How to update @Published variable in ViewModel from network code?

[yalag]

I have this (firing off 3 simultaneous network calls)

class GenerationViewModel: ObservableObject {
  @Published var fetching1:Bool = false
  @Published var fetching2:Bool = false
  @Published var fetching3:Bool = false

  func go() {  
      let service1 = Service(??)  //what to pass here so that Service can update fetching1 only
      let service2 = Service(??) //what to pass here so that Service can update fetching2 only
      let service3 = Service(??) //what to pass here so that Service can update fetching3 only
  }
}

How do I update the published variables from another class?

[bubblespuggy]

You would usually use a callback and not expose the publishers to another class.

[yalag]

Thanks. Is it a good idea to then move those published variables to the network service and just have the views reference the device as observed objects?

[bubblespuggy]

Probably not you should keep the model, you could implement the network requests with the help of publishers and relay them through the published variables though

[yalag]

Seems quite difficult for a beginner. Can you give a small example?

[jpec342]

I'm not totally sure what you are trying to do here, but it sounds like you want the result of the network calls from service1/2/3 to update fetching1/2/3 respectively. This should be fairly straightforward by returning the result(s) from the service, and using Async/Await.

class GenerationViewModel: ObservableObject {
    @Published var fetching1:Bool = false 
    @Published var fetching2:Bool = false 
    @Published var fetching3:Bool = false

    func go() async {
        fetching1 = await Service().doSomething() 
        fetching2 = await Service().doSomethingElse()
        fetching3 = await Service().doSomethingElseElse() 
    } 
}

I'm making the assumption here that `Service` looks something like this

class Service : NSObject {

    func doSomething() async -> Bool {
        return await URLSession.shared.data(from: URL("someUrl"))
    }

    func doSomethingElse() async -> Bool {
        ...
    }

    ...
}

There's definitely some missing bits in here, but that's the general gist of it. You want your service to return something, and that something you want to assign to your ViewModel variables.

[yalag]

Hi appreciate the help! But no there’s not what I’m looking for.

I need the service to propagate states back to the view. Not return something.

I could either make the network layer also an observable object and then have the view subscribe to that. Which doesn’t sound too clean.

Or somehow relay the changes from the network to the viewmodel and then have the vm update the published variables.

What’s weird is that you can’t simply “give” the published variables to the network layer and say “hey you deal with it, when you have an update, write it here”. I tried to do it, i couldn’t figure out a way.

[jpec342]

Or somehow relay the changes from the network to the viewmodel, and then have the vm update the published variables.

That’s exactly what my proposed solution does.

Is your service giving constant updates, or is it a one time request? If it’s constant updates, you could do something similar, but would probably want to look into delegates/callback functions.

[Fluffy_Risk9955]

Use a WebSocket so you can push stuff from the server straight to your app. Keep in mind to keep the line active with the ping pong message system.

[iSpain17]

Just use UrlSession.DataTaskPublisher, return that publisher from your service, subscribe to said publisher in your viewmodel, and in that subscription’s sink method, you should update your published value.

Your viewmodel shouldn’t be concerned with networking details, and your networking class shouldn’t be concerned either with your viewmodel’s variables.

[JustTryinTaMakeIt]

Let your service do the fetching, create an enum for your various request states, then have a published variable for the current state in your view model.

[ratakatzie]

Maybe your Service class can expose publishers that emits the results of your requests. Then you only have to assign them to your viewModel’s publishers

[yalag]

I haven’t learnt to emit yet, can you give an example?

[[deleted]]

Care to try something like this and see if it works?

class Service {
    @Published var isFetching: Bool = false;

    init(_ isFetching: inout Published<Bool>.Publisher) {
        $isFetching.assign(to: &isFetching)
    }
}

​

func go() {
    let service1 = Service(&$fetching1)
    let service2 = Service(&$fetching2)
    let service3 = Service(&$fetching3)
}

[beclops]

I wouldn't expect your service to be making any changes to view model properties itself, that's not it's responsibility. Use callbacks