Changes to a struct not being published in SwiftUI

Question

I'm loading data from an API, and expecting my app to show the data once it's loaded.

In my View Model file, here's the code: It calls a WeatherService to get the data, and populates the weather property. Weather is a struct in this case.

class WeatherViewModel: ObservableObject {

    let webService = WeatherService.shared
    @Published var weather:Weather?
    
    init() {
        
    }

    func getWeather() {
        webService.getWeather { weather in
            if let weather = weather {
                self.weather = weather
            }
        }
    }
    
}

In my SwiftUI view, here's the code:

I instantiate an instance of the View Model as an ObservedObject
In the inAppear, I call the method in the view model to get the data
The first time the screen launches (using a tab bar), I see "Loading weather..." and it never goes away
If I navigate to a different tab and back, I see the weather. I can't tell if this is data from the old API call, or from the new one.

struct WeatherView: View {
    
    @ObservedObject var weatherViewModel = WeatherViewModel()
    @State var areDetailsHidden = true
    
    var body: some View {
        VStack(alignment: .leading, spacing: 0) {
            
            if(weatherViewModel.weather == nil) {
                Text("Loading weather...")
            } else {
                Text("Display the weather here")
            }
        }
        .onAppear{
            self.weatherViewModel.getWeather()
        }
    }
}

The weird thing is, if I remove the getWeather() from the onAppear and add it to the init() of the View Model, it works (although for some reason getWeather() gets called twice...). However, I want the weather info to be refreshed every time the screen is loaded.

The problem you have hear is that the weatherViewModel is owned by the WeatherView. When it changes that causes the view to be rendered again which creates a new weatherViewModel and you are in an endless loop; You need to move the model out of the view. — Upholder Of Truth
@UpholderOfTruth that did it! I turned the weatherViewModel into an environment object and that solved it. I was under the impression that the body was got rendered again, and the properties remain as is. Interesting. If you submit this as an answer I'll mark it as such. — Khuffie

Upholder Of Truth Upholder Of Truth · Accepted Answer · 2020-07-04T09:27:39

This is caused by the:

@ObservedObject var weatherViewModel = WeatherViewModel()

being owned by the WeatherView itself.

So what happens is the weather view model changes which forces a re-render of the view which creates a new copy of the weather view model, which changes forces a re-render...

So you end up with an endless loop.

To fix it you need to move the weather view model out of the view itself so either use an @Binding and pass it in or an @EnvironmentObject and access it that way.

Changes to a struct not being published in SwiftUI

1 Answers