SwiftUI: How can I catch changing value from observed object when I execute function

Question

I have a problem with observed object in SwiftUI. I can see changing values of observed object on the View struct. However in class or function, even if I change text value of TextField(which is observable object) but "self.codeTwo.text still did not have changed.

here's my code sample (this is my ObservableObject)

class settingCodeTwo: ObservableObject {

private static let userDefaultTextKey = "textKey2"
@Published var text: String = UserDefaults.standard.string(forKey: settingCodeTwo.userDefaultTextKey) ?? ""

private var canc: AnyCancellable!

     init() {
        canc = $text.debounce(for: 0.2, scheduler: DispatchQueue.main).sink { newText in
        UserDefaults.standard.set(newText, forKey: settingCodeTwo.userDefaultTextKey)

    }
}


   deinit {
     canc.cancel()
   }

}

and the main problem is... "self.codeTwo.text" never changed!

class NetworkManager: ObservableObject {


@ObservedObject var codeTwo = settingCodeTwo()
@Published var posts = [Post]()


func fetchData() {
    var urlComponents = URLComponents()
    urlComponents.scheme = "http"



    urlComponents.host = "\(self.codeTwo.text)" //This one I want to use observable object




    urlComponents.path = "/mob_json/mob_json.aspx"
    urlComponents.queryItems = [
        URLQueryItem(name: "nm_sp", value: "UP_MOB_CHECK_LOGIN"),
        URLQueryItem(name: "param", value: "1000|1000|\(Gpass.hahaha)")
    ]

    if let url = urlComponents.url {
        print(url)
        let session = URLSession(configuration: .default)
        let task = session.dataTask(with: url) { (data, response, error) in
            if error == nil {
                let decoder = JSONDecoder()
                if let safeData = data {
                    do {
                        let results = try decoder.decode(Results.self, from: safeData)

                        DispatchQueue.main.async {
                            self.posts = results.Table
                        }   
                    } catch {
                        print(error)
                    }
                }
            }
        }
        task.resume()
    }

}
}

and this is view, I can catch change of the value in this one

import SwiftUI
import Combine



struct SettingView: View {

  @ObservedObject var codeTwo = settingCodeTwo()
  var body: some View {
    ZStack {
        Rectangle().foregroundColor(Color.white).edgesIgnoringSafeArea(.all).background(Color.white)
        VStack {


            TextField("test", text: $codeTwo.text).textFieldStyle(BottomLineTextFieldStyle()).foregroundColor(.blue)

            Text(codeTwo.text)
        }
    }
}
}

Help me please.

Answer 1

Non-SwiftUI Code

• Use ObservedObject only for SwiftUI, your function / other non-SwiftUI code will not react to the changes.
• Use a subscriber like Sink to observe changes to any publisher. (Every @Published variable has a publisher as a wrapped value, you can use it by prefixing with $ sign.

Reason for SwiftUI View not reacting to class property changes:

• struct is a value type so when any of it's properties change then the value of the struct has changed
• class is a reference type, when any of it's properties change, the underlying class instance is still the same.
  • If you assign a new class instance then you will notice that the view reacts to the change.

Approach:

• Use a separate view and that accepts codeTwoText as @Binding that way when the codeTwoText changes the view would update to reflect the new value.
• You can keep the model as a class so no changes there.

Example

class Model : ObservableObject {

    @Published var name : String //Ensure the property is `Published`.

    init(name: String) {
        self.name = name
    }
}

struct NameView : View {

    @Binding var name : String

    var body: some View {

        return Text(name)
    }
}

struct ContentView: View {

    @ObservedObject var model : Model

    var body: some View {
        VStack {
            Text("Hello, World!")
            NameView(name: $model.name) //Passing the Binding to name
        }
    }
}

Testing

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {

        let model = Model(name: "aaa")

        DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
            model.name = "bbb"
        }

        return ContentView(model: model)
    }
}

Answer 2

It is used two different instances of SettingCodeTwo - one in NetworkNamager another in SettingsView, so they are not synchronised if created at same time.

Here is an approach to keep those two instances self-synchronised (it is possible because they use same storage - UserDefaults)

Tested with Xcode 11.4 / iOS 13.4

Modified code below (see also important comments inline)

extension UserDefaults { 
    @objc dynamic var textKey2: String { // helper keypath 
        return string(forKey: "textKey2") ?? ""
    }
}

class SettingCodeTwo: ObservableObject { // use capitalised name for class !!!

    private static let userDefaultTextKey = "textKey2"
    @Published var text: String = UserDefaults.standard.string(forKey: SettingCodeTwo.userDefaultTextKey) ?? ""

    private var canc: AnyCancellable!
    private var observer: NSKeyValueObservation!

    init() {
        canc = $text.debounce(for: 0.2, scheduler: DispatchQueue.main).sink { newText in
            UserDefaults.standard.set(newText, forKey: SettingCodeTwo.userDefaultTextKey)
        }
        observer = UserDefaults.standard.observe(\.textKey2, options: [.new]) { _, value in
            if let newValue = value.newValue, self.text != newValue { // << avoid cycling on changed self
                self.text = newValue
            }
        }
    }
}

class NetworkManager: ObservableObject {

    var codeTwo = SettingCodeTwo() // no @ObservedObject needed here
    ...