ability to stack onClose and onSubscribe options

options.go

@@ -1,11 +1,11 @@
 package gubgub
 
-import (
-	"sync"
-)
+import "sync"
 
 // TopicOptions holds common options for topics.
 type TopicOptions struct {
+	mu sync.Mutex
+
 	// onClose is called after the Topic is closed and all messages have been delivered. Even
 	// though you might call Close multiple times, topics are effectively closed only once thus
 	// this should be called only once.
@@ -15,16 +15,63 @@ type TopicOptions struct {
 	onSubscribe func()
 }
 
+func (to *TopicOptions) TriggerClose() {
+	to.mu.Lock()
+	defer to.mu.Unlock()
+
+	if to.onClose == nil {
+		return
+	}
+
+	to.onClose()
+}
+
+func (to *TopicOptions) TriggerSubscribe() {
+	to.mu.Lock()
+	defer to.mu.Unlock()
+
+	if to.onSubscribe == nil {
+		return
+	}
+
+	to.onSubscribe()
+}
+
+func (to *TopicOptions) Apply(opts ...TopicOption) {
+	to.mu.Lock()
+	defer to.mu.Unlock()
+
+	for _, opt := range opts {
+		opt(to)
+	}
+}
+
 type TopicOption func(*TopicOptions)
 
 func WithOnClose(fn func()) TopicOption {
 	return func(opts *TopicOptions) {
-		opts.onClose = sync.OnceFunc(fn)
+		if opts.onClose == nil {
+			opts.onClose = fn
+		} else {
+			oldFn := opts.onClose // preserve previous onClose handler
+			opts.onClose = func() {
+				fn()
+				oldFn()
+			}
+		}
 	}
 }
 
 func WithOnSubscribe(fn func()) TopicOption {
 	return func(opts *TopicOptions) {
-		opts.onSubscribe = fn
+		if opts.onSubscribe == nil {
+			opts.onSubscribe = fn
+		} else {
+			oldFn := opts.onSubscribe // preserve previous onSubscribe handler
+			opts.onSubscribe = func() {
+				fn()
+				oldFn()
+			}
+		}
 	}
 }