Send and receive messages through NSNotificationCenter in Objective-C?

I am attempting to send and receive messages through `NSNotificationCenter` in Objective-C. However, I haven't been able to find any examples on how to do this. How do you send and receive messages through `NSNotificationCenter`?

This one helped me:

// Add an observer that will respond to loginComplete
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(showMainMenu:)
name:@"loginComplete" object:nil];


// Post a notification to loginComplete
[[NSNotificationCenter defaultCenter] postNotificationName:@"loginComplete" object:nil];


// the function specified in the same class where we defined the addObserver
- (void)showMainMenu:(NSNotification *)note {
NSLog(@"Received Notification - Someone seems to have logged in");
}

Source:

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@implementation TestClass

- (void) dealloc
{
// If you don't remove yourself as an observer, the Notification Center
// will continue to try and send notification objects to the deallocated
// object.
[[NSNotificationCenter defaultCenter] removeObserver:self];
[super dealloc];
}

- (id) init
{
self = [super init];
if (!self) return nil;

// Add this instance of TestClass as an observer of the TestNotification.
// We tell the notification center to inform us of "TestNotification"
// notifications using the receiveTestNotification: selector. By
// specifying object:nil, we tell the notification center that we are not
// interested in who posted the notification. If you provided an actual
// object rather than nil, the notification center will only notify you
// when the notification was posted by that particular object.

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

return self;
}

- (void) receiveTestNotification:(NSNotification *) notification
{
// [notification name] should always be @"TestNotification"
// unless you use this method for observation of other notifications
// as well.

if ([[notification name] isEqualToString:@"TestNotification"])
NSLog (@"Successfully received the test notification!");
}

@end

... somewhere else in another class ...

- (void) someMethod
{

// All instances of TestClass will be notified
[[NSNotificationCenter defaultCenter]
postNotificationName:@"TestNotification"
object:self];

}

There is also the possibility of using blocks:

NSOperationQueue *mainQueue = [NSOperationQueue mainQueue];
[[NSNotificationCenter defaultCenter]
addObserverForName:@"notificationName"
object:nil
queue:mainQueue
usingBlock:^(NSNotification *notification)
{
NSLog(@"Notification received!");
NSDictionary *userInfo = notification.userInfo;

// ...
}];

[Apple's documentation][1]

[1]:

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:

if you're using NSNotificationCenter for updating your view, don't forget to send it from the main thread by calling `dispatch_async`:

dispatch_async(dispatch_get_main_queue(),^{
[[NSNotificationCenter defaultCenter] postNotificationName:@"my_notification" object:nil];
});

To expand upon [dreamlax's example](

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)... If you want to send data along with the notification

In posting code:

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

In observing code:

- (void) receiveTestNotification:(NSNotification *) notification {

NSDictionary *userInfo = notification.userInfo;
MyObject *myObject = [userInfo objectForKey:@"someKey"];
}

**SWIFT 5.1 of selected answer for newbies**

class TestClass {
deinit {
// If you don't remove yourself as an observer, the Notification Center
// will continue to try and send notification objects to the deallocated
// object.
NotificationCenter.default.removeObserver(self)
}

init() {
super.init()

// Add this instance of TestClass as an observer of the TestNotification.
// We tell the notification center to inform us of "TestNotification"
// notifications using the receiveTestNotification: selector. By
// specifying object:nil, we tell the notification center that we are not
// interested in who posted the notification. If you provided an actual
// object rather than nil, the notification center will only notify you
// when the notification was posted by that particular object.

NotificationCenter.default.addObserver(self, selector: #selector(receiveTest(_:)), name: NSNotification.Name("TestNotification"), object: nil)
}

@objc func receiveTest(_ notification: Notification?) {
// [notification name] should always be @"TestNotification"
// unless you use this method for observation of other notifications
// as well.

if notification?.name.isEqual(toString: "TestNotification") != nil {
print("Successfully received the test notification!")
}
}
}

... somewhere else in another class ...

func someMethod(){
// All instances of TestClass will be notified
NotificationCenter.default.post(name: NSNotification.Name(rawValue: "TestNotification"), object: self)
}