What is parameter `object` in NSNotification addObserver:?

Question

One of my class named Message.m is posting a notification with an object sentObject as below

NSDictionary *sentObject = [NSDictionary dictionaryWithObjectsAndKeys:draftData.arr,@"data", nil];

//Post notification to inform a receiver to reload data     
[[NSNotificationCenter defaultCenter] postNotificationName:@"reloadDuringSave" object:self userInfo:sentObject];

DraftData.m will be be the receiver to catch the notification as follow

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

For posting notification, userInfo can be nil or can be an object (like sentObject as type of NSDictionary in this example).

Question:

What are other params for object in addObserver method? Can they be anything other than nil, and if so what?

Answer 1

That "object" parameter to "addObserver" is an optional filter. Upon posting a notification you can set an object to the sender of the notification, and will then only be notified of that sender's events. If set to "nil" you will get all notification of this type (regardless who sent them).

Answer 2

You can use it to pass any object with the notification. The receiver of the notification will then be able to access that object. For example, you could implement dataReloaded like this:

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

    NSLog(@"%@", notification.object); // this will log the object you passed in addObserver:selector:name:object:

}

It can be useful when you want to pass on data with your notification, so that the receiver of a notification can use that data too.

Answer 3

For anyone interested in apple's documentation. This is what it says:

notificationSender

The object whose notifications the observer wants to receive; that is, only notifications sent by this sender are delivered to the observer. If you pass nil, the notification center doesn’t use a notification’s sender to decide whether to deliver it to the observer.