@ObservedObject
When a view creates its own @ObservedObject instance it is recreated every time a view is discarded and redrawn:
struct ContentView: View {
@ObservedObject var viewModel = ViewModel()
}
On the contrary a @State variable will keep its value when a view is redrawn.
@StateObject
A @StateObject is a combination of @ObservedObject and @State – the instance of the ViewModel will be kept and reused even after a view is discarded and redrawn:
struct ContentView: View {
@StateObject var viewModel = ViewModel()
}
Performance
Although an @ObservedObject can impact the performance if the View is forced to recreate a heavy-weight object often, it should not matter much when the @ObservedObject is not complex.
When to use @ObservedObject
It might appear there is no reason now to use an @ObservedObject, so when should it be used?
You should use @StateObject for any observable properties that you
initialize in the view that uses it. If the ObservableObject instance
is created externally and passed to the view that uses it mark your
property with @ObservedObject.
Note there are too many use-cases possible and sometimes it may be desired to recreate an observable property in your View. In that case it’s better to use an @ObservedObject.
Useful links: