当使用NavigationLink，@ EnvironmentObject和List时，Xcode 11 beta 3崩溃

Question

I've got a strange crash in SwiftUI / Xcode 11 beta 3 with code like the one below (I've kept only the bare minimum to show the behavior):

import SwiftUI
import Combine

final class AppData: BindableObject  {
    let didChange = PassthroughSubject<AppData, Never>()

    init() { }
}

struct ContentView : View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: DetailView() ) {
                Text("link")
            }
        }
    }
}

struct DetailView : View {
    @EnvironmentObject var appData: AppData
//  @ObjectBinding var appData = AppData() -> Works 

    var body: some View {
        List {
            Text("A")
            Text("B")
            Text("C")
        }
    }
}

The BindableObject is injected in SceneDelegate.swift like this:

....
        // Use a UIHostingController as window root view controller
        if let windowScene = scene as? UIWindowScene {
            let window = UIWindow(windowScene: windowScene)
            window.rootViewController = UIHostingController(rootView: ContentView()
                                           .environmentObject(AppData()))
            self.window = window
            window.makeKeyAndVisible()
        }
....

When following the NavigationLink it crashes with

Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

If I remove the List view from the detail view it works OK. The same if I use @ObjectBinding instead (like in the commented line in my code).

The same code used to work in previous betas.

Answer 1

It's a bug in Xcode 11 beta 3. The old behaviour will likely return.

From https://developer.apple.com/tutorials/swiftui/handling-user-input as of July 4th 2019:

Step 4

In Xcode 11 beta 3, the LandmarkDetail view doesn't automatically access the UserData object in the view hierarchy's environment. The workaround for this is to add the environmentObject(_:) modifier to the LandmarkDetail view.

Answer 2

I think that this is by design. When you create DetailView(), it is disconnected from the hierarchy, and hence it does not inherit the same environment.

If you change your ContentView to the following, it won't crash. I think I remember having a similar problem with modals:

struct ContentView : View {
    @EnvironmentObject var appData: AppData

    var body: some View {
        NavigationView {
            NavigationLink(destination: DetailView().environmentObject(appData) ) {
                Text("link")
            }
        }
    }
}