once(type, listenerFn, opt_scope)code »
Registers a one-time listener which will be called only the first time an
event is emitted, after which it will be removed.
Parameters
- type
string
The type of event to listen for.
- listenerFn
Function
The function to invoke when the event is fired.
- opt_scope
?Object=
The object in whose scope to invoke the listener.