package server import ( "log" "math/rand" "sync" ) // topic represents a channel to which subscribers can subscribe, and publishers // can publish a message type topic struct { ID string subscribers map[int]subscriber mu sync.Mutex } // subscriber is a function that is called for every new message on a topic type subscriber func(v *visitor, msg *message) error // newTopic creates a new topic func newTopic(id string) *topic { return &topic{ ID: id, subscribers: make(map[int]subscriber), } } // Subscribe subscribes to this topic func (t *topic) Subscribe(s subscriber) int { t.mu.Lock() defer t.mu.Unlock() subscriberID := rand.Int() t.subscribers[subscriberID] = s return subscriberID } // Unsubscribe removes the subscription from the list of subscribers func (t *topic) Unsubscribe(id int) { t.mu.Lock() defer t.mu.Unlock() delete(t.subscribers, id) } // Publish asynchronously publishes to all subscribers func (t *topic) Publish(v *visitor, m *message) error { go func() { t.mu.Lock() defer t.mu.Unlock() for _, s := range t.subscribers { if err := s(v, m); err != nil { log.Printf("error publishing message to subscriber") } } }() return nil } // Subscribers returns the number of subscribers to this topic func (t *topic) Subscribers() int { t.mu.Lock() defer t.mu.Unlock() return len(t.subscribers) }