Cocoa Programming for OS X: The Big Nerd Ranch Guide

For the More Curious: More on Key-Value Observing

How did the label become an observer of the temperature key in the MainWindowController object? The code to become an observer of this key might look something like this:

let mainWindowController = ...
mainWindowController.addObserver(self,
                     forKeyPath: "temperature",
                        options: NSKeyValueObservingOptions.Old,
                        context: &MyKVOContext)

This method is defined in NSObject. It is how you say, “Hey! Let me know whenever temperature changes.” The options and context determine what extra data is sent along when temperature changes. The method that is triggered looks like this:

override func observeValueForKeyPath(keyPath: String!, ofObject object: AnyObject!, change: [NSObject : AnyObject]!, ...