SwiftUI Preview doesn't work with FetchRequest and Init

Question

I have a problem with SwiftUI Preview not working sometimes (but not giving me any errors, just a blank Canvas). I have narrowed down the problem - it doesn't work when I'm using fetchRequest with init. But I don't know what to do next.

Preview works with this code:

import SwiftUI

struct ListView: View {

    var fetchRequest = FetchRequest<NPBooking>(entity: NPBooking.entity(), sortDescriptors: [])
    var bookings: FetchedResults<NPBooking> { fetchRequest.wrappedValue }

    var body: some View {
        ForEach(bookings, id: \.self) { booking in
            Text("item")
        }
    }

}

struct ListView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        //Test data
        let testBooking = NPBooking.init(context: context)
        testBooking.date = Date()
        testBooking.name = "name"
        return ListView().environment(\.managedObjectContext, context)
    }
}

Preview doesn't work with this code:

import SwiftUI

struct ListView: View {

    var fetchRequest: FetchRequest<NPBooking>
    var bookings: FetchedResults<NPBooking> { fetchRequest.wrappedValue }

    var body: some View {
        ForEach(bookings, id: \.self) { booking in
            Text("item")
        }
    }

    init(startDateOfMonth: Date) {
        fetchRequest = FetchRequest<NPBooking>(entity: NPBooking.entity(), sortDescriptors: [
            NSSortDescriptor(keyPath: \NPBooking.date, ascending: true)
        ], predicate: NSPredicate(format: "date >= %@", startDateOfMonth as NSDate))
    }

}

struct ListView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        //Test data
        let testBooking = NPBooking.init(context: context)
        testBooking.date = Date()
        testBooking.name = "name"
        return ListView(startDateOfMonth: Date()).environment(\.managedObjectContext, context)
    }
}

I guess I need to add some test data for this init or fetchRequest or both? Tried few things and I cannot manage to make it work.

I think the problem is that when you call the initialiser (in `previews`), you are using `Date()` for both the `startDateOfMonth` and `endDateOfMonth`. You need to **calculate** the start and end of month - for which see [this answer](https://stackoverflow.com/a/33606288/3985749). — pbasdf, Jan 29 '20 at 09:58
Thanks, @pbasdf, but that's not it. I have just edited the code to use just startDateOfMonth alone and it still doesn't work. — mallow, Jan 29 '20 at 11:33
Thanks, @pbasdf. It seems you were on the right track. This part with Date() was causing the problems. CranialDev provided the correct solution. Thanks for your help too :) — mallow, Jan 31 '20 at 08:09

score 3 · Accepted Answer · edited Jan 31 '20 at 08:42

3

The Code works fine if you change your call in previews to

 ListView(startDateOfMonth: Date().addingTimeInterval(-86400 * 30)).environment(\.managedObjectContext, context)

Your startDateOfMonth var gets set to 30 days before today with this lines

edited Jan 31 '20 at 08:42

mallow

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answered Jan 29 '20 at 15:27

lorem ipsum

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Thank you! It works. I understand this change in previews. But why VStack needed to be added? I think I have sometimes used ForEach alone. But maybe I'm wrong... hmmm – mallow Jan 31 '20 at 07:38
Actually, only thing needed was this addingTimeInterval in Previews part. With this code added everything works. Thanks! Adding VStack was not necessary and it just caused some problems - all the items rendered 3 times. I have edited your answer and left just the code that solved my problem. Thank you very much for your help :) – mallow Jan 31 '20 at 08:07

SwiftUI Preview doesn't work with FetchRequest and Init

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