goemitter/main.go

// Package goemitter implements an event emitter pattern similar to Node.js EventEmitter.
// It provides a way to register event listeners and emit events with data.
//
// Basic usage example:
//
//	emitter := goemitter.NewEmitter()
//
//	// Register an event listener
//	handle := emitter.On("userConnected", func(args ...interface{}) {
//	    user := args[0].(string)
//	    fmt.Printf("User connected: %s\n", user)
//	})
//
//	// Emit an event
//	emitter.Emit("userConnected", "john_doe")
//
//	// Remove the listener when no longer needed
//	handle.Remove()
package goemitter

import (
	"sync"
)

// EventEmitter provides event subscription and publishing functionality.
// It is safe for concurrent use thanks to internal synchronization.
type EventEmitter struct {
	callbacks     map[string]map[int]func(args ...interface{}) // Persistent event callbacks indexed by ID
	onceCallbacks map[string][]func(args ...interface{})       // One-time event callbacks
	lock          sync.Mutex                                   // Mutex to ensure thread safety
	counter       int                                          // Counter for generating unique callback IDs
}

// EventHandle represents a handle to a registered event listener.
// It can be used to remove the listener when it's no longer needed.
type EventHandle struct {
	id      int           // Unique identifier for the callback
	event   string        // Event name
	emitter *EventEmitter // Reference to the parent emitter
}

// Remove unregisters the event listener associated with this handle.
// After removal, the listener will no longer receive events.
func (eh *EventHandle) Remove() {
	eh.emitter.lock.Lock()
	defer eh.emitter.lock.Unlock()
	if _, ok := eh.emitter.callbacks[eh.event]; ok {
		delete(eh.emitter.callbacks[eh.event], eh.id)
	}
}

// On registers a new event listener for the specified event.
// It returns an EventHandle that can be used to remove the listener later.
// The callback function can accept any number of arguments passed during event emission.
func (e *EventEmitter) On(event string, callback func(args ...interface{})) *EventHandle {
	e.lock.Lock()
	defer e.lock.Unlock()
	e.counter++
	if _, ok := e.callbacks[event]; !ok {
		e.callbacks[event] = make(map[int]func(args ...interface{}))
	}
	e.callbacks[event][e.counter] = callback
	return &EventHandle{
		id:      e.counter,
		event:   event,
		emitter: e,
	}
}

// Once registers a one-time event listener for the specified event.
// The listener will be automatically removed after being triggered once.
// Multiple one-time listeners can be registered for the same event.
func (e *EventEmitter) Once(event string, callback func(args ...interface{})) {
	e.lock.Lock()
	defer e.lock.Unlock()

	if _, ok := e.onceCallbacks[event]; !ok {
		e.onceCallbacks[event] = []func(args ...interface{}){callback}
	} else {
		e.onceCallbacks[event] = append(e.onceCallbacks[event], callback)
	}
}

// Emit triggers an event asynchronously with the provided arguments.
// All registered callbacks for the event will be executed in separate goroutines.
// One-time callbacks are cleared after emission.
func (e *EventEmitter) Emit(event string, data ...interface{}) {
	e.lock.Lock()
	defer e.lock.Unlock()

	// Execute persistent callbacks asynchronously
	if callbacks, ok := e.callbacks[event]; ok {
		for _, callback := range callbacks {
			go callback(data...)
		}
	}

	// Execute and clear one-time callbacks
	if onceCallbacks, ok := e.onceCallbacks[event]; ok {
		for _, callback := range onceCallbacks {
			go callback(data...)
		}
		e.onceCallbacks[event] = []func(args ...interface{}){}
	}
}

// EmitSync triggers an event synchronously with the provided arguments.
// All registered callbacks for the event will be executed in the current goroutine.
// One-time callbacks are cleared after emission.
func (e *EventEmitter) EmitSync(event string, data ...interface{}) {
	e.lock.Lock()
	defer e.lock.Unlock()

	// Execute persistent callbacks synchronously
	if callbacks, ok := e.callbacks[event]; ok {
		for _, callback := range callbacks {
			callback(data...)
		}
	}

	// Execute and clear one-time callbacks
	if onceCallbacks, ok := e.onceCallbacks[event]; ok {
		for _, callback := range onceCallbacks {
			callback(data...)
		}
		e.onceCallbacks[event] = []func(args ...interface{}){}
	}
}

// NewEmitter creates and initializes a new EventEmitter instance.
// It initializes the internal maps for storing callbacks and returns
// a pointer to the new instance.
func NewEmitter() *EventEmitter {
	ret := &EventEmitter{}
	ret.callbacks = make(map[string]map[int]func(args ...interface{}))
	ret.onceCallbacks = make(map[string][]func(args ...interface{}))
	return ret
}