For
protocol ItemDetailViewControllerDelegate: class {
func itemDetailViewControllerDidCancel(_ controller: ItemDetailViewController)
func itemDetailViewController(_ controller: ItemDetailViewController, didFinishAdding item: ChecklistItem)
func itemDetailViewController(_ controller: ItemDetailViewController, didFinishEditing item: ChecklistItem)
}
Why is the first function “did cancel” so different from the others? It only has 1 parameter, and the naming is different than the other 2 functions. Are these functions already provided by Swift, or was this a function we made up?
These names were made up by the programmer (me) but they follow the Cocoa naming conventions (which admittedly are a bit strange sometimes).
It’s customary to place a reference to the “thing” that sends the protocol messages inside the message, which is why we pass along the controller: ItemDetailViewController parameter.
Now, the didFinishAdding and didFinishEditing messages have a second parameter, the ChecklistItem object. But didCancel does not have another parameter, only the reference to controller.
It would have been more consistent to write:
func itemDetailViewController(_ controller: ItemDetailViewController, didCancel)
but that makes didCancel a parameter, but with no value, which does not make sense in Swift.
I hope this explains it a bit better. 
If not, let me know and I’ll try harder!