KVO vs NSNotification vs protocol/delegates?

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.

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

Delegation is a design pattern that you use when you want some other object to modify the sender's behavior. Example: terminal windows avoid showing any lines or characters that are clipped by the window's edges, because the terminal window's delegate alters the size of the window to ensure this.

Notification is a pattern to use when you don't need a response. Example: you get a notification that the system is about to go to sleep. The sender of that notification doesn't care what you do about it.