switch.go
func (sw *Switch) AddListener(l Listener) {
	sw.listeners = append(sw.listeners, l)
}
func (sw *Switch) AddListener(l Listener) {
	sw.listeners = append(sw.listeners, l)
}
func ( sw * Switch ) AddListener ( l Listener ) { sw . listeners = append ( sw . listeners , l ) }
// Not goroutine safe.
Not goroutine safe .