PerformanceTracker and Architecture

[Alex0Klain]

Hello!
I can't fully understand whether it is necessary to transfer a technical metric, such as, for example, PerformanceTracker (from Firebase), which measures the time from init to onAppear in the Reducer, or can I leave this logic in the View?

[acosmicflamingo]

TCA encourages the practice of using dependencies within a reducer to ensure the whole structure comprises a single entry-point system (https://pointfreeco.github.io/swift-dependencies/main/documentation/dependencies/singleentrypointsystems/). This is to ensure that if you overwrite a dependency at the top-level of your app, it will propagate down to any view (assuming all views are powered by some sort of store and dependencies are accessed there).

From what I've seen, I think the answer you're going to get is something to the extent of having .onAppear send an action to store and then in your reducer's action, you'd make that call you're doing in the initializer. Hopefully I'm not providing completely incorrect information, others can chime in if I'm wrong :)

[Alex0Klain]

@acosmicflamingo With this approach, can't calling action affect the correctness of the metric itself? (She counts the time from init to onAppear)

[acosmicflamingo]

Yes you're right. I answered more with respect to when dependencies should be used in a reducer, but my answer does not actually apply here since you care about getting the most accurate performance metrics.

[mbrandonw]

which measures the time from init to onAppear in the Reducer

Hi @Alex0Klain, you definitely do not want to instrument a view's init. Views in SwiftUI are lightweight structs that can be created dozens or hundreds of times in a view's lifecycle. You should not perform any real work in the init of a view.

[Alex0Klain]

@mbrandonw But this metric measures the time from calling init to onAppear. Or is this also incorrect?

[mbrandonw]

Yes, that is not correct. It does not make sense to discuss the time from init to onAppear because init can be called hundreds of times when onAppear is only called a single time. A view struct being created has no semantic meaning in your application.

[Alex0Klain]

@mbrandonw Can you please tell me in which case init at the screen can be called again? At the moment, according to the logs, it is called 1 time and onAppear only when the screen is displayed :\

[mbrandonw]

This demonstrates how it is possible:

struct ParentView: View {
  @State var count = 0
  var body: some View {
    Form {
      Text(self.count.description)
      Button("+") { self.count += 1 }
      ChildView()
    }
  }
}
struct ChildView: View {
  init() { print("ChildView.init") }
  var body: some View {
    Text("Hi")
      .onAppear { print("ChildView.onAppear") }
    }
  }
}

Every time the "+" button is tapped the ChildView struct is initialized again. Structs are not long-living values. They are created and discarded dozens or hundreds of times. You cannot depend on the init being called in any reasonable way in a SwiftUI app.