Key-Value Observing

Key-value observing is a mechanism for transparently notifying observers of changes in object properties. At the beginning of the “Key-Value Coding” section, you built a table view cell that could display any object. In that example, the data was static. If you changed the data, the cell wouldn’t update. Let’s improve that now. You can make the cell automatically update whenever its object changes. You need a changeable object, so use the current date and time. You use key-value observing to get a callback every time a property you care about changes.

KVO has a lot of similarities to NSNotificationCenter. You start observing using addObserver:forKeyPath:options:context:. To stop observing, you use removeObserver:forKeyPath:. The callback is always observeValueForKeyPath:ofObject:change:context:. Here are the modifications required to create 1,000 rows of date cells that automatically update every second.