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How to send state updates manually using objectWillChange

Paul Hudson    @twostraws   

Updated for Xcode 13.0 beta 2

Although using @Published is the easiest way to control state updates, you can also do it by hand if you need something specific. For example, you might want the view to refresh only if you’re happy with the values you’ve been given.

All observable objects automatically get access to an objectWillChange property, which itself has a send() method we can call whenever we want observing views to refresh.

For example:

// Create an observable object class that announces 
// changes to its only property
class UserAuthentication: ObservableObject {
    var username = "Taylor" {
        willSet {
            objectWillChange.send()
        }
    }
}

struct ContentView: View {
    // Create an instance of our object
    @StateObject var user = UserAuthentication()

    var body: some View {
        VStack(alignment: .leading) {
            TextField("Enter your name", text: $user.username)
            Text("Your username is: \(user.username)")
        }
    }
}

Download this as an Xcode project

Notice how we have a willSet property observer attached to the username property of UserAuthentication, allowing us to can run code whenever that value changes. In our example code, we call objectWillChange.send() whenever username changes, which is what tells the objectWillChange publisher to put out the news that our data has changed so that any subscribed views can refresh.

Tip: This example is no different from using @Published on the property, but now that we have a custom call to objectWillChange.send() we can add extra functionality – we could save the value to disk, for example.

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