KVO and Constraint Solvers
Marcel Weiher: The Siren Call of KVO and (Cocoa) Bindings.
You probably already figured it out: bindings are one-way dataflow constraints, specifically with the equation limited to y = x1. More complex equations can be obtained by using NSValueTransformers. KVO is more of an implicit invocation mechanism that is used primarily to build ad-hoc dataflow constraints.