feat: add listen method

interfaces/registry.go

@@ -23,5 +23,6 @@ type RegistryService interface {
 	Get(pk []byte) (SignedRegistryEntry, error)
 	BroadcastEntry(sre SignedRegistryEntry, receivedFrom net.Peer) error
 	SendRegistryRequest(pk []byte) error
+	Listen(pk []byte, cb func(sre SignedRegistryEntry)) (func(), error)
 	Service
 }

service/registry.go

@@ -260,6 +260,39 @@ func (r *RegistryImpl) Get(pk []byte) (interfaces.SignedRegistryEntry, error) {
 	return nil, nil
 }
 
+func (r *RegistryImpl) Listen(pk []byte, cb func(sre interfaces.SignedRegistryEntry)) (func(), error) {
+	key, err := encoding.NewMultihash(pk).ToString()
+	if err != nil {
+		return nil, err
+	}
+
+	cbProxy := func(event *emitter.Event) {
+		sre, ok := event.Args[0].(interfaces.SignedRegistryEntry)
+		if !ok {
+			r.logger.Error("Failed to cast event to SignedRegistryEntry")
+			return
+		}
+
+		cb(sre)
+	}
+
+	if !r.streams.Contains(key) {
+		em := emitter.New(0)
+		r.streams.Put(key, em)
+		err := r.SendRegistryRequest(pk)
+		if err != nil {
+			return nil, err
+		}
+	}
+	streamVal, _ := r.streams.Get(key)
+	stream := streamVal.(*emitter.Emitter)
+	channel := stream.On("event", cbProxy)
+
+	return func() {
+		stream.Off("event", channel)
+	}, nil
+}
+
 func (r *RegistryImpl) getFromDB(pk []byte) (sre interfaces.SignedRegistryEntry, err error) {
 	err = r.node.Db().View(func(txn *bbolt.Tx) error {
 		bucket := txn.Bucket([]byte(registryBucketName))