KVO vs NSNotification vs protocol/delegates?

I have some idea of which to use when but the exact usage is still not clear to me. Can someone explain with example?

Use a delegate if you want to talk to only one object. For example, a tableView has a delegate - only one object should be responsible for dealing with it.

Use notifications if you want to tell everyone that something has happened. For example in low memory situations a notification is sent telling your app that there has been a memory warning. Because lots of objects in your app might want to lower their memory usage it's a notification.

I don't think KVO is a good idea at all and try not to use it but, if you want to find out if a property has changed you can listen for changes.

Hope that helps.

PS This sums up why I think KVO is broken

Use a delegate when there is a "master/slave" relationship (delegate knows about the class and class knows about the delegate), with one class higher up the control hierarchy, and when it is clear that there won't be situations where other elements (mostly UI) will be interested in knowing what the class has to say.

Use notifications when the class is not interested in knowing who listens and how many they are, anybody and any number can register for the notifications.

KVO is useful to listen "without the class knowing", although of course that's not the case, the class on which KVO is applied does not need to be changed.