Objective-C Fundamentals

198 CHAPTER 9 Memory management
new notifications and observers to register their interest in receiving particular types
of notifications. A notification center is similar in concept to an email distribution list.
One or more email addresses (observers) are registered with the mailing list (notification
center), and when a new email (notification) is sent by a sender, the mailing list
distributes it to all currently registered email addresses. A given email address may not
receive all emails sent to the server because it’s unlikely the address will be registered
with every email list offered by the mail server.
In Objective-C the first step in handling a notification is to define a method you
want invoked whenever a suitable notification occurs. For example, you could define
the following method in the CTRentalProperty class:
- (void)HandleLowMemoryWarning:(NSNotification *)notification
{
NSLog(@"HandleLowMemoryWarning was called");
}
Once the method is defined, you must inform the NSNotificationCenter that you’re
interested in observing a particular type of notification. This can be done by sending
an addObserver:selector:name:object: message, as shown in the following listing.
Listing 9.3
Registering intent to observe memory warning notifications
- (id)initWithAddress:(NSString *)newAddress
rentalPrice:(float)newRentalPrice
andType:(PropertyType)newPropertyType {
if ((self = [super init])) {
self.address = newAddress;
self.rentalPrice = newRentalPrice;
self.propertyType = newPropertyType;
}
[[NSNotificationCenter defaultCenter]
addObserver:self
selector:@selector(HandleLowMemoryWarning)
name:UIApplicationDidReceiveMemoryWarningNotification
object:nil];
}
return self;
The observer and selector arguments specify which object should act as the observer
and the message that object wishes to be sent when a suitable notification occurs. The
last two arguments specify the particular notification you’re interested in observing.
This example specifies that you want to observe a notification called UIApplication-
DidReceiveMemoryWarningNotification. Passing nil for the object argument means
that you’re interested in observing this event no matter who the sender is. If you’re
interested only in notifications generated by a particular object, you can specify that
object instead to filter out notifications generated by other objects.
Registering with a notification center indicates your intent to observe a notification
event, so it’s natural to need to unregister your intent when you no longer want to