This is a regression bug. I had filed openradar.appspot.com/FB8724843 back in September.
In testing this again today with Xcode 12.1.1 Release Candidate, it seems the user needs to be on iOS 14.2 for it to not occur. Just building the app against the 14.2 SDK doesn't seem to fix the bug if run on a 14.1 device.
Video showing bug at twitter.com/jamiemcd/status/1308132262635016199
jamiemcdaniel
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Update: The bug might not be with SwiftData or Observation framework, but rather in SwiftUI registering tracking for the observable.
If I add this to the body above I do get the view updating.
Section {
Text("Planes departing = \(airport.numberOfAirplanesDeparting)")
Text("Planes in flight = \(airport.numberOfAirplanesInFlight)")
Text("Planes landing = \(airport.numberOfAirplanesLanding)")
}
And it updates the text in the footer as well.
But with only the 3 Text views in the footer SwiftUI will not update when the airport's computed properties change.