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Khoa Pham
Khoa Pham

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Instance property vs parameter in Swift

Original post https://github.com/onmyway133/blog/issues/72

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)
  }
}
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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 ⚠️
  }
}
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Even with just 1 line change in MainController.swift, the whole UI breaks, as all the views were added to the tab bar. Strange 😡 .

It didn't take long until I remember that parameter takes precedence over instance property if they have same name. So in this case, the compiler, without warning, assume you're dealing with view from TabBarView ⚠️

That's why you often use self to disambiguate.

struct User: Codable, Equatable {
  let firstName: String
  let lastName: String

  init(firstName: String, lastName: String) {
    self.firstName = firstName
    self.lastName = lastName
  }
}
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Back to our code. The workaround is to specify self to specify view of MainController

self.view.addSubview(wholeAppContentView)
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Well, you may say, who should add view again in case of tab bar changes 😬 This is a bad example, but the lesson is learned 😇

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