How to update a SwiftUI view state from outside (UIViewController for example)

Question

I have a SwiftUI view:

struct CatView : View {      @State var eyesOpened: Bool = false      var body: some View {        Image(uiImage: eyesOpened ? #imageLiteral(resourceName: "OpenedEyesCat") : #imageLiteral(resourceName: "ClosedEyesCat"))     } } 

I'm trying to integrate it with in a regular UIViewController.

let hostingVC = UIHostingController<CatView>(rootView: cat) addChild(hostingVC) view.addSubview(hostingVC.view) hostingVC.view.pinToBounds(of: view) 

Now in the UIViewController if I try to set the eyesOpened property I get a

Thread 1: Fatal error: Accessing State<Bool> outside View.body

How are we supposed to make this work? Are SwiftUI views not supposed to work in this scenario?

Answer 1

@State is the wrong thing to use here. You'll need to use @ObservedObject.

@State: Used for when changes occur locally to your SwiftUI view - ie you change eyesOpened from a toggle or a button etc from within the SwiftUI view it self.

@ObservedObject: Binds your SwiftUI view to an external data source - ie an incoming notification or a change in your database, something external to your SwiftUI view.

I would highly recommend you watch the following WWDC video - Data Flow Through SwiftUI