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What is the @ObservedObject property wrapper?

Paul Hudson    @twostraws   

Updated for Xcode 12.0

SwiftUI gives us the @ObservedObject property wrapper so that views can watch the state of an external object, and be notified when something important has changed.

For example, we might use something like this:

class Order: ObservableObject {
    @Published var items = [String]()
}

struct ContentView: View {
    @ObservedObject var order = Order()

    var body: some View {
        // your code here
    }
}

That Order class uses @Published so it will automatically send change announcements when items changes, and ContentView uses @ObservedObject to watch for those announcements. Without @ObservedObject the change announcements would be sent but ignored.

Although that looks straightforward enough, it’s worth digging into a few specifics.

First, any type you mark with @ObservedObject must conform to the ObservableObject protocol, which in turn means it must be a class rather than a struct. This isn’t optional – SwiftUI requires us to use a class here.

Second, observed objects are specifically designed for data that is external to your view, which means it might be shared across more than one view. The @ObservedObject property wrapper will automatically make sure the property is watched closely so that important changes will reload any views using it.

Third, not all properties in an observed object cause views to refresh – you need to decide which properties should send change notifications, either using @Published or custom announcements. Types that conform to ObservableObject are given a default objectWillChange publisher to make custom announcements as needed.

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