CoreData in SwiftUI: How to fetch a property distinctly? - ios

Question

I'm spending hours to figure out how to fetch values uniquely from a property of a data model (CoreData).
For example, if I have 3 records that have "Apple", "Banana" and "Apple" respectively in their wrappedName property, the ListView shows "Apple", "Banana" and "Apple" (3 rows).
But I want to show only "Apple" and "Banana" (2 rows).
How can I achieve this?
//ListView.swift
import SwiftUI
import CoreData
struct ListView: View {
#FetchRequest(entity: Data.entity(), sortDescriptors: []) var data: FetchedResults<Data>
var body: some View {
NavigationView{
List{
ForEach(data, id: \.self) { d in
NavigationLink(destination: ChartView(title: d.wrappedName) {
Text(d.wrappedName)
}
}
}
}
}
}
//Data+CoreDataProperties.swift
import Foundation
import CoreData
extension Data {
#nonobjc public class func fetchRequest() -> NSFetchRequest<Data> {
return NSFetchRequest<Data>(entityName: "Data")
}
#NSManaged public var date: Date?
#NSManaged public var id: UUID?
#NSManaged public var name: String?
public var wrappedName: String {
name ?? "Unknown"
}
}

Answer 1

A Custom Notification that sets a Published var would be better
public init() {
self.uniqueDict = [NSDictionary]()
let notificationCenter = NotificationCenter.default
notificationCenter.addObserver(self, selector: #selector(setUniqueDictValue), name: NSNotification.Name.NSManagedObjectContextObjectsDidChange, object: managedObjectContext)
//set the uniqueDict variable
setUniqueDictValue()
}
private func getEntityFetchRequest() -> NSFetchRequest<NSDictionary>
{
let fetchRequest = NSFetchRequest<NSDictionary>(entityName: "YourModel")
let sortDescriptor = NSSortDescriptor(key: "name", ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]
fetchRequest.resultType = NSFetchRequestResultType.dictionaryResultType
fetchRequest.propertiesToFetch = ["name"]
fetchRequest.returnsDistinctResults = true
return fetchRequest
}
///Single Fetch of objects willSet uniqueDict variable
func setUniqueDictValue() {
do {
try self.uniqueDict = managedObjectContext.fetch(getEntityFetchRequest())
} catch {
fatalError("Failed to fetch entity: \(error)")
}
}
Distinct Results Sample Project
https://www.youtube.com/watch?v=xl3KVmBJrSg

Answer 2

You need to change your Core Data model. The model should be designed how you want to display it in your UI. Don't design it like it is a database.
In your case an another Entity called Fruit, with an int fruitType that is a unique key, 1 for Apple and 2 for Banana. In your Data entity have a relation fruit but instead of fetching Data now fetch for Fruit. If you want to show something about Data in your list cell then simply add a property to the Fruit class to retrieve it from the related datas. And using KVO observing you can make it so the context detects a change to Fruit when something in its related Data changes.