Writing testable UI related code with modern Swift

[vanvoorden (Rick van Voorden)]

@grynspan

When you're using @Observable, you can use withObservationTracking(_:onChange:) in your test to observe changes:

@MainActor @Test func criticalBatteryEventually() {
  let viewModel = ...
  withObservationTracking {
    // ...
  } onChange: {
    #expect(viewModel. criticalBattery)
  }
}

Unfortunately… the public API on Observation is not willSet (which delivers the new value) or didSet (which took place after the value was set). The Observation API available outside Apple just tells you "some change is going to happen"… but engineers then have to work around that with an additional dispatch main or something to actually see the new value.

[Accepted with revision] SE-0395: Observability

I actually brought this up to a few engineers from Apple at the SwiftUI summit these last two days. Because the withObservationTracking that accepts willSet and didSet are @_spi(SwiftUI) (for now)… it's not completely clear what a "evolution" looks like to then just remove that spi and launch this as a legit "public" API. Would that be something that would need an internal SwiftUI approval? Would the community just make that decision to approve and then SwiftUI just has to go along with what th…