Change Notification from Observable Object as a Nested Object in SwiftUI

Question

I have a following scenario.

I have AppState which consists of an object of type Foo. Foo has a counter variable and I want to call objectWillChange when counter value updates so I can update the UI.

At present nothing happens. The increment function gets called but the UI never gets updated.




import Foundation
import Combine

class Foo: ObservableObject {
    @Published var counter: Int = 999
    
    func increment() {
        counter += 1 // how to get notified when counter value changes
    }
}

class AppState: ObservableObject {
    
    @Published var foo: Foo = Foo()
}

// usage in Scene Delegate as Environment Object
let appState = AppState()

// Use a UIHostingController as window root view controller.
if let windowScene = scene as? UIWindowScene {
    let window = UIWindow(windowScene: windowScene)
    window.rootViewController = UIHostingController(rootView: accountSummaryScreen.environmentObject(appState))

UPDATE

class Foo: ObservableObject {
    @Published var counter: Int = 999 {
        didSet {
            objectWillChange.send() 
        }
    }
    
    func increment() {
        counter += 1 // how to get notified when counter value changes
    }
}

That's because `Foo` doesn't actually change - it's a reference-type - so `@Published` doesn't help here, since it's always the same reference — New Dev, Aug 04 '20 at 17:05
Yes that is clear! What is the solution for this, apart from making Foo a struct. How can I get changes from Foo object. — john doe, Aug 04 '20 at 17:06
See if this helps: https://stackoverflow.com/a/62988407/968155, but in a nutshell - you'd need to manually subscribe to changes in `.counter` and call your own `objectWillChange.send` — New Dev, Aug 04 '20 at 17:07
I added update. Calling manually inside the Foo class but even that does not do anything. — john doe, Aug 04 '20 at 17:11

New Dev · Accepted Answer · 2020-08-04T17:23:53.000

3

Changes aren't detected because Foo, being a reference-type, doesn't actually change - it's the same reference, so @Published doesn't help here.

AppState would need to manually subscribe to changes and call its own objectWillChange.send:

class AppState: ObservableObject {
    
    var foo: Foo = Foo() {
       didSet {
          cancellables = []
          foo.$counter
             .map { _ in } // ignore actual values
             .sink(receiveValue: self.objectWillChange.send)
             .store(in: &cancellables)
       }
    }

    private var cancellables: Set<AnyCancellable> = []
}

edited Aug 04 '20 at 17:23

answered Aug 04 '20 at 17:11

New Dev

48,427
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@johndoe - I amended the answer to account for the case when `foo` itself is updated – New Dev Aug 04 '20 at 17:24
In my real app I have AppState which consists an array of items and each item has another item which has a balance property. Seems like this will be a LOT of work if I go that route. Easy way is to simply use the struct ; – john doe Aug 04 '20 at 17:33
@johndoe - definitely. If they are values, not living objects, then they should be structs – New Dev Aug 04 '20 at 17:34

score 1 · Answer 2 · answered Mar 01 '22 at 15:37

1

In SwiftUI our model types are structs, e.g.

struct Foo: Identifiable {
    let id = UUID()
    var counter: Int = 999 
    
    mutating func increment() {
        counter += 1 
    }
}

Now it can be used with @Published in an ObservableObject.

answered Mar 01 '22 at 15:37

malhal

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Change Notification from Observable Object as a Nested Object in SwiftUI

2 Answers2

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