optimize some code and fix imports

common/observer/dispatcher/direct_event_dispatcher.go

@@ -38,10 +38,12 @@ type DirectEventDispatcher struct {
 	observer.BaseListenable
 }
 
+// NewDirectEventDispatcher ac constructor of DirectEventDispatcher
 func NewDirectEventDispatcher() observer.EventDispatcher {
 	return &DirectEventDispatcher{}
 }
 
+// Dispatch event directly
 func (ded *DirectEventDispatcher) Dispatch(event observer.Event) {
 	if event == nil {
 		logger.Warnf("[DirectEventDispatcher] dispatch event nil")

common/observer/dispatcher/direct_event_dispatcher_test.go

@@ -19,11 +19,14 @@ package dispatcher
 
 import (
 	"fmt"
-	"github.com/apache/dubbo-go/common/observer"
 	"reflect"
 	"testing"
 )
 
+import (
+	"github.com/apache/dubbo-go/common/observer"
+)
+
 func TestDirectEventDispatcher_Dispatch(t *testing.T) {
 	ded := NewDirectEventDispatcher()
 	ded.AddEventListener(&TestEventListener{})

common/observer/event_dispatcher.go

@@ -51,7 +51,7 @@ func SetAndInitGlobalDispatcher(name string) {
 	if globalEventDispatcher != nil {
 		logger.Warnf("EventDispatcher already init. It will be replaced")
 	}
-	if dispatchers[name] == nil {
+	if dp, ok := dispatchers[name]; !ok || dp == nil {
 		panic("EventDispatcher for " + name + " is not existing, make sure you have import the package.")
 	}
 	globalEventDispatcher = dispatchers[name]()