How to retrieve and show data on screen in SwiftUI? - ios

Question

I am storing simple data in core data which i want to retrieve and show on screen, but I am not sure how and where should I write that code to show it on screen as I am always get out of bound error..
Also not all data is saving at time its only saving when i scroll til bottom
import SwiftUI
import CoreData
struct ContentView: View {
#Environment(\.managedObjectContext) private var viewContext
#FetchRequest(
sortDescriptors: [NSSortDescriptor(keyPath: \Item.title, ascending: true)],
animation: .default)
private var items: FetchedResults<Item>
#StateObject private var viewModel = HomeViewModel()
var body: some View {
GeometryReader { geometry in
NavigationView {
ScrollView {
LazyVGrid(columns: Array(repeating: .init(.flexible()),
count: UIDevice.current.userInterfaceIdiom == .pad ? 4 : 2)) {
ForEach(viewModel.results, id: \.self) {
let viewModel = ResultVM(model: $0)
NavigationLink(destination: {
DetailView(data: viewModel.trackName)
}, label: {
SearchResultRow(resultVM: viewModel, coreDM: PersistenceController())
})
}
}
}
}
.onAppear(perform: {
viewModel.performSearch()
})
}
}
}
struct SearchResultRow: View {
let resultVM: ResultVM
let coreDM: PersistenceController
var body: some View {
HStack {
RoundedRectangle(cornerRadius: 16).fill(.yellow)
.frame(maxWidth: .infinity).aspectRatio(1, contentMode: .fit)
.overlay(Text(resultVM.trackName)) // want to show data from Core data here
}.padding()
.background(Color.red)
.onAppear(perform: {
coreDM.saveResult(title: resultVM.trackName)
})
}
}
Data showing from API which same data storing in CoreData
Method to save and retrieve (which is working fine)
func saveResult(title: String) {
let result = Item(context: container.viewContext)
result.title = title
do {
try container.viewContext.save()
}
catch {
print("error")
}
}
func getResult() -> [Item] {
let fetchRequest: NSFetchRequest<Item> = Item.fetchRequest()
do {
return try container.viewContext.fetch(fetchRequest)
}
catch {
return []
}
}
API call
import Foundation
import CoreData
class HomeViewModel: ObservableObject {
#Published var results = [ResultItem]()
func performSearch() {
guard let gUrl = URL(
string: "https://api.artic.edu/api/v1/artworks"
) else { return }
Task {
do {
let (data, _) = try await URLSession.shared.data(from: gUrl)
let response = try JSONDecoder()
.decode(ResponseData.self, from: data)
DispatchQueue.main.async { [weak self] in
self?.results = response.data ?? []
}
} catch {
print("*** ERROR ***")
}
}
}
}

Answer 1

Remove the view model we don't use view model objects in SwiftUI. The View struct stores the view data and does dependency tracking, and the property wrappers give it external change tracking features like an object. If you add an actual object on top you'll get consistency bugs that SwiftUI was designed to eliminate. So first remove this:
#StateObject private var viewModel = HomeViewModel()
You already have the fetch request property wrapper and as I said that gives the View change tracking, so when the items change (e.g. an item is added, removed or moved), the body will be called, so simply use it in your ForEach like this:
ForEach(items) { item in
NavigationLink(destination: {
DetailView(item: item)
}, label: {
SearchResultRow(item: item)
})
}
Then use #ObservedObject in your subview, this gives the View the ability to track changes to the item so body will be called when the item's properties change, like item.title. Do the same in the DetailView.
struct SearchResultRow: View {
#ObservedObject var item: Item
It seems to me you are trying to download and cache data in Core Data. That is unnecessary since you can simply set a NSURLCache on the NSURLSession and it will automatically cache it for you, it even works if offline. However if you really want to cache it yourself in Core Data then the architecture should be your UI fetches it from Core Data and then you have another object somewhere responsible for syncing down the remote data into Core Data. Normally this would be at the App struct level not in the View appearing. When the data is saved to core data the managed object context context will be changed which the #FetchRequest is listening for and body will be called.