Preview Error

When you're using CoreData in a SwiftUI View, you'll get erros when resuming the preview with the following code.

struct ContentView: View {
  @Environment(\.managedObjectContext) var moc

  var body: some View {
    Text("hello World")
  }
}

struct ContentView_Previews: PreviewProvider {
  static var previews: some View {
    ContentView()
  }
}

The reason is that your preview ContentView() does not have a managed object context.

Solution

One solution I found is to create a wrapper view which adds test data to the managed object context and passes it back to the view.

struct PreviewCoreDataWrapper<Content: View>: View {
  let content: (NSManagedObjectContext) -> Content

  var body: some View {
    let managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext

    let todo = Todo(context: managedObjectContext)
    todo.title = "I Am Legend"

    return self.content(managedObjectContext)
  }

  init(@ViewBuilder content: @escaping (NSManagedObjectContext) -> Content) {
    self.content = content
  }
}

Now, you can embed every view which needs access to the managed object context within the PreviewCoreDataWrapper view and access the managedObjectContext to pass it into the enviroment.

struct MoviesView_Previews: PreviewProvider {
  static var previews: some View {
    PreviewCoreDataWrapper { managedObjectContext in
      MoviesView().environment(\.managedObjectContext, managedObjectContext)
    }
  }
}

Now you can resume your preview and it should render just fine without any errors.

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