main.go

package bus

import (
	"fmt"
	"sync"
	"sync/atomic"
)

var mapper sync.Map // holds key (event name - string) versus topic values

// we allow developers to override event names. They should be careful about name collisions
type iEventName interface {
	EventID() string //
}

// if developers implement this interface, we're spinning a goroutine if the event says it is async
type iAsync interface {
	Async() bool
}

// Listener is being returned when you subscribe to a topic, so you can unsubscribe or access the parent topic
type Listener[T any] struct {
	parent   *Topic[T]     // so we can call unsubscribe from parent
	callback func(event T) // the function that we're going to call
}

// Topic keeps the subscribers of one topic
type Topic[T any] struct {
	subs      []*Listener[T] // list of listeners
	rwMu      sync.RWMutex   // guards subs
	lisnsPool sync.Pool      // a pool of listeners
}

// NewTopic creates a new topic for a specie of events
func NewTopic[T any]() *Topic[T] {
	result := &Topic[T]{}
	result.lisnsPool.New = func() any {
		return &Listener[T]{
			parent: result,
		}
	}
	return result
}

// Sub adds a callback to be called when an event of that type is being published
func (b *Topic[T]) Sub(callback func(v T)) *Listener[T] {
	result := b.lisnsPool.Get().(*Listener[T])
	result.callback = callback
	result.parent = b

	b.rwMu.Lock()
	b.subs = append(b.subs, result)
	b.rwMu.Unlock()

	return result
}

// cancel is private to the topic, but can be accessed via Listener
func (b *Topic[T]) cancel(who *Listener[T]) {
	b.rwMu.Lock()
	for i := range b.subs {
		if b.subs[i] != who {
			continue
		}

		b.subs[i] = b.subs[len(b.subs)-1]
		b.subs[len(b.subs)-1] = nil
		b.subs = b.subs[:len(b.subs)-1]
		break
	}
	b.rwMu.Unlock()

	who.callback = nil
	b.lisnsPool.Put(who)
}

// NumSubs in case you need to perform tests and check the number of subscribers of this particular topic
func (b *Topic[T]) NumSubs() int {
	b.rwMu.RLock()
	result := len(b.subs)
	b.rwMu.RUnlock()
	return result
}

// Cancel forgets the indicated callback
func (s *Listener[T]) Cancel() {
	s.parent.cancel(s)
}

// Topic gives access to the underlying topic
func (s *Listener[T]) Topic() *Topic[T] {
	return s.parent
}

// Pub allows you to publish an event in that topic
func (b *Topic[T]) Pub(event T) {
	b.rwMu.RLock()

	isAsync := false
	switch m := any(event).(type) {
	case iAsync:
		isAsync = m.Async()
	}

	for sub := range b.subs {
		if isAsync {
			go b.subs[sub].callback(event)
			continue
		}

		b.subs[sub].callback(event)
	}

	b.rwMu.RUnlock()
}

func (b *Topic[T]) PubAsync(event T) {
	b.rwMu.RLock()

	for sub := range b.subs {
		go b.subs[sub].callback(event)
	}

	b.rwMu.RUnlock()
}

// Bus is being returned when you subscribe, so you can manually Cancel
type Bus[T any] struct {
	listener *Listener[T]
	stop     atomic.Uint32 // flag for unsubscribing after receiving one event
}

// Cancel allows callers to manually unsubscribe, in case they don't want to use SubCancel
func (o *Bus[T]) Cancel() {
	if o.stop.CompareAndSwap(0, 1) {
		go o.listener.Cancel()
	}
}

// SubCancel can be used if you need to unsubscribe immediately after receiving an event, by making your function return true
func SubCancel[T any](callback func(event T) bool) *Bus[T] {
	var (
		event T
		key   string
	)

	switch m := any(event).(type) {
	case iEventName:
		key = m.EventID()
	default:
		key = fmt.Sprintf("%T", event)
	}

	topic, ok := mapper.Load(key)
	if !ok || topic == nil {
		topic, _ = mapper.LoadOrStore(key, NewTopic[T]())
	}

	var result Bus[T]

	result.listener = topic.(*Topic[T]).Sub(func(v T) {
		if result.stop.Load() == 1 {
			return
		}

		shouldCancel := callback(v)
		if shouldCancel {
			result.Cancel()
		}

	})

	return &result
}

// Sub subscribes a callback function to listen for a specie of events
func Sub[T any](callback func(event T)) *Bus[T] {
	var (
		event T
		key   string
	)

	switch m := any(event).(type) {
	case iEventName:
		key = m.EventID()
	default:
		key = fmt.Sprintf("%T", event)
	}

	topic, ok := mapper.Load(key)
	if !ok || topic == nil {
		topic, _ = mapper.LoadOrStore(key, NewTopic[T]())
	}

	var result Bus[T]

	result.listener = topic.(*Topic[T]).Sub(func(v T) {
		if result.stop.Load() == 1 {
			return
		}
		callback(v)
	})

	return &result
}

// Pub publishes an event which will be dispatched to all listeners
func Pub[T any](event T) {
	var key string

	switch m := any(event).(type) {
	case iEventName:
		key = m.EventID()
	default:
		key = fmt.Sprintf("%T", event)
	}

	topic, ok := mapper.Load(key)
	if !ok || topic == nil { // create a new topic, even if there are no listeners (otherwise we will have to panic)
		topic, _ = mapper.LoadOrStore(key, NewTopic[T]())
	}

	topic.(*Topic[T]).Pub(event)
}

// PubAsync publishes an event which will be dispatched to all listeners
func PubAsync[T any](event T) {
	var key string

	switch m := any(event).(type) {
	case iEventName:
		key = m.EventID()
	default:
		key = fmt.Sprintf("%T", event)
	}

	topic, ok := mapper.Load(key)
	if !ok || topic == nil { // create a new topic, even if there are no listeners (otherwise we will have to panic)
		topic, _ = mapper.LoadOrStore(key, NewTopic[T]())
	}
	topic.(*Topic[T]).PubAsync(event)
}

// Range gives access to mapper Range
func Range(f func(k, v any) bool) {
	mapper.Range(f)
}