Objective-C: Date picker and level indicator

I'm using an NSDatePicker and NSLevelIndicator to try and set/display certain values of an object. I don't want to use bindings. My first thought would be to try and set a delegate of the date picker/level indicator to be my controller class so that I can be notified when either of those is changed. However, NSDatePicker and NSLevelIndicator don't have a delegate (at least, none that I can see in interface开发者_开发百科 builder). How then do I keep track of when these things are changed?

NSControl and its subclasses use the target / action mechanism to alert you when their value changes. Some delegate protocols work in a similar manner, but in general delegates are used to modify the behavior of an object, while target / action alerts your controller of a change in a UI control.