@ObservedObject v @StateObject

The Youtube algorithm thinks I need to watch more MVVM videos, and it turns out it’s probably right. A day or two ago in an MVVM post using a super simple example, I stored the view model as a property of the view using the @ObservedObject wrapper, as I created it.

struct ContentView: View { @ObservedObject var light = LightViewModel()

var body: some View {
    VStack{
        Spacer()
        if light.isOn(){
            drawLitBulb
        }
        else{
            Image(systemName: "lightbulb.fill").font(.system(size: 72))
        }

But then today, Youtube served me up this video from BeyondOnesAndZeros

They start off with @ObservableObject, but then say that if the View Model is instantiated there, that this is repeated every time the View is recreated (which happens every time it’s redrawn).

I don’t really understand the property wrappers (like @ObservableObject) but I assume this was a type property - ie static. Apparently not. The Apple documentation on managing data says:

SwiftUI might create or recreate a view at any time, so it’s important that initializing a view with a given set of inputs always results in the same view. As a result, it’s unsafe to create an observed object inside a view. Instead, SwiftUI provides the StateObject attribute for this purpose.

So that code should use @StateObject, and @ObservedObject should be used where I pass it down into other view structs in the hierarchy.

Paul Hudson gives a good example in his explanation on this topic here .