Objective-C Fundamentals

Responding to low-memory warnings
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- (NSString *)initWithString:(NSString *)str;
+ (NSString *)stringWithString:(NSString *)str;
The first line initializes a string you’ve explicitly allocated with a call to alloc. The second
performs a similar task, but it doesn’t need a string to be allocated beforehand.
Internally this method allocates a new string object and initializes it as required before
sending it an autorelease message and returning it.
9.6 Responding to low-memory warnings
The iPhone operating system implements a cooperative approach when it comes to
memory management and provides a warning to each application when the device
finds itself in a low-memory situation.
If the amount of free memory drops below a threshold, the operating system
attempts to release any excess memory that it holds, and if this doesn’t free up enough
memory, it also sends currently running applications a warning. If your application
receives a low-memory warning, it’s your chance to free up as much memory as possible
by deallocating unneeded objects or by clearing out cached information that can
easily be regenerated when next required.
The UIKit application framework provides access to the low-memory warning via a
number of mechanisms:
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Implementing the applicationDidReceiveMemoryWarning: message in your
application delegate
Overriding the didReceiveMemoryWarning message in your UIViewController
subclass
Registering for the UIApplicationDidReceiveMemoryWarningNotification
notification
We cover each of these scenarios in turn. It’s important that your application responds
to these warnings. If your application doesn’t respond, or you don’t manage to free
enough memory, the operating system may take even more drastic measures such
as “randomly” and abruptly exiting your application in its effort to gain additional
free memory.
9.6.1 Implementing the UIApplicationDelegate protocol
As discussed in chapter 7, protocols can have optional messages, which you can selectively
decide to implement. The UIApplicationDelegate protocol (implemented by
the RentalManagerAppDelegate class in the Rental Manager application) has an
optional message called applicationDidReceiveMemoryWarning:, which is sent whenever
the application detects a low-memory situation. This message can be implemented
as follows:
- (void)applicationDidReceiveMemoryWarning:(UIApplication *)application
{
NSLog(@"Hello, we are in a low-memory situation!");
}