SwiftUI and Core Data: Using a fetch request with instance member as argument inside a view

Question

I seem to be caught in a Catch-22 situation. Perhaps my approach is entirely wrong here. I hope someone can help. I want to create a star-based rating display using feedback from users who visit a particular real world landmark. In CoreData I have an entity called Rating with attributes called rating (Int32) and landmark (String). I want to get the average for all rating(s) associated with a given landmark in order to display stars in the view for each. Here is the code for the View:

struct TitleImageView: View {
    @Environment(\.managedObjectContext) var viewContext : NSManagedObjectContext
    let landmark: Landmark 
    var body: some View {
        
        Image(landmark.imageName)
            .resizable()
            .shadow(radius: 10 )
            .border(Color.white)
            .scaledToFit()
            .padding([.leading, .trailing], 40)
            .layoutPriority(1)
            .overlay(TextOverlay(landmark: landmark))
            .overlay(RatingsOverlay(rating: stars))
    }
}

Here is the fetch (which works as expected when the argument for the fetch is hard coded):

let fetchRequest = Rating.fetchRequestForLandmark(landmark: landmark.name)
    var ratings: FetchedResults<Rating> {
            fetchRequest.wrappedValue
    }
        var sum: Int32 {
            ratings.map { $0.rating }.reduce(0, +)
        }
        var stars : Int32 {
            sum / Int32(ratings.count)
        }

The problem is this: When I insert the fetch before the body of the view, I get the warning

"Cannot use instance member 'landmark' within property initializer; property initializers run before 'self' is available"

When I place the fetch after the body, I get:

"Closure containing a declaration cannot be used with function builder 'ViewBuilder'" (with reference to var ratings)

Is there an easy way out of this conundrum or must I go back to the proverbial drawingboard? Thanks.

You need to initalise landmark object first before accessing it’s name property. In your case it’s not yet initialised, and you are trying to access it’s property — Tushar Sharma, Feb 13 '21 at 10:01
Oops... sorry for the delayed response. (Life sometimes gets in the way.) Short answer is yes and no. @Helperbug 's proposed solution perfectly cleared up the warnings. But the fetch request ends up coming back empty (though it retrieves the correct data when run elsewhere). I will keep at it and report back when I have a solution. — Hyoryusha, Feb 21 '21 at 10:02

score 0 · Answer 1 · answered Feb 13 '21 at 21:44

How about wrapping the fetch in a calculated property that returns a sum & stars tuple? Slightly different shape with the same results. Calculated properties can reference other properties, so the swift-init hurdle is cleared!

var metrics : (sum: Int, stars: Int) {
let fetchRequest = Rating.fetchRequestForLandmark(landmark: landmark.name)
var ratings: FetchedResults<Rating> {
    fetchRequest.wrappedValue
}

let sum = ratings.map { $0.rating }.reduce(0, +)
let stars = sum / ratings.count

return (sum: sum, stars: stars)

}

score 0 · Answer 2 · answered May 16 '21 at 00:45

Thanks to those who offered suggestions. After a long hiatus, I returned to this problem and arrived at this solution (based on a HackingWithSwift tutorial: https://www.hackingwithswift.com/books/ios-swiftui/dynamically-filtering-fetchrequest-with-swiftui) Here's the code:

struct RatingsView: View {
    var fetchRequest: FetchRequest<Rating>
    
    var body: some View {

        HStack(spacing: 0.4){
            ForEach(1...5) { number in
                if number > stars {
                    //Image(systemName: "star")
                } else {
                    Image(systemName: "star.fill")
                }
            }
        }
    }
    
    init(filter: String) {
        fetchRequest = FetchRequest<Rating>(entity: Rating.entity() , sortDescriptors: [], predicate: NSPredicate(format: "%K == %@", "landmark" , filter))
    }
    var sum: Int16 {
        fetchRequest.wrappedValue.reduce(0) { $0 + $1.rating }
        }
    
    var stars : Int {
        var starCount:Int
        if fetchRequest.wrappedValue.count > 0 {
            starCount = Int(sum) / fetchRequest.wrappedValue.count
        } else {
            starCount = 0
        }
        return starCount
    }
}

I'm not sure if this is the absolute best solution, but it's working as intended. Hope this can help others.

SwiftUI and Core Data: Using a fetch request with instance member as argument inside a view

2 Answers2