Figuring out SwiftUI bindings

Has anyone figured out how and when SwiftUI bindings trigger view updates?

Here's an example I fail to understand. In this example, ParkingStore is a BindableObject that fetches data from a server and sends a didChange event every 5 seconds. When this happens, the map should update and add annotations for new data received from the server.

import MapKit
import SwiftUI

struct MapView: UIViewRepresentable {
    
    @ObjectBinding var store: ParkingStore

    // class RandomClass { }
    // let x = RandomClass()
    
    func makeCoordinator() -> MapViewCoordinator {
        MapViewCoordinator()
    }
    
    func makeUIView(context: UIViewRepresentableContext<MapView>) -> MKMapView {
        // Create and return a map
    }
    
    func updateUIView(_ view: MKMapView, context: UIViewRepresentableContext<MapView>) {
        // Adjust map annotations using the ParkingStore
    }
}

When this view loads, it calls makeCoordinator, then makeUIView, then updateUIView twice. However, this happens immediately and before store has any data or sends any didChange event. Afterwards, updateUIView is never called.

Strangely, when I add a stored property such as the one currently commented out, updateUIView does get called every time store sends a didChange event. This only seems to work if the property is of a reference type.

Is this a bug, or am I missing something?

1 Like

I was banging my head for hours, experiencing the same issue with beta 7. Your workaround works, thanks for this. Have you filed a radar that I can reference to report the same issue?

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