Introduce interface for topic event subscriber

daprdocs/content/en/go-sdk-docs/go-service/grpc-service.md

@@ -83,6 +83,35 @@ if err != nil {
 }
 ```
 
+You can also create a custom type that implements the `TopicEventSubscriber` interface to handle your events:
+
+```go
+type EventHandler struct {
+	// any data or references that your event handler needs.
+}
+
+func (h *EventHandler) Handle(ctx context.Context, e *common.TopicEvent) (retry bool, err error) {
+    log.Printf("event - PubsubName:%s, Topic:%s, ID:%s, Data: %v", e.PubsubName, e.Topic, e.ID, e.Data)
+    // do something with the event
+    return true, nil
+}
+```
+
+The `EventHandler` can then be added using the `AddTopicEventSubscriber` method:
+
+```go
+sub := &common.Subscription{
+    PubsubName: "messages",
+    Topic:      "topic1",
+}
+eventHandler := &EventHandler{
+// initialize any fields
+}
+if err := s.AddTopicEventSubscriber(sub, eventHandler); err != nil {
+    log.Fatalf("error adding topic subscription: %v", err)
+}
+```
+
 ### Service Invocation Handler
 To handle service invocations you will need to add at least one service invocation handler before starting the service:
 

daprdocs/content/en/go-sdk-docs/go-service/http-service.md

@@ -78,6 +78,35 @@ if err != nil {
 }
 ```
 
+You can also create a custom type that implements the `TopicEventSubscriber` interface to handle your events:
+
+```go
+type EventHandler struct {
+	// any data or references that your event handler needs.
+}
+
+func (h *EventHandler) Handle(ctx context.Context, e *common.TopicEvent) (retry bool, err error) {
+    log.Printf("event - PubsubName:%s, Topic:%s, ID:%s, Data: %v", e.PubsubName, e.Topic, e.ID, e.Data)
+    // do something with the event
+    return true, nil
+}
+```
+
+The `EventHandler` can then be added using the `AddTopicEventSubscriber` method:
+
+```go
+sub := &common.Subscription{
+    PubsubName: "messages",
+    Topic:      "topic1",
+}
+eventHandler := &EventHandler{
+// initialize any fields
+}
+if err := s.AddTopicEventSubscriber(sub, eventHandler); err != nil {
+    log.Fatalf("error adding topic subscription: %v", err)
+}
+```
+
 ### Service Invocation Handler
 To handle service invocations you will need to add at least one service invocation handler before starting the service:
 

service/common/service.go

@@ -27,18 +27,23 @@ const (
 )
 
 // Service represents Dapr callback service.
+//
+//nolint:interfacebloat
 type Service interface {
 	// AddHealthCheckHandler sets a health check handler, name: http (router) and grpc (invalid).
 	AddHealthCheckHandler(name string, fn HealthCheckHandler) error
 	// AddServiceInvocationHandler appends provided service invocation handler with its name to the service.
 	AddServiceInvocationHandler(name string, fn ServiceInvocationHandler) error
 	// AddTopicEventHandler appends provided event handler with its topic and optional metadata to the service.
-	// Note, retries are only considered when there is an error. Lack of error is considered as a success
+	// Note, retries are only considered when there is an error. Lack of error is considered as a success.
 	AddTopicEventHandler(sub *Subscription, fn TopicEventHandler) error
+	// AddTopicEventSubscriber appends the provided subscriber with its topic and optional metadata to the service.
+	// Note, retries are only considered when there is an error. Lack of error is considered as a success.
+	AddTopicEventSubscriber(sub *Subscription, subscriber TopicEventSubscriber) error
 	// AddBindingInvocationHandler appends provided binding invocation handler with its name to the service.
 	AddBindingInvocationHandler(name string, fn BindingInvocationHandler) error
 	// RegisterActorImplFactory Register a new actor to actor runtime of go sdk
-	// Deprecated: use RegisterActorImplFactoryContext instead
+	// Deprecated: use RegisterActorImplFactoryContext instead.
 	RegisterActorImplFactory(f actor.Factory, opts ...config.Option)
 	// RegisterActorImplFactoryContext Register a new actor to actor runtime of go sdk
 	RegisterActorImplFactoryContext(f actor.FactoryContext, opts ...config.Option)
@@ -49,7 +54,7 @@ type Service interface {
 	Start() error
 	// Stop stops the previously started service.
 	Stop() error
-	// Gracefully stops the previous started service
+	// Gracefully stops the previous started service.
 	GracefulStop() error
 }
 
@@ -60,3 +65,13 @@ type (
 	JobEventHandler          func(ctx context.Context, in *JobEvent) error
 	HealthCheckHandler       func(context.Context) error
 )
+
+type TopicEventSubscriber interface {
+	Handle(ctx context.Context, e *TopicEvent) (retry bool, err error)
+}
+
+// Handle converts TopicEventHandler into an adapter that implements
+// TopicEventSubscriber.
+func (h TopicEventHandler) Handle(ctx context.Context, e *TopicEvent) (retry bool, err error) {
+	return h(ctx, e)
+}

service/grpc/topic.go

@@ -31,11 +31,20 @@ import (
 
 // AddTopicEventHandler appends provided event handler with topic name to the service.
 func (s *Server) AddTopicEventHandler(sub *common.Subscription, fn common.TopicEventHandler) error {
+	if fn == nil {
+		return errors.New("topic handler required")
+	}
+
+	return s.AddTopicEventSubscriber(sub, fn)
+}
+
+// AddTopicEventSubscriber appends the provided subscriber to the service.
+func (s *Server) AddTopicEventSubscriber(sub *common.Subscription, subscriber common.TopicEventSubscriber) error {
 	if sub == nil {
 		return errors.New("subscription required")
 	}
 
-	return s.topicRegistrar.AddSubscription(sub, fn)
+	return s.topicRegistrar.AddSubscription(sub, subscriber)
 }
 
 // ListTopicSubscriptions is called by Dapr to get the list of topics in a pubsub component the app wants to subscribe to.
@@ -142,7 +151,7 @@ func (s *Server) OnTopicEvent(ctx context.Context, in *runtimev1pb.TopicEventReq
 				in.GetPath(), in.GetPubsubName(), in.GetTopic(),
 			)
 		}
-		retry, err := h(ctx, e)
+		retry, err := h.Handle(ctx, e)
 		if err == nil {
 			return &runtimev1pb.TopicEventResponse{Status: runtimev1pb.TopicEventResponse_SUCCESS}, nil
 		}

service/http/topic.go

@@ -241,6 +241,15 @@ func (s *Server) registerBaseHandler() {
 
 // AddTopicEventHandler appends provided event handler with it's name to the service.
 func (s *Server) AddTopicEventHandler(sub *common.Subscription, fn common.TopicEventHandler) error {
+	if fn == nil {
+		return errors.New("topic handler required")
+	}
+
+	return s.AddTopicEventSubscriber(sub, fn)
+}
+
+// AddTopicEventSubscriber appends the provided subscriber to the service.
+func (s *Server) AddTopicEventSubscriber(sub *common.Subscription, subscriber common.TopicEventSubscriber) error {
 	if sub == nil {
 		return errors.New("subscription required")
 	}
@@ -249,7 +258,7 @@ func (s *Server) AddTopicEventHandler(sub *common.Subscription, fn common.TopicE
 	if sub.Route == "" {
 		return errors.New("handler route name")
 	}
-	if err := s.topicRegistrar.AddSubscription(sub, fn); err != nil {
+	if err := s.topicRegistrar.AddSubscription(sub, subscriber); err != nil {
 		return err
 	}
 
@@ -306,7 +315,7 @@ func (s *Server) AddTopicEventHandler(sub *common.Subscription, fn common.TopicE
 			w.WriteHeader(http.StatusOK)
 
 			// execute user handler
-			retry, err := fn(r.Context(), &te)
+			retry, err := subscriber.Handle(r.Context(), &te)
 			if err == nil {
 				writeStatus(w, common.SubscriptionResponseStatusSuccess)
 				return

service/internal/topicregistrar.go

@@ -14,11 +14,11 @@ type TopicRegistrar map[string]*TopicRegistration
 // TopicRegistration encapsulates the subscription and handlers.
 type TopicRegistration struct {
 	Subscription   *TopicSubscription
-	DefaultHandler common.TopicEventHandler
-	RouteHandlers  map[string]common.TopicEventHandler
+	DefaultHandler common.TopicEventSubscriber
+	RouteHandlers  map[string]common.TopicEventSubscriber
 }
 
-func (m TopicRegistrar) AddSubscription(sub *common.Subscription, fn common.TopicEventHandler) error {
+func (m TopicRegistrar) AddSubscription(sub *common.Subscription, fn common.TopicEventSubscriber) error {
 	if sub.Topic == "" {
 		return errors.New("topic name required")
 	}
@@ -40,7 +40,7 @@ func (m TopicRegistrar) AddSubscription(sub *common.Subscription, fn common.Topi
 	if !ok {
 		ts = &TopicRegistration{
 			Subscription:   NewTopicSubscription(sub.PubsubName, sub.Topic),
-			RouteHandlers:  make(map[string]common.TopicEventHandler),
+			RouteHandlers:  make(map[string]common.TopicEventSubscriber),
 			DefaultHandler: nil,
 		}
 		ts.Subscription.SetMetadata(sub.Metadata)

service/internal/topicregistrar_test.go

@@ -13,12 +13,12 @@ import (
 )
 
 func TestTopicRegistrarValidation(t *testing.T) {
-	fn := func(ctx context.Context, e *common.TopicEvent) (retry bool, err error) {
+	fn := common.TopicEventHandler(func(ctx context.Context, e *common.TopicEvent) (retry bool, err error) {
 		return false, nil
-	}
+	})
 	tests := map[string]struct {
 		sub common.Subscription
-		fn  common.TopicEventHandler
+		fn  common.TopicEventSubscriber
 		err string
 	}{
 		"pubsub required": {
@@ -75,9 +75,9 @@ func TestTopicRegistrarValidation(t *testing.T) {
 }
 
 func TestTopicAddSubscriptionMetadata(t *testing.T) {
-	handler := func(ctx context.Context, e *common.TopicEvent) (retry bool, err error) {
+	handler := common.TopicEventHandler(func(ctx context.Context, e *common.TopicEvent) (retry bool, err error) {
 		return false, nil
-	}
+	})
 	topicRegistrar := internal.TopicRegistrar{}
 	sub := &common.Subscription{
 		PubsubName: "pubsubname",