add locks on the MemoryService of the vue mvc example, keep it simple but runnable:P

Former-commit-id: 5d661f401e486a26d66f5c0c297f5cf897fc1834

_examples/tutorial/vuejs-todo-mvc/src/todo/service.go

@@ -1,20 +1,33 @@
 package todo
 
+import (
+	"sync"
+)
+
 type Service interface {
 	Get(owner string) []Item
 	Save(owner string, newItems []Item) error
 }
 
 type MemoryService struct {
+	// key = session id, value the list of todo items that this session id has.
 	items map[string][]Item
+	// protected by locker for concurrent access.
+	mu sync.RWMutex
 }
 
 func NewMemoryService() *MemoryService {
-	return &MemoryService{make(map[string][]Item, 0)}
+	return &MemoryService{
+		items: make(map[string][]Item, 0),
+	}
 }
 
-func (s *MemoryService) Get(sessionOwner string) (items []Item) {
-	return s.items[sessionOwner]
+func (s *MemoryService) Get(sessionOwner string) []Item {
+	s.mu.RLock()
+	items := s.items[sessionOwner]
+	s.mu.RUnlock()
+
+	return items
 }
 
 func (s *MemoryService) Save(sessionOwner string, newItems []Item) error {
@@ -26,6 +39,8 @@ func (s *MemoryService) Save(sessionOwner string, newItems []Item) error {
 		}
 	}
 
+	s.mu.Lock()
 	s.items[sessionOwner] = newItems
+	s.mu.Unlock()
 	return nil
 }