How do I make it so editing text fields updates display as I type?

It seems that text fields only do their actions when I press Return or focus out of them. Then I saw a simple binding example (a concept I still don't master) and the bound objects still only updated when I focused out. Yet I see lots of other applications where actions happen as soon as I type. How do I do it? By Key-Value Observing? I tried to observe NSTextField's stringValue, but it didn't work.

 

Thanks! I'll try it. It wasn't quite obvious, as I was bumping into other similarly named structures, which did nothing.