Difference between @StateObject and @ObservedObject

Views in SwiftUI can be redrawn frequently whenever @Published values are being observed by the view. So it is up to the developers when to use what .

@ObservedObject When a view is redrawn, the entire View is redrawn again, which means it will reset all the variables to their default ones except those which are marked in @State .

@StateObject When a view is redrawn, objects marked with @StateObject does not reinstantiated . This means the first instance of the object which we create will be persist/retained and be used whenever View is redrawn.

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Consider the below example

struct RandomNumberView: View {
    @State var randomNumber = 0

    var body: some View {
        VStack {
            Text("Random number is: \(randomNumber)")
            Button("Randomize number") {
                randomNumber = (0..<1000).randomElement()!
            }
        }.padding(.bottom)

        CounterView()
    }
}

struct RandomStringView: View {
    @StateObject var viewModel = RandomStringViewModel()

    var body: some View {
        VStack {
            Text("Random String is \(viewModel.updateName)")
            Button("Random String") {
                viewModel.randomString(length: 4)
            }
        }
    }
}

final class RandomStringViewModel: ObservableObject {
    @Published var updateName: String = ""

    func randomString(length: Int) {
      let letters = "abcdefghijklmnopqrstuvwxyz"
        updateName = String((0..<length).map{ _ in letters.randomElement()! })
    }
}

Here in the above example, if the view model is declared as below with @StateObject, the random string which was generated in the session will persist even though the view is redrawn .

@StateObject var viewModel = RandomStringViewModel()

It will reset to default when it is declared as @ObservedObject

@ObservedObject var viewModel = RandomStringViewModel()