previewProvider on SwiftUI does not load

Question

today I'm wondering if it's possible to work designing a view on SwiftUI using PreviewProvider It's difficult when you have models because you need to "Initialize" every Model and Attribute to see the UI Emulator. Ex

    struct SimpleView: View {
    
    @State private var number : Int 
    
    
    var body: some View {
        ScrollView{
            VStack{
                Text(String(number))
            }
        }
    }
}


struct SimpleView_Previews: PreviewProvider {
    static var previews: some View {
        SimpleView(Int: 5)
    }
}

UI Preview Works good!

But what if I need to use a custom object with 40-70 properties... :(

Is there a way to work without "hardcode" in the beginning?

Thank you so much

I normally either create a MockViewModel subclass of the relevant ViewModel or if the view model is a struct I create a static function on te struct that returns an appropriate "mock" instance of the struct. — Paulw11
– Paulw11, Commented Dec 10, 2020 at 1:00
with 40-70 properties - don't do that, ever - separate on small parts, the smaller is the better. — Asperi
– Asperi, Commented Dec 10, 2020 at 4:08
I'll try to separate that models because it's annoying to load an entire model (unfortunately there are so many models that works together! I mean, an Address Form with 20 attributes like zip code, email, name, telephone... It's a mess) — Antonio Labra
– Antonio Labra, Commented Dec 10, 2020 at 5:55

Paulw11 · Accepted Answer · 2020-12-10 07:33:20Z

One approach is to create a mockViewModel class property that returns an instance with fixed values that you can use for previews.

In this example, the backing model object is a Core Data managed object.

In normal use you call the ProjectDetailViewModel(project:) initialiser.

For the preview you can use the mockViewModel property

class ProjectDetailViewModel: ObservableObject {
    
    @Published var name: String {
        didSet {
            self.project.name = name
        }
    }
    
    @Published var notes: String {
        didSet {
            self.project.notes = notes
        }
    }
    
    let project: Project!
    
    init(project: Project) {
        self.project = project
        name = project.name ?? ""
        notes = project.notes ?? ""
    }
    
    private init() {
        project = nil
        name = "Mock project"
        threePhase = false
        notes = "Mock Notes"
    }
    
    static var mockViewModel:ProjectDetailViewModel {
        return ProjectDetailViewModel()
    }
    
}


struct ProjectDetailView_Previews: PreviewProvider {
    static var previews: some View {
        ProjectDetailView(project: ProjectDetailViewModel.mockViewModel)
    }
}

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