once method
Implementation
void once (String name, {dynamic param, EventCallBack? cb}){
if(!events.containsKey(name))
events[name] = [];
events[name]!.add(_Event(cb,param,true));
}
void once (String name, {dynamic param, EventCallBack? cb}){
if(!events.containsKey(name))
events[name] = [];
events[name]!.add(_Event(cb,param,true));
}