How to trigger an action as soon as a SwiftUI observed variable changes its value

Question

I've got a List in my View where its elements are going to be updated as soon as the list argument will change. This is my View:

struct ContentView: View {
@ObservedObject var users = Utenti()
@State private var isSharePresented: Bool = false

var body: some View {
    VStack(spacing: 20) {
        HStack(alignment: .center) {
            Spacer()
            Text("Internal Testers")
                .font(.title)
                .foregroundColor(.primary)
            Spacer()
            Button(action: {
                self.isSharePresented.toggle()
            }) {
                Image(systemName: "square.and.arrow.up").font(.title).padding(.trailing)
            }.sheet(isPresented: self.$isSharePresented, onDismiss: {
                print("Dismissed")
            }, content: {
                ActivityViewController(activityItems: self.users.listaUtenti)
            })
        }
        List(self.users.listaUtenti, id: \.self) { user in
            Text(user)
        }
    }
}

}

The variable users is an @ObservedObject, so the list content is updated automatically as soon as it changes in the Model.

My questions are: how can I catch the 'update' event concerning the users variable ? And how can I trigger an action (e.g. call a function) after catching it ?

do it in your model! if your View have to reflect some final state... expose the state via property of your model. SwiftUI is a presentation layer — user3441734, Feb 02 '20 at 18:30

score 6 · Accepted Answer · answered Feb 02 '20 at 14:03

6

Assuming that listaUtenti is @Published property you can catch its publisher as below

List(self.users.listaUtenti, id: \.self) { user in
    Text(user)
}
.onReceive(self.users.$listaUtenti) { newValue in
     // do this what's needed
}

answered Feb 02 '20 at 14:03

Asperi

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Does it exist a kind of willReceive() that catches the published value just before the published variable will change ? – Giuseppe Roberto Rossi Feb 07 '20 at 23:46
A bit late, but for reference. Yes, you must use the newValue as it contains the updated change. Do not use the self.users.$listaUtenti in the block or in any function you call from the block as it does not yet contain the change. – Brett Feb 26 '21 at 05:11

score 0 · Answer 2 · edited Jun 28 '22 at 19:47

0

class Utenti : ObservableObject {

    @Published var listaUtenti = ["anna", "smith", "david"]  {
         didSet {
             // your func here
         }
    }
}

In the above code, whenever listaUtenti element changes your function will execute.

edited Jun 28 '22 at 19:47

pkamb

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answered Feb 02 '20 at 12:52

koreaMacGuy

75
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Thanks, however in my case I need to monitor the changed variable in the View rather than in the Model. How could I do that ? – Giuseppe Roberto Rossi Feb 02 '20 at 13:40
@GiuseppeRobertoRossi hmmmm i think whenever model change, view refresh. Therefore... view can detect model.Do u add property that is get property of model value. – koreaMacGuy Feb 03 '20 at 13:25
I adjusted the above because ObservableObject only applies to class, not struct. – James Toomey Jan 29 '22 at 05:28

How to trigger an action as soon as a SwiftUI observed variable changes its value

2 Answers2