkManager: NetworkManager

    var body: some View {
        Text("Canvas not working")
    }
}

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

Update:

It doesn’t load the preview as well when i am using a binding:

struct ContentView : View {
    @EnvironmentObject var networkManager: NetworkManager
    @Binding var test123: String

    var body: some View {
        Text("Canvas not working")
    }
}

#if DEBUG
struct ContentView_Previews: PreviewProvider {
    @State static var test1 = ""
    static var previews: some View {
        ContentView(test123: $test1).environmentObject(NetworkManager())
    }
}
#endif

Answer 1

I'm assuming based on the code you provided that your SceneDelegate looks like this:

if let windowScene = scene as? UIWindowScene {
    let window = UIWindow(windowScene: windowScene)
    window.rootViewController = UIHostingController(rootView: ContentView())
    self.window = window
    window.makeKeyAndVisible()
}

I'm not going to pretend I know exactly what the canvas is doing behind the scenes when it generates a preview, but based on the fact that the error specifically states that the app may have crashed, I'm assuming that it's attempting to launch the entire app when it tries to generate a preview. Maybe it needs to use the SceneDelegate to launch the preview, maybe it's something else entirely - I can't say for sure.

Regardless, the reason the app is crashing is because you aren't passing an environment object in your SceneDelegate. Your SceneDelegate should look like this:

if let windowScene = scene as? UIWindowScene {
    let window = UIWindow(windowScene: windowScene)
    window.rootViewController = UIHostingController(rootView: ContentView().environmentObject(NetworkManager()))
    self.window = window
    window.makeKeyAndVisible()
}