extension: Implement an EventBroker (#315)

Signed-off-by: James Hillyerd <james@hillyerd.com>

Signed-off-by: James Hillyerd <james@hillyerd.com>

pkg/extension/broker.go

@@ -0,0 +1,57 @@
+package extension
+
+import "sync"
+
+// EventBroker maintains a list of listeners interested in a specific type
+// of event.
+type EventBroker[E any, R comparable] struct {
+	sync.RWMutex
+	listenerNames []string     // Ordered listener names.
+	listenerFuncs []func(E) *R // Ordered listener functions.
+}
+
+// Emit sends the provided event to each registered listener in order, until
+// one returns a non-nil result.  That result will be returned to the caller.
+func (eb *EventBroker[E, R]) Emit(event *E) *R {
+	eb.RLock()
+	defer eb.RUnlock()
+
+	for _, l := range eb.listenerFuncs {
+		// Events are copied to minimize the risk of mutation.
+		if result := l(*event); result != nil {
+			return result
+		}
+	}
+
+	return nil
+}
+
+// AddListener registers the named listener, replacing one with a duplicate
+// name if present.  Listeners should be added in order of priority, most
+// significant first.
+func (eb *EventBroker[E, R]) AddListener(name string, listener func(E) *R) {
+	eb.Lock()
+	defer eb.Unlock()
+
+	eb.lockedRemoveListener(name)
+	eb.listenerNames = append(eb.listenerNames, name)
+	eb.listenerFuncs = append(eb.listenerFuncs, listener)
+}
+
+// RemoveListener unregisters the named listener.
+func (eb *EventBroker[E, R]) RemoveListener(name string) {
+	eb.Lock()
+	defer eb.Unlock()
+
+	eb.lockedRemoveListener(name)
+}
+
+func (eb *EventBroker[E, R]) lockedRemoveListener(name string) {
+	for i, entry := range eb.listenerNames {
+		if entry == name {
+			eb.listenerNames = append(eb.listenerNames[:i], eb.listenerNames[i+1:]...)
+			eb.listenerFuncs = append(eb.listenerFuncs[:i], eb.listenerFuncs[i+1:]...)
+			break
+		}
+	}
+}