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There are two methods for removing observers: -removeObserver: and -removeObserver:name:object.
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It's important for objects to remove observers before they're deallocated, in order to prevent further messages from being sent. [center addObserverForName: nil object: nil queue: nil usingBlock:^( NSNotification *notification) *See for yourself! An ordinary iOS app fires dozens of notifications just in the first second of being launched-many that you've probably never heard of before, nor will ever have to think about again. However, if both name and object are nil, then all notifications posted will trigger.
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So, if both name and object are set, only notifications with that name and the specified object will trigger. If name is set, only notifications with that name will trigger, but if nil is set, then all names will match. The name and object parameters of both methods are used to decide whether the criteria of a posted notification match the observer. Any concerns in this respect are the same as for any other block-based API. Just make sure to understand memory management rules when referencing self in blocks. It's perfectly safe and suitable for use in applications. Unlike its similarly named counterpart, this method actually returns the constructed observer object, which is necessary for unregistering the observer, as discussed in the next section.Ĭontrary to a recent article claiming otherwise, –addObserverForName:object:queue:usingBlock: should not be considered harmful. Instead of registering an existing object as an observer for a notification, this method creates its own anonymous object to be the observer, which performs a block on the specified queue (or the calling thread, if nil) when a matching notification is posted. The modern, block-based API for adding notification observers is –addObserverForName:object:queue:usingBlock. The traditional way to add an observer is –addObserver:selector:name:object:, in which an object (usually self) adds itself to have the specified selector performed when a matching notification is posted. * But like a tree falling in the woods, a notification is moot unless there's something listening for it. Adding ObserversĪll sorts of notifications are constantly passing through NSNotificationCenter. In the case of UIKeyboardWillShowNotification, frame positioning and animation timing are passed in userInfo, while the notification's associated object is nil. The object associated with that notification is the text field itself. Each time an event occurs, the notification goes through its dispatch table, and messages any registered observers for that event.Įach running Cocoa program manages its own default notification center, so it's unusual for a new notification center to be instantiated separately.Įach NSNotification object has a name, with additional context optionally provided by an associated object and userInfo dictionary.įor example, UITextField posts an NSNotification with the name UITextFieldTextDidChangeNotification each time its text changes. Observers register with a notification center to respond to particular events with a specified action. NSNotificationCenter provides a centralized hub through which any part of an application may notify and be notified of changes from any other part of the application. This week, we'll expand our look at the available options, with NSNotificationCenter & NSNotification. We've discussed the importance of how events are communicated in APIs previously in our article on Key-Value Observing. In Cocoa, there are a number of approaches to communicating between objects, with different characteristics of intimacy and coupling: This is as true of humans as it is within a computer process. Not being able to distinguish between these two circumstances leads to awkward situations. Sometimes communication is one-to-one, like an in-person conversation, while at other times, such as a television broadcast, it's one-to-many. One of the first lessons of socialization is to know one's audience. A medium defines the form and scale of significance in such a way to shape the very meaning of an idea. Any idea is inextricably linked to how it's communicated.