Which memory is released by didReceiveMemoryWarning/viewDidUnload?

Question

My iPhone application is running into problems when deployed to the device, principally because I haven't handled memory warnings thus far (no problem in the simulator, with 4GB of ram on my dev machine!). No problem there, I just need to handle these warnings more skilfully (less suckily...).

Question is, which memory does the runtime release... just the view and subviews? I suspect that it is just these, but want to be sure that the runtime will not dereference any of the objects or memory in my controller (ie. not in the view).

Subquestion: if it is just the view and subviews, do I need to do anything special in viewDidLoad to make sure that when the view is brought back into memory the view shows correct data, or is it all handled automatically via my IBOutlet-s?

Answer 1

There are potentially many things that the view, or its subviews, may cache - such as image data. These are the sort of things that will be purged. Anything specific to your application that you can afford to flush you'll need to do yourself by handling that callback.

However, this may be more of an indication that you are either leaking memory or not being as efficient with it as you might be. It's certainly worth running the app in Instruments with the Leaks tool, as well as running with the CLANG compiler's static code analyser. Also, examine your code to see if you are holding on to blocks of memory you don't really need - whether you can compress images more etc.

Remember, before the 3GS or the latest iPod touch, system memory was 128Mb but you should only count on having about 25-30Mb available to your application

Answer 2

Neither viewDidUnload nor didRecieveMemoryWarning automatically release anything. You need to override both of these methods.

Generally, viewDidUnload should release anything in the view that wasn't created in IB, and that you can reasonably deal with reloading when the view is loaded up again.

And didRecieveMemoryWarning is a message that is sent to your app when the system is running low on memory. When your app receives this message, you should release anything and everything that you don't immediately need, or risk the system forcibly shutting down your app.