Naming Convention

Notifications are identified by global NSString objects whose names are composed in this way:

Name of associated class + Did | Will + UniquePartOfName + Notification

For example:

• NSApplicationDidBecomeActiveNotification
• NSWindowDidMiniaturizeNotification
• NSTextViewDidChangeSelectionNotification
• NSColorPanelColorDidChangeNotification

Swift 2.3

NSNotificationCenter.defaultCenter().addObserver(self, 
                                                 selector: #selector(self.testNotification(_:)), 
                                                 name: "TestNotification", 
                                                 object: nil)

Swift 3

NSNotificationCenter.default.addObserver(self, 
                                         selector: #selector(self.testNotification(_:)), 
                                         name: NSNotification.Name(rawValue: "TestNotification"), 
                                         object: nil)

Objective-C

[[NSNotificationCenter defaultCenter] addObserver:self 
                                      selector:@selector(testNotification:) 
                                      name:@"TestNotification" 
                                      object:nil];

PS: It is also worth noting that the number of times an observer has been added has to be exactly the number of times the observer is removed. A rookie mistake is to add the observer in the viewWillAppear: of a UIViewController, but removing the observer in viewDidUnload:, will cause an uneven number of pushes and thus leaking the observer and the notification selector getting called in a superfluous manner.

Swift 2.3

//Remove observer for single notification
NSNotificationCenter.defaultCenter().removeObserver(self, name: "TestNotification", object: nil)
    
//Remove observer for all notifications
NotificationCenter.defaultCenter().removeObserver(self)

Swift 3

//Remove observer for single notification
NotificationCenter.default.removeObserver(self, name: NSNotification.Name(rawValue: "TestNotification"), object: nil)

//Remove observer for all notifications
NotificationCenter.default.removeObserver(self)

Objective-C

//Remove observer for single notification
[[NSNotificationCenter defaultCenter] removeObserver:self name:@"TestNotification" object:nil];
    
//Remove observer for all notifications
[[NSNotificationCenter defaultCenter] removeObserver:self];

Swift

NSNotificationCenter.defaultCenter().postNotificationName("TestNotification", object: self)

Objective-C

[[NSNotificationCenter defaultCenter] postNotificationName:@"TestNotification" object:nil];

Swift

let userInfo: [String: AnyObject] = ["someKey": myObject]
NSNotificationCenter.defaultCenter().postNotificationName("TestNotification", object: self, userInfo: userInfo)

Objective-C

NSDictionary *userInfo = [NSDictionary dictionaryWithObject:myObject forKey:@"someKey"];
[[NSNotificationCenter defaultCenter] postNotificationName: @"TestNotification" object:nil userInfo:userInfo];

Swift

func testNotification(notification: NSNotification) {
    let userInfo = notification.userInfo
    let myObject: MyObject = userInfo["someKey"]
}

Objective-C

- (void)testNotification:(NSNotification *)notification {
    NSDictionary *userInfo = notification.userInfo;
    MyObject *myObject = [userInfo objectForKey:@"someKey"];
}

Instead of adding an observer with a selector, a block can be used:

id testObserver = [[NSNotificationCenter defaultCenter] addObserverForName:@"TestNotification"
                                                                    object:nil 
                                                                     queue:nil 
                                                                usingBlock:^(NSNotification* notification) {
    NSDictionary *userInfo = notification.userInfo;
    MyObject *myObject = [userInfo objectForKey:@"someKey"];
}];

The observer can then be removed with:

[[NSNotificationCenter defaultCenter] removeObserver:testObserver 
                                                name:@"TestNotification"
                                              object:nil];

// Add observer
let observer = NSNotificationCenter.defaultCenter().addObserverForName("nameOfTheNotification", object: nil, queue: nil) { (notification) in
    // Do operations with the notification in this block
}

// Remove observer
NSNotificationCenter.defaultCenter().removeObserver(observer)