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events.go
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events.go
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package main
import (
"encoding/json"
"github.com/gin-gonic/gin"
)
// Broker represents a server-sent events (SSE) route.
//
// Once created with NewBroker, use ServeHTTP as a route handler and Broadcast
// to send events.
type Broker struct {
clients map[chan string]struct{}
openChan chan chan string
closeChan chan chan string
sendChan chan string
}
// NewBroker creates a ready-to-use broker. It starts a new goroutine that
// handles incoming messages and connections.
func NewBroker() Broker {
b := Broker{
clients: make(map[chan string]struct{}),
openChan: make(chan chan string),
closeChan: make(chan chan string),
sendChan: make(chan string, 4),
}
go b.listen()
return b
}
// ServeHTTP is an SSE handler function that sends any broadcasted message.
func (b *Broker) ServeHTTP(c *gin.Context) {
msgs := b.Subscribe()
defer b.Unsubscribe(msgs)
w := c.Writer
closed := w.CloseNotify()
// Don't use c.Stream(): it would block the connection until a new message
// is received whereas we should listen for disconnection simultaneously.
for {
select {
case msg := <-msgs:
c.SSEvent("", msg)
w.Flush()
case <-closed:
return
}
}
}
// Subscribe creates, registers and returns a new channel that can be used to
// receive incoming messages.
func (b *Broker) Subscribe() chan string {
c := make(chan string)
b.openChan <- c
return c
}
// Unsubscribe closes a channel that was created with Subscribe. It is
// mandatory to unsubscribe when a client disconnects.
func (b *Broker) Unsubscribe(c chan string) {
b.closeChan <- c
}
// Broadcast sends a message to every connected client. The sent event is
// marshalled into JSON beforehand.
func (b *Broker) Broadcast(event interface{}) {
msg, err := json.Marshal(event)
if err != nil {
panic(err)
}
b.sendChan <- string(msg)
}
// listen handles client's connection and disconnection, and broadcasts any
// incoming message to all clients.
func (b *Broker) listen() {
for {
select {
case c := <-b.openChan:
b.clients[c] = struct{}{}
case c := <-b.closeChan:
close(c)
delete(b.clients, c)
case msg := <-b.sendChan:
for client := range b.clients {
b.send(client, msg)
}
}
}
}
// send writes a message to a client's channel. It returns when the message is
// sent or if the client disconnected.
func (b *Broker) send(client chan string, msg string) {
// A client could disconnect while we're trying to send a message.
// So we MUST listen to the closeChan to avoid a (sneaky) deadlock!
for {
select {
case client <- msg:
return
case c := <-b.closeChan:
close(c)
delete(b.clients, c)
if c == client {
return
}
}
}
}