IPhone memory problems...

Question

Hi, I am working on an App that is already been made but Memory Management was not considered in the development stages. So what can I do to keep the App memory usage low as soon as I get a memory warning? Is there any general tool or some piece of code that I can use to release any unused memory?

Answer 1

You should have a look at the method

- (void)didReceiveMemoryWarning

of your UIViewControllers. This method is called when you receive a memory warning. Here you can release objects currently not used. But it's your part to determine what is used and what not.

The "Build and Analyze" feature of XCode is a tool you could use to see if the code contains any obvious memory leaks.

You should have a look at the small section "Memory Management" in the UIViewController class reference: http://developer.apple.com/iphone/library/documentation/UIKit/Reference/UIViewController_Class/Reference/Reference.html

Also an important document is the "Memory Management Programming Guide": http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html

Answer 2

Xcode can be combined with the Instruments tool to show you the places where your application is leaking memory, i.e. where reserved memory is not released properly. CIMGF has a solid tutorial on this: http://www.cimgf.com/2008/04/02/cocoa-tutorial-fixing-memory-leaks-with-instruments/

Answer 3

Two things.

First, if you're using any sort of view hierarchy (tab bar, UINavigationController, or something you've rolled on your own), the message is going to propagate upward. That means one handler for memory messages in your root-level UIViewController subclass can handle memory events for the whole app, which is very handy.

Second, you want to go after the low-hanging fruit. In the app I'm currently working on I have a couple different arrays of dictionaries that contain my app data, and each of those dicts contains both a thumbnail and a larger image. Obviously those make up the bulk of the bits I'm keeping in memory. So the first thing my root view controller does when it gets a memory warning is go through those data sets and set those images to nil. Because they're retained properties, they get released when the setter is called and the images are freed from memory. Then I have functions in my view controllers to notice the nil-ness of those image fields and reload them from the server.

By the way (okay... two things and a "by-the-way"), memory warnings aren't a problem. Some people seem to feel bad about getting them, want to redesign everything about their app so they never get one. That's really not necessary; even the best-designed app will get warned about memory occasionally, just because of the unpredictability of the background apps on the device. The important thing is to handle them well.