What's the nicest way to do observer/observable in objective-c (iphone version)

There are two built-in ways of doing observation in Cocoa: Key-Value Observing and notifications. In neither system do you need to maintain or notify a collection of observers yourself; the framework will handle that for you.

Key-Value Observing (KVO) lets you observe a property of an object — including even a property that represents a collection — and be notified of changes to that property. You just need to send the object

-addObserver:forKeyPath:options:context:

passing
the object you want to receive updates, the key path of the property (relative to the receiver) for which you want to receive updates, and the types of updates you want to receive. (There are similar methods you can use if you want to observe a property representing
a collection.)

Notifications are older and heavier-weight. You register with an

NSNotificationCenter

—
usually the default center — an object and selector pair to be passed a notification when an event occurs. The notification object itself can contain arbitrary data via its

userInfo

property,
and you can choose to observe all notifications of a specific name rather than those that apply to a particular object.

Which should you use in any particular case? In general, if you care about changes to a specific property of a specific object, use Key-Value Observing. That's what it's designed for and it's intentionally lightweight. (Among other uses, it is the foundation
on which Cocoa Bindings are built.) If you care about a change in state that isn't represented by a property, then notifications are more appropriate.

For example, to stay in sync when the user changes the name of a model object, I'd use KVO. To know when an entire object graph was saved, I'd use notifications.
http://stackoverflow.com/questions/165790/whats-the-nicest-way-to-do-observer-observable-in-objective-c-iphone-version