Module Test::Unit::UI::Util::Observable
In: test/unit/ui/util/observable.rb
This is a utility class that allows anything mixing it in to notify a set of listeners about interesting events.
add_listener    notify_listeners    remove_listener   
Public Instance methods
add_listener(channel_name, listener_key=NOTHING, &listener) {|value| ...}
Adds the passed proc as a listener on the channel indicated by channel_name. listener_key is used to remove the listener later; if none is specified, the proc itself is used.

Whatever is used as the listener_key is returned, making it very easy to use the proc itself as the listener_key:

remove_listener(channel_name, listener_key)
Removes the listener indicated by listener_key from the channel indicated by channel_name. Returns the registered proc, or nil if none was found.
notify_listeners(channel_name, value=NOTHING)
Calls all the procs registered on the channel indicated by channel_name. If value is specified, it is passed in to the procs, otherwise they are called with no arguments.