Refactor field models and logic into separate package.

field/notifier.go

@@ -0,0 +1,57 @@
+// Copyright 2014 Team 254. All Rights Reserved.
+// Author: pat@patfairbank.com (Patrick Fairbank)
+//
+// Publish-subscribe model for nonblocking notification of server events to websocket clients.
+
+package field
+
+import (
+	"log"
+)
+
+// Allow the listeners to buffer a small number of notifications to streamline delivery.
+const notifyBufferSize = 3
+
+type Notifier struct {
+	// The map is essentially a set; the value is ignored.
+	listeners map[chan interface{}]struct{}
+}
+
+func NewNotifier() *Notifier {
+	notifier := new(Notifier)
+	notifier.listeners = make(map[chan interface{}]struct{})
+	return notifier
+}
+
+// Registers and returns a channel that can be read from to receive notification messages. The caller is
+// responsible for closing the channel, which will cause it to be reaped from the list of listeners.
+func (notifier *Notifier) Listen() chan interface{} {
+	listener := make(chan interface{}, notifyBufferSize)
+	notifier.listeners[listener] = struct{}{}
+	return listener
+}
+
+// Sends the given message to all registered listeners, and cleans up any listeners that have closed.
+func (notifier *Notifier) Notify(message interface{}) {
+	for listener, _ := range notifier.listeners {
+		notifier.notifyListener(listener, message)
+	}
+}
+
+func (notifier *Notifier) notifyListener(listener chan interface{}, message interface{}) {
+	defer func() {
+		// If channel is closed sending to it will cause a panic; recover and remove it from the list.
+		if r := recover(); r != nil {
+			delete(notifier.listeners, listener)
+		}
+	}()
+
+	// Do a non-blocking send. This guarantees that sending notifications won't interrupt the main event loop,
+	// at the risk of clients missing some messages if they don't read them all promptly.
+	select {
+	case listener <- message:
+		// The notification was sent and received successfully.
+	default:
+		log.Println("Failed to send a notification due to blocked listener.")
+	}
+}