Xcode 12 & SwiftUI: Cannot preview in this file — Failed to update preview

Question

Seems like previews stopped working on Xcode 12! Trying to preview the SwiftUI file template and getting the error below. Any ideas how to fix this? Tried cleaning the build folder, deleting derived data and restarting Xcode to no avail!

On Catalina 10.15.6.

RemoteHumanReadableError: Failed to update preview.

The preview process appears to have crashed.

Error encountered when sending 'prepare' message to agent.

==================================

|  RemoteHumanReadableError: The operation couldn’t be completed. (BSServiceConnectionErrorDomain error 3.)
|  
|  BSServiceConnectionErrorDomain (3):
|  ==BSErrorCodeDescription: OperationFailed

The code I am trying to preview (from SwiftUI new file template):

import SwiftUI

struct SwiftUIView: View {
    var body: some View {
        Text("Hello, World!")
    }
}

struct SwiftUIView_Previews: PreviewProvider {
    static var previews: some View {
        SwiftUIView()
    }
}

Answer 1

Apparently it was a bug with Firestore, which is most likely the cause: XCode 12 Preview Crashes when adding Firebase SDK Swift UI 2.0. The best solution at this point is to call, pod update which should solve the issue. I verified on Xcode 12, iOS 13+.

Answer 2

if you use SwiftUI (ObservableObject, @EnvironmentObject)

add to previews

.environmentObject(ModelData())

--

struct LandmarkDetail_Previews: PreviewProvider {
        static var previews: some View {
    
            LandmarkDetail(landmark: ModelData().landmarks[1])
                .environmentObject(ModelData())
        }
    }

my ModelData

final class ModelData:ObservableObject
{
   @Published var landmarks: [Landmark] = load("landmarkData.json")
}

Answer 3

If your SwiftUIView use ObservableObject as environmentObject, try this:

struct SwiftUIView_Previews: PreviewProvider {
    static var previews: some View {
        SwiftUIView().environmentObject(YourObservableObjectClass())
    }
}