Instance property vs parameter in Swift

The other day I was refactor my code. I have

extension MainController: TabBarViewDelegate {
  func buttonDidPress index: Int) {
    let initialIndex = tabBarView.selectedIndex
    let wholeAppContentView = updateWholeAppContentView()
    view.addSubview(wholeAppContentView)
  }
}

The delegate method does not look right, as it’s hard to tell between required delegate method, or just instance method. Also it lacks a subject. I like this post API Design, you can read section Rule 19: Always say who’s talking

This is a simple rule, and an equally simple mistake to make. In your delegate methods, always pass the sender as a parameter. Always. Even for singletons. Even for things you cannot conceive would ever be used more than once simultaneously. No exceptions.

So I refactor the delegate, and conform to it.

extension MainController: TabBarViewDelegate {
  func tabBarView(_ view: TabBarView, buttonDidPress index: Int) {
    let initialIndex = tabBarView.selectedIndex
    let wholeAppContentView = updateWholeAppContentView()
    view.addSubview(wholeAppContentView) // This is the culprit ⚠️
  }
}

Even with just 1 line change in MainController.swift, the whole UI breaks, as all the views were added to the…