How to deal with low memory warnings on the iPhone?

Question

I'm having problems with my application receiving low memory warnings while the user is deep within a navigation controller stack of views. After the user browses through a bunch of hierarchical options in subsequent UITableViews, he can open a PDF document in a UIWebView (in a different view controller).

Everything works fine, the PDF loads and the user can flip through the pages. However, when the document is a bit large, or has several pages, and the user taps on the "Back" button in the navigation controller, he app crashes as the previous view controller in the navigation controller stack has been deallocated.

After searching around for ways to deal with low memory warnings, and dealing with this type of problem, I found several posts that just advise people to release the objects that can be released, and then lazy-load them later on when the user tries to load a view that has been deallocated. One of these posts is Craig Hockenberry's Dealing with memory loss: the cleanup post.

While that's a bit helpful, it doesn't give me much information to work from.

Can someone provide a simple guide on how to deal with low memory warnings, and how to implement "lazy loading" of objects?

Answer 1

Your first task is to figure out what exactly is causing the problem. Which object is being deallocated? Usually, fixing this is simply a matter of making sure the object is retained properly.

I suggest you use NSZombiesEnabled to try and track down the object you're having trouble with. Once you have that, you can make sure you're retaining it properly.

Answer 2

When memory is low, the system sends out a series of memory-related messages. Any instantiated view controllers will get the -didReceiveMemoryWarning message. If you don't implement this, the default action (assuming the view controller is not front-most) is to release the controller's view member. If you've got hooks into it, or perhaps into its subviews, that could cause issues when your controller gets returned to the top of the stack.