NSNotificationCenter in iOS

Question

[[NSNotificationCenter defaultCenter] postNotificationName:@"TapNewProduct" object:self.productID];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(tapNewProduct:) name:@"TapNewProduct" object:nil];
[[NSNotificationCenter defaultCenter] removeObserver:self name:@"TapNewProduct" object:nil];

What is the difference between the three sections of the code inside the "object"?

Answer 1

Adding the Observer:

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

You add the observer to "listen for things". The statement above is saying, "If you hear a TapNewProduct notification, then execute the method (selector) tapNewProduct.

Posting Notifications to the Observer:

// This can be on a different viewController or the same viewController the
// observer lives
[[NSNotificationCenter defaultCenter] postNotificationName:@"TapNewProduct" object:self.productID];

You'd post a notification whenever you want the tapNewProduct method (selector) to execute.

Removing the Observer:

This line posts the notification. You'd probably have this in didSelectRowAtIndexPath, a UIButton, or a UICollectionView.

[[NSNotificationCenter defaultCenter] removeObserver:self name:@"TapNewProduct" object:nil];

This line would go in dealloc of the UIViewController where you added the observer. When it's deallocated, the observer is removed.

Steps:

1) Add the observer first.

2) THEN you post the notification, which the observer picks up and then executes the selector you specified when you added the observer.

3) When you deallocate the class where you originally added the observer, you remove the observer (put it in dealloc, not viewDidDisappear).

See this answer for a more detailed explanation:

https://stackoverflow.com/a/2191802/4475605

Answer 2

This "object"s in the three of sections are the same: The object posting the notification.

[[NSNotificationCenter defaultCenter] postNotificationName:@"TapNewProduct" object:self.productID];

This "object" is the object posting the notification.

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

If set the "object", the observer will only response the notification when the notification's object is the "object"

[[NSNotificationCenter defaultCenter] removeObserver:self name:@"TapNewProduct" object:nil];

Just like the above, it only remove the observer whose object is the "object"

In a word, the "object" is notification sender, meanwhile, if set, means a more criterion