Why doesn't this SwiftUI View update when changing a #State variable? - swiftui

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)
}
}

Answer 1

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)
}
}

Answer 2

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.