feat: observer pattern for statistics

pkg/emitter/emitter.go

@@ -0,0 +1,54 @@
+package emitter
+
+import "git.maximhutz.com/max/lambda/pkg/set"
+
+type Observer[T any] struct {
+	fn      func(T)
+	message string
+	emitter *Emitter[T]
+}
+
+type Emitter[T any] struct {
+	listeners map[string]*set.Set[*Observer[T]]
+}
+
+func Ignore[T any](fn func()) func(T) {
+	return func(T) { fn() }
+}
+
+func (e *Emitter[T]) On(message string, fn func(T)) *Observer[T] {
+	observer := &Observer[T]{
+		fn:      fn,
+		message: message,
+		emitter: e,
+	}
+
+	if e.listeners == nil {
+		e.listeners = map[string]*set.Set[*Observer[T]]{}
+	}
+
+	if e.listeners[message] == nil {
+		e.listeners[message] = set.New[*Observer[T]]()
+	}
+
+	e.listeners[message].Add(observer)
+	return observer
+}
+
+func (o *Observer[T]) Off() {
+	if o.emitter.listeners[o.message] == nil {
+		return
+	}
+
+	o.emitter.listeners[o.message].Remove(o)
+}
+
+func (e *Emitter[T]) Emit(message string, value T) {
+	if e.listeners[message] == nil {
+		return
+	}
+
+	for listener := range *e.listeners[message] {
+		listener.fn(value)
+	}
+}