https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/BuildingCocoaApps/AdoptingCocoaDesignPatterns.html#//apple_ref/doc/uid/TP40014216-CH7-XID_10

Key-Value Observing
Information forthcoming.

後回しにしてるだけで、そのうち来るでしょ