Why doesn't this SwiftUI View update when changing a @State variable?

Question

I've got a SwiftUI View that takes a Hacker News API submission ID, and then fetches the details for that item in fetchStory().

When fetchStory() completed its HTTP call, it updates the @State private var url on the View, however the View never re-renders to show the new value -- it always shows the initial empty value.

Why?

import Foundation
import SwiftUI

struct StoryItem: Decodable {
    let title: String
    let url: String?
}

struct StoryView: View {
    public var _storyId: Int

    @State private var url: String = "";

    init(storyId: Int) {
        self._storyId = storyId
        self.fetchStory()
    }

    var body: some View {
        VStack {
            Text("(Load a webview here for the URL of Story #\(self._storyId))")
            Text("URL is: \(self.url)") // this never changes!
        }
    }

    private func fetchStory() {
        let url = URL(string: "https://hacker-news.firebaseio.com/v0/item/\(self._storyId).json")!

        let task = URLSession.shared.dataTask(with: url) {(data, response, error) in
            guard let data = data else {
                print(String(error.debugDescription))
                return
            }

            do {
                let item: StoryItem = try JSONDecoder().decode(StoryItem.self, from: data)

                if let storyUrl = item.url {
                    self.url = storyUrl
                } else {
                    print("No url")
                }
            } catch {
                print(error)
            }
        }

        task.resume()
    }
}

struct StoryView_Previews: PreviewProvider {
    static var previews: some View {
        StoryView(storyId: 22862053)
    }
}

workingdog support Ukraine · Accepted Answer · 2020-04-15 02:57:21Z

something like this:

import SwiftUI

struct StoryItem: Decodable {
let title: String
let url: String?
}

class ObservedStoryId: ObservableObject {
@Published var storyId: String = ""
init(storyId: String) {
    self.storyId = storyId
}
}

struct StoryView: View {

@ObservedObject var storyId: ObservedStoryId
@State var url: String = ""

var body: some View {
    VStack {
        Text("(Load a webview here for the URL of Story #\(self.storyId.storyId))")
        Text("URL is: \(self.url)")
        Text(self.storyId.storyId)
    }.onReceive(storyId.$storyId) { _ in self.fetchStory() }
}

private func fetchStory() {
    let url = URL(string: "https://hacker-news.firebaseio.com/v0/item/\(self.storyId.storyId).json")!
    let task = URLSession.shared.dataTask(with: url) {(data, response, error) in
        guard let data = data else {
            print(String(error.debugDescription))
            return
        }
        do {
            let item: StoryItem = try JSONDecoder().decode(StoryItem.self, from: data)
            if let storyUrl = item.url {
                self.url = storyUrl
            } else {
                print("No url")
            }
        } catch {
            print(error)
        }
    }
    task.resume()
}
}

and call it like this:

struct ContentView: View {

    @ObservedObject var storyId = ObservedStoryId(storyId: "22862053")

    var body: some View {
     StoryView(storyId: storyId)
    }
}

workingdog support Ukraine · Accepted Answer · 2020-04-15 02:29:28Z

1

to fix your problem:

init(storyId: Int) {
    self._storyId = storyId
}

var body: some View {
    VStack {
        Text("(Load a webview here for the URL of Story #\(self._storyId))")
        Text("URL is: \(self.url)") // this now works
    }.onAppear(perform: fetchStory)
}

why does this works and not your code, my guess is this: "self.url" can only be updated/changed within the special SwiftUI View functions, such as onAppear(), elsewhere it does not change a thing.

answered Apr 15, 2020 at 2:29

workingdog support Ukraine

38.9k5 gold badges47 silver badges52 bronze badges

4 Comments

Bryce Over a year ago

This is almost perfect, however it only works once. If the parent view changes the storyId prop, then onAppear does not re-trigger (I guess since it's "appeared" once already)

workingdog support Ukraine Over a year ago

in that case maybe you could use an ObservableObject instead.

Bryce Over a year ago

I'll give that a go. If you post that recommendation as an answer I'll accept it

pauln Over a year ago

The accepted answer (using an ObservableObject) is certainly valid, but for many (simpler) cases this approach will resolve the problem with less complexity.

Collectives™ on Stack Overflow

Why doesn't this SwiftUI View update when changing a @State variable?

2 Answers 2

Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related