No Observable Object cover image

No Observable Object

David Kanenwisher • May 5, 2020


I ran into some trouble after I opened up the app I've been toying with in Xcode. The SwiftUI preview wouldn't render. I clicked "Diagnostics" and it said:

|  Error Code=12 "Rendering service was interrupted" UserInfo={NSLocalizedDescription=Rendering service was interrupted}

I have no idea what the ultraviolet service is, what it means that the code is 12, and it seems pretty clear the "Rendering service was interrupted". I did manage to find some more information by looking real close.

Depending on the size of your screen, on mine it was rather small, there's a little "i" with a circle around after the crashed message. I clicked that then clicked "Show Crash Logs".

After scrolling through a bunch of information I don't understand I struck gold. I saw something I could understand:

Fatal error: No ObservableObject of type AppData found. A View.environmentObject(_:) for AppData may be missing as an ancestor of this view.: file SwiftUI, line 0

Thankfully, I'd recently gained a higher understanding of ObservableObjects plus AppData is a name of a variable in my project. Thankfully and reasonably, AppData is an ObservableObject. How do I know it's an ObservableObject? Why I made it implement ObservableObject protocol:

import SwiftUI

class AppData: ObservableObject {
    @Published var imageCache = ImageCache()

So the fatal error seems to be telling me that AppData is missing from the current view. This was the preview for my ContentView so the first place I go is the SceneDelegate. It looks like that's passing AppData:

window.rootViewController = UIHostingController(rootView: contentView.environmentObject(appData))

"Ah but this is the preview!", I cry with a finger pointed firmly and vertically in the air.

I take a look at PreviewProvider in ContentView.

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {

"Well there's your problem" points out the surly mechanic in my head.

If you are like the surly mechanic in my head then you likely noticed that ContentView() is bereft of a call to "environmentObject". This is the source of the fatal error reported earlier. A quick touch up and it's good to go:

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {

That did the trick! I can see my exceptional preview.

What hard to decipher error messages are you getting from SwiftUI? Let me know by dropping me a line on the contact page.