In FrTime (and probably other FRP systems) an inverse depedency graph is used to update signals efficiently. When an event occurs, new values are inserted into the signals at the root of the (inverse) graph - if a signal's value has changed, any signals that depend on that signal will also be updated, propagating all the way to the leaves of that inverse dependency graph. Finally, the runtime can observe the new values of those final signals to update the outside world.