I used @AppStorage before and it works perfect in that app but i can't get it to work in this app

import SwiftUI
import MapKit
import Foundation

struct SupermarketFinder {

    @AppStorage("supermarkets") var supermarkets: [MKMapItem] = []

    func fetchSupermarkets(for currentLocation: CLLocationCoordinate2D, radius: CLLocationDistance) {
            getSupermarkets(for: currentLocation, radius: radius) { supermarkets in
                self.supermarkets = supermarkets
            }
        }

    func getSupermarkets(for currentLocation: CLLocationCoordinate2D, radius: CLLocationDistance, completion: @escaping ([MKMapItem]) -> Void) {
        let request = MKLocalSearch.Request()
        request.naturalLanguageQuery = "supermarket"
        request.region = MKCoordinateRegion(center: CLLocationCoordinate2D(latitude: currentLocation.latitude, longitude: currentLocation.longitude), latitudinalMeters: radius, longitudinalMeters: radius)

        let search = MKLocalSearch(request: request)
        search.start { response, error in
            guard let response = response else {
                completion([])
                return
            }
            completion(response.mapItems)
        }
    }
}

AppStorage and SceneStorage use the UserDefaults system behind the scenes. UserDefaults only allows a few simple types of data: Bool, Int, Float, Double, URL, String, Date, and a few more I can't recall off the top of my head.

More complex types, like arrays and dictionaries, are allowed as long as their elements are one of the allowed simple types. (Although I think Dictionary is only allowed with the additional caveat that the key must be a String or Int. Don't quote me on that, though.)

MKMapItem is not one of the allowed types. If you can convert MKMapItem into a Data object, that is one of the types that can be stored.

if that the case then if the type is String then this should work!

@AppStorage("supermarkets") var supermarkets: [String] = [] // No exact matches in call to initializer

Yes can be used in UserDefaults

UserDefaults.standard.array(forKey: "supermarkets")

PS Dictionary is [String: Any]

Ah, well, you learn something every day. I guess I never tried to stick an array in AppStorage and just assumed since it uses UserDefaults behind the scenes that you can use all the same types.

There's probably a way you could do it by converting your array to JSON or a Property List and then storing the resulting string, or converting it to Data and storing that. But that may be more trouble than it's worth.