Core Components of a Notification

A notification encapsulates data exchanged between objects. It consists of three primary elements:

• name: An identifier for the notification type.
• object: A reference to the entity that posted the notification.
• userInfo: A dictionary containing supplementary data passed from poster to receiver.

@property (readonly, copy) NSString *name;
@property (nullable, readonly, retain) id object;
@property (nullable, readonly, copy) NSDictionary *userInfo;

Creating Notification Instances

Notifications can be instantiated using factory methods or designated initializers.

// Class factory methods
+ (instancetype)notificationWithName:(NSString *)notificationName object:(nullable id)sender;
+ (instancetype)notificationWithName:(NSString *)notificationName object:(nullable id)sender userInfo:(nullable NSDictionary *)additionalData;

// Initializer
- (instancetype)initWithName:(NSString *)notificationName object:(nullable id)sender userInfo:(nullable NSDictionary *)additionalData;

Posting Notifications to the Center

An object can dispatch notifications via the default notification center.

// Post a pre-configured notification instance
- (void)postNotification:(NSNotification *)notification;

// Post by specifying name and sender
- (void)postNotificationName:(NSString *)notificationName object:(nullable id)sender;

// Post with name, sender, and additional data
- (void)postNotificationName:(NSString *)notificationName object:(nullable id)sender userInfo:(nullable NSDictionary *)additionalData;

Registering Observers

Objects interested in specific notifications must register with the notification center.

Selector-Based Registration

The observer provides a method to be called when the notification arrives.

- (void)addObserver:(id)receiver
           selector:(SEL)handlerMethod
               name:(nullable NSString *)notificationName
             object:(nullable id)sender;

Block-Based Registration

A block is executed upon notification receipt.

- (id)addObserverForName:(nullable NSString *)notificationName
                  object:(nullable id)sender
                   queue:(nullable NSOperationQueue *)targetQueue
              usingBlock:(void (^)(NSNotification *note))eventHandler;

The targetQueue specifies where the block runs; nil implies the current queue.

Unregistering Observers

Observers must deregister before being deallocated to prevent crashes.

// Remove all registrations for an observer
- (void)removeObserver:(id)receiver;

// Remove specific registrations for an observer
- (void)removeObserver:(id)receiver
                  name:(nullable NSString *)notificationName
                object:(nullable id)sender;

Example deallocation code:

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

System-Generated Notifications

UIDevice Events

The singleton UIDevice instance broadcasts state changes.

UIDevice *device = [UIDevice currentDevice];

Relevant notifications include:

• UIDeviceOrientationDidChangeNotification
• UIDeviceBatteryStateDidChangeNotification
• UIDeviceBatteryLevelDidChangeNotification
• UIDeviceProximityStateDidChangeNotification

Keyboard Activity

The system posts notfiications related to keyboard appearence and geometry.

• UIKeyboardWillShowNotification
• UIKeyboardDidShowNotification
• UIKeyboardWillHideNotification
• UIKeyboardDidHideNotification
• UIKeyboardWillChangeFrameNotification
• UIKeyboardDidChangeFrameNotification

Accompanying userInfo dictionary keys:

• UIKeyboardFrameBeginUserInfoKey
• UIKeyboardFrameEndUserInfoKey
• UIKeyboardAnimationDurationUserInfoKey
• UIKeyboardAnimationCurveUserInfoKey

Comparison with Delegation

Both delegation and notifications facilitate communication between objects. However, delegation establishes a one-too-one relationship, where as notifications support a many-to-many pattern.