Objective-C Fundamentals

194 CHAPTER 9 Memory management
Figure 9.3 The Simulate Memory
Warning option found in the Hardware
menu of the iPhone Simulator can be
used to test how well your application
responds to low-memory scenarios.
For reasons outlined previously, the iPhone Simulator isn’t a representative example
of the kinds of memory constraints your applications will find themselves in while running
on a real device. To help developers test their applications in low-memory conditions,
the iPhone Simulator provides a way to generate a simulated low-memory
situation. This feature can be initiated by selecting Simulate Memory Warning in the
Hardware menu (see figure 9.3). The simulator fakes a low-memory situation, no matter
how much memory is currently available on your computer.
When you select the Simulate Memory Warning option, you should see that your
application delegate is sent an applicationDidReceiveMemoryWarning: message, as
demonstrated by the following log messages appearing in the Xcode debugger console:
Received simulated memory warning.
Hello, we are in a low-memory situation!
Notice that the iPhone Simulator also emits a log message stating that the low-memory
situation was faked. Obviously, if a real low-memory situation occurs on a device, you
wouldn’t see this additional log message.
In a typical application it’s possible that your UIApplicationDelegate implementation
may not have easy access to significant resources that it can free on a moment’s
notice. It’s more probable that the individual UIViews and UIViewControllers that
make up the visual aspects of your application will have access to the memory
resources that can be returned to the operating system. Therefore, the UIView-
Controller class provides an alternative mechanism to access low-memory warnings
in the form of a method called didReceiveMemoryWarning that can be overridden by
interested subclasses.
9.6.2 Overriding didReceiveMemoryWarning
In a UIViewController subclass, such as the RootViewController class in the Rental
Manager application, you can handle the low-memory warning condition by implementing
a method named didReceiveMemoryWarning:
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
[cityMappings release];