{"pageProps":{"contents":[{"slug":"swiftui-preview-with-coredata","title":"SwiftUI Preview with CoreData","description":"The SwiftUI preview errors when using CoreData managed object context.","content":"\n## Preview Error\n\nWhen you're using CoreData in a SwiftUI `View`, you'll get erros when resuming the preview with the following code.\n\n```swift\nstruct ContentView: View {\n  @Environment(\\.managedObjectContext) var moc\n\n  var body: some View {\n    Text(\"hello World\")\n  }\n}\n\nstruct ContentView_Previews: PreviewProvider {\n  static var previews: some View {\n    ContentView()\n  }\n}\n\n```\n\nThe reason is that your preview `ContentView()` does not have a managed object context.\n\n## Solution\n\nOne 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.\n\n```swift\nstruct PreviewCoreDataWrapper<Content: View>: View {\n  let content: (NSManagedObjectContext) -> Content\n\n  var body: some View {\n    let managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext\n\n    let todo = Todo(context: managedObjectContext)\n    todo.title = \"I Am Legend\"\n\n    return self.content(managedObjectContext)\n  }\n\n  init(@ViewBuilder content: @escaping (NSManagedObjectContext) -> Content) {\n    self.content = content\n  }\n}\n\n```\n\nNow, you can embed every view which needs access to the managed object context within the `PreviewCoreDataWrapper` view\nand access the `managedObjectContext` to pass it into the enviroment.\n\n```swift\nstruct MoviesView_Previews: PreviewProvider {\n  static var previews: some View {\n    PreviewCoreDataWrapper { managedObjectContext in\n      MoviesView().environment(\\.managedObjectContext, managedObjectContext)\n    }\n  }\n}\n```\n\nNow you can resume your preview and it should render just fine without any errors.\n\n---\n\n<small>\n  If you have feedback or questions, please feel free to get in touch via Twitter.\n</small>\n","type":"tip","tags":["swiftui","coredata","preview"],"publishedAt":"2020-04-26T18:06:07.227Z","image":null,"updatedAt":null,"readingTime":{"text":"2 min read","minutes":1.125,"time":67500,"words":225},"data":{"title":"SwiftUI Preview with CoreData","description":"The SwiftUI preview errors when using CoreData managed object context.","tags":["swiftui","coredata","preview"],"publishedAt":"2020-04-26T18:06:07.227Z","updatedAt":null,"type":"tip"}}],"tag":"preview"},"__N_SSG":true}