iPhone Delegates - How to use them correctly

Question

I have 2 or 3 views in my iPhone application where I have various pieces of functionality that use delegates. In all cases the delegates are assigned to "self", responding specifically to that view's actions and interacting with instance variables.

However, if I do something that takes a bit of time with a delegate, and leave the view, obviously it crashes my app as the delegate methods get called on a view I've left.

Typically in my delegate methods I am doing things like interacting with IBOutlets, calling other instance methods, saving data to Core Data etc...

How can I work with delegates better? Is what I'm doing typical, or not?

Thanks for any guidance!

Answer 1

Depends on the use case. If, for example, you've got a UINavigationController that manages a ViewController that use something such as Location Services, when you pop the View Controller off of the stack you're going to want to set the CLLocationManager's delegate to nil. You can do this in the dealloc method.

Answer 2

Can you give a specific example of an issue you're facing?

I've encountered this situation once when dealing with MapKit (race conditions involving delegate callbacks and delegate deallocation). However, in general, I think it's an indication of a bad design decision when your delegate becomes invalidated as a result of race conditions, but I could be wrong.

Typically, the objects that make use of your delegate should exist within the context of the delegate itself. So, for example, the same class that contains various IBOutlets that you want to manage with delegate callbacks should also be the delegate of those IBOutlets. That way, when the class (i.e., the delegate) is deallocated, the IBOutlets are (hopefully) also deallocated, so they won't be making callbacks to anything.

bpapa's right. More generally, if you have a potentially lengthy delegate callback, either make sure that 1) the delegate is an object that won't be deallocated during the lifecycle of the delegator (e.g., UINavigationController managing UIViewControllers) or 2) the delegator's delegate object is set to nil during the delegate's deallocation.

... That last sentence was a mouthful. :)