wings/events/events.go

133 lines
3.7 KiB
Go

package events
import (
"encoding/json"
"github.com/pkg/errors"
"reflect"
"strings"
"sync"
)
type Event struct {
Data string
Topic string
}
type EventBus struct {
mu sync.RWMutex
callbacks map[string][]*func(Event)
}
func New() *EventBus {
return &EventBus{
callbacks: make(map[string][]*func(Event)),
}
}
// Publish data to a given topic.
func (e *EventBus) Publish(topic string, data string) {
t := topic
// Some of our topics for the socket support passing a more specific namespace,
// such as "backup completed:1234" to indicate which specific backup was completed.
//
// In these cases, we still need to the send the event using the standard listener
// name of "backup completed".
if strings.Contains(topic, ":") {
parts := strings.SplitN(topic, ":", 2)
if len(parts) == 2 {
t = parts[0]
}
}
e.mu.RLock()
defer e.mu.RUnlock()
// Acquire a read lock and loop over all of the channels registered for the topic. This
// avoids a panic crash if the process tries to unregister the channel while this routine
// is running.
if _, ok := e.callbacks[t]; ok {
evt := Event{Data: data, Topic: topic}
for _, callback := range e.callbacks[t] {
go func(evt Event, callback func(Event)) {
callback(evt)
}(evt, *callback)
}
}
}
// Publishes a JSON message to a given topic.
func (e *EventBus) PublishJson(topic string, data interface{}) error {
b, err := json.Marshal(data)
if err != nil {
return errors.WithStack(err)
}
e.Publish(topic, string(b))
return nil
}
// Register a callback function that will be executed each time one of the events using the topic
// name is called.
func (e *EventBus) On(topic string, callback *func(Event)) {
e.mu.Lock()
defer e.mu.Unlock()
// Check if this topic has been registered at least once for the event listener, and if
// not create an empty struct for the topic.
if _, exists := e.callbacks[topic]; !exists {
e.callbacks[topic] = make([]*func(Event), 0)
}
// If this callback is not already registered as an event listener, go ahead and append
// it to the array of callbacks for this topic.
if e.index(topic, reflect.ValueOf(callback)) < 0 {
e.callbacks[topic] = append(e.callbacks[topic], callback)
}
}
// Removes an event listener from the bus.
func (e *EventBus) Off(topic string, callback *func(Event)) {
e.mu.Lock()
defer e.mu.Unlock()
i := e.index(topic, reflect.ValueOf(callback))
// If i < 0 it means there was no index found for the given callback, meaning it was
// never registered or was already unregistered from the listeners. Also double check
// that we didn't somehow escape the length of the topic callback (not sure how that
// would happen, but lets avoid a panic condition).
if i < 0 || i >= len(e.callbacks[topic]) {
return
}
// We can assume that the topic still exists at this point since we acquire an exclusive
// lock on the process, and the "e.index" function cannot return a value >= 0 if there is
// no topic already existing.
e.callbacks[topic] = append(e.callbacks[topic][:i], e.callbacks[topic][i+1:]...)
}
// Removes all of the event listeners that have been registered for any topic.
func (e *EventBus) RemoveAll() {
e.mu.Lock()
defer e.mu.Unlock()
e.callbacks = make(map[string][]*func(Event))
}
// Finds the index of a given callback in the topic by comparing all of the registered callback
// pointers to the passed function. This function does not aquire a lock as it should only be called
// within the confines of a function that has already acquired a lock for the duration of the lookup.
func (e *EventBus) index(topic string, v reflect.Value) int {
if _, ok := e.callbacks[topic]; ok {
for i, handler := range e.callbacks[topic] {
if reflect.ValueOf(handler).Pointer() == v.Pointer() {
return i
}
}
}
return -1
}