Delegate pattern and UIKit view controllers, what about -viewDidUnload?

Question

I'm pretty new to programming and I started learning ObjC and the CocoaTouch Framework. I've learnt about the delegation pattern and I'm pretty comfortable using it, but maybe I'm using it too much.

There is one thing I do not understand and it's specific to UIKit view controllers, I've read a lot of posts about it but I'm not able to find a clear answer.
Memory management with delegates?
Why are Objective-C delegates usually given the property assign instead of retain?
Let's suppose that I have a navigation controller and I'm pushing view controllers through it, imagine that the visible controller has as a delegate (assign not retain) a non-visible controller in the stack. A memory warning is incoming and all the view controllers (except the visible one ) are unloaded using the viewDidUnload and dealloc method, the delegate will be unloaded and the "callback" never sent.
If the new view controller is not pushed but presented modally the "connection" between delegate and the view controller is never lost, viewDidUnload is never called in the parent view.

Here is my question:
Is it correct to use delegation pattern between two view controllers?

Answer 1

It is certainly a correct design, depending on your controllers semantics. I used this when having a "master" view controller managing (and receiving delegate calls) from "slave" view controllers.

As to your analysis of what happens in case a memory warning is sent, there is possibly a slight misunderstanding, in that the view controller actually receives the viewDidUnload, but what this means is that the view controlled by it has been unloaded to get back some memory, so the controller can do its part of cleaning (as usual). The view controller is not itself "unloaded" or released or whatever. So the callback will always be sent. The only thing is that if the view had been previously unloaded, you would need to restore it.

Keep also in mind that, if is utterly impossible for you to recreate your unloaded views, you can prevent a specific view from being released by not calling super in your didReceiveMemoryWarning override. Take this suggestion "cum granum salis", however!

In the end, no need to use a Modal View.

Indeed, this is what Apple Docs say:

• (void)didReceiveMemoryWarning : The default implementation of this method checks to see if the view controller can safely release its view. This is possible if the view itself does not have a superview and can be reloaded either from a nib file or using a custom loadView method. If the view can be released, this method releases it and calls the viewDidUnload method.

• (void)viewDidUnload: This method is called as a counterpart to the viewDidLoad method. It is called during low-memory conditions when the view controller needs to release its view and any objects associated with that view to free up memory. Because view controllers often store references to views and other view-related objects, you should use this method to relinquish ownership in those objects so that the memory for them can be reclaimed. You should do this only for objects that you can easily recreate later, either in your viewDidLoad method or from other parts of your application. You should not use this method to release user data or any other information that cannot be easily recreated.

Answer 2

Your UIViewController is fairly lightweight and should never be unloaded in a low memory condition. The UIView owned by the view controller is very heavyweight and will definitely be unloaded in low memory conditions. Your controller should be ready to recreate the view if necessary, but you'll never need to regenerate your stack of view controllers.