RAA - observable

A variation on the observer pattern.

Allows the state of a variable to be exposed to observers. An observer registers code to execute when the variable changes (and also a pattern to detect changes of interest). Notification happens when the writer method is called.

New in 0.5:

• Fixed a harmless warning about an uninitialized __observer_map variable when assigning to an observable var before setting up any when clauses. Thanks to Hadmut Danisch.
• If an observer proc takes 3 args, then the third arg is assigned the object being observed. If it takes 4 args, then the fourth is assigned the name of the attribute. This is useful if there are a lot of objects or attrs. See the examples/mem-usage-*.rb for examples and discussion. Thanks again to Hadmut.