Question 129925 in Xcode Question posted in
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Xcode – How do you preview @StateObject with Binding without simulator

I am trying to code an app using samples from Apple, but I realised that Scrumdinger app preview doesn’t work in Xcode preview. I started a simulator and there it works. So I understand that this is because of @StateObject. How do you code without preview or how do you make it work?

Here’s the sample code:

@main
struct LandLordApp: App {
    @StateObject var store = EstateStore()
    
    var body: some Scene {
        WindowGroup {
            EstatesList(estates: $store.estates)
        }
    }
}

import Foundation

struct Estate: Identifiable, Codable, Hashable {
    var id = UUID()
    var name: String = ""
    var address: String = ""
    var isRented: Bool = false
}

class EstateStore: ObservableObject {
    @Published var estates: [Estate] = []
}

import SwiftUI

struct EstatesList: View {
    
    @Binding var estates: [Estate]
    
    var body: some View {
        NavigationStack {
            List(estates) {estate in
                NavigationLink {
                    Label(estate.name, systemImage: "building.2")
                } label: {
                    Label(estate.name, systemImage: "building.2")
                }
            }
            .toolbar {
                ToolbarItem {
                    Button {
                        estates.append(Estate(name: "Krautzplaz, 10"))
                    } label: {
                        Label("Add", systemImage: "plus")
                    }
                }
            }
        }
    }
}

struct EstatesList_Previews: PreviewProvider {
    
    static var previews: some View {
        EstatesList(estates: .constant(EstateStore().estates))
    }
}

If you run in a simulator it works. In the Preview it just doesn’t work.

Tags:

2

Answers


  1. 0

    You can replace @StateObject with @ObservedObject, which has similar behavior and is supported in Xcode previews.

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  2. 0

    Usually we use .environmentObject for the data model store object as follows:

    @main
struct LandLordApp: App {

// Because this is @StateObject it won't be init during previewing. 
// @StateObject is init before body is called and App's body isn't called when previewing.
    @StateObject var store = EstateStore() 
    
    var body: some Scene {
        WindowGroup {
            EstatesList()
                 .environmentObject(store) // we could also use EstateStore.shared
        }
    }
}

    struct EstatesList: View {
    
    @EnvironmentObject var store: EstateStore
    
    var body: some View {
        NavigationStack {
            List($store.estates) { $estate in

    struct EstatesList_Previews: PreviewProvider {
    
    static var previews: some View {
        EstatesList()
            .environmentObject(EstateStore(previewing: true)) // or EstateStore.preview
    }
}

    class EstateStore: ObservableObject {
    @Published var estates: [Estate] = []

    init(previewing: Bool = false) {
        if previewing {
            estates = [Estate( /* sample data */  )]
        }
    }

    // or singletons if you prefer not to use @StateObject in the App because say you don't want the scene to be recomputed all the time when not using any estate properties.
    //static var shared = EstateStore()
    //static var preview = EstateStore(previewing: true)
}
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