Is there an analogue of the withObservationTracking for use outside View?

[riley-usagi]

Hello.

Tell me, please, is it possible to track changes in the parameters outside the View hierarchy?

[mbrandonw]

Hi @riley-usagi, yes, we provide a withPerceptionTracking method that works exactly as Swift's native withObservationTracking.

[PatrickDanino]

Keep in mind that you need to be careful using withPerceptionTracking from within a class. After some time spent debugging, I filed this Radar with Apple: https://openradar.appspot.com/FB13556571

TL,DR: withPerceptionTracking seems to break SwiftUI views if you reference a property from within the class that is not itself a Perceptible/Observable object. Here's a very simple example that breaks any SwiftUI view tracking count:

@Perceptible class Test {
  var count: Int = 0
  init() {
     withPerceptionTracking { _ = self.count }
     onChange: { [weak self] in print("Count: \(self?.count ?? 0)") }
  }
}

A view that is displaying an instance of Test.count will not work. If you comment out the code inside init, it will work. That wasn't intuitive at all.

I'll note that I was able to repro this on iOS 17 using Observable exclusively, so this is not due to the Perception framework.

The workaround, which worked for me, was to rely on didSet instead. You can also use withPerceptionTracking to track changes for properties that support Perceptible/Observable, similar to the example Apple has in the SDK docs.

[mbrandonw]

Hi @PatrickDanino, this doesn't have to do with withObservationTracking or withPerceptionTracking. It can happen without:

@Perceptible class Test {
  var count: Int = 0
  init() {
    _ = self.count
  }
}

This is just a gotcha with using @Observable in general. If you access an observable field in the init, and if the class is created in a parent view's body, then that parent view will observe the field. We filed the issue with Apple and they consider it to be working as expected, although it is surprising.