"Two-way flow aka data binding binds two pieces of state: in most cases, one inside the controller (e. g. some variable), and one inside the view (e. g. contents of textbox). Binding means that, when one piece changes, the other piece changes as well and gets the same value, so you can pretend that there's only one piece of state involved (while there's two actually). Write events are going back and forth between controllers and views - thus two-way"

Think of watcher as a pub/sub system that syncs view and data

Ref: stackoverflow.com/a/47652784
vasanthk.gitbooks.io/react-bits/content/pat..