Question

I have a doubt about classes that designate a delegate.

Sometimes the delegate object needs to implement a protocol, sometimes not. For example, if you want to display an UIAlertView, you create it specifying a delegate like self. Then, the class that is calling the UIAlertView must implement the UIAlertViewDelegate protocol.

If you create other classes like NSUrlConnection with the connectionWithRequest:delegate: message, you designate a delegate, which will receive the delegate messages sent from NSUrlConnection as the connection progresses, but the object designated as a delegate do not have to implement a protocol.

Am I missing something? How do I understand when I should implement a protocol? Is it always clear from the API reference as it is for UIAlertViewDelegate?

I’ve noticed that UIAlertView reference only presents instance methods and tells you that you have to implement the UIAlertViewDelegate protocol, while NSUrlConnection does not talk about protocols to implement, but it has a delegate methods section.

Could NSUrlConnection have been defined like the UIAlertView class, without delegate methods but with a NSUrlConnectionDelegate protocol.

Or am I missing the point? It seems to me two different ways for similar purposes, but maybe I am wrong. I hope I’ve been quite clear, I just want to fully understand the rationale behind this stuff.

Answer 1

NSURLConnection itself implements the delegate methods, providing some default behaviors for the those events.

UIAlertView, by default, simply does not handle those events. And allow you to make a delegate to customize the handling.

The difference is whether default behaviors are needed.