Delegate or notification — when to choose which?

Middle PRO

The point

These are three different ways to say "something happened", and the choice depends on the number of receivers and the strength of coupling:

• Delegate — one receiver, an explicit protocol contract, a weak reference (AnyObject protocol); the sender knows a receiver exists and can get an answer back (shouldSelectRow returns a Bool). Examples: UITableViewDelegate, URLSessionDelegate.
• Closure / callback — one receiver, a one-off or simple event (onConfirm, a completion). Shorter than a delegate, but [weak self] is needed when stored.
• NotificationCenter — many receivers, the sender knows none of them, loose coupling, no answer back. Examples: keyboardWillShow, theme change, logout, system notifications.

On "synchronicity": a delegate and a closure are called directly, and notifications sent with post are also delivered synchronously on the sender's thread — this is not a selection criterion. The criterion is coupling and the number of subscribers.