[#143] object: Add notification setters and getters

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

object/raw.go

@@ -161,3 +161,8 @@ func (o *RawObject) ResetRelations() {
 func (o *RawObject) InitRelations() {
 	o.initRelations()
 }
+
+// SetNotification writes NotificationInfo to the object structure.
+func (o *RawObject) SetNotification(ni NotificationInfo) {
+	o.setNotification(ni)
+}

object/rw.go

@@ -249,6 +249,57 @@ func (o *rwObject) setChildren(v ...*oid.ID) {
 	})
 }
 
+// NotificationInfo groups information about object notification
+// that can be written to object.
+//
+// Topic is an optional field.
+type NotificationInfo struct {
+	ni object.NotificationInfo
+}
+
+// Epoch returns object notification tick
+// epoch.
+func (n NotificationInfo) Epoch() uint64 {
+	return n.ni.Epoch()
+}
+
+// SetEpoch sets object notification tick
+// epoch.
+func (n *NotificationInfo) SetEpoch(epoch uint64) {
+	n.ni.SetEpoch(epoch)
+}
+
+// Topic return optional object notification
+// topic.
+func (n NotificationInfo) Topic() string {
+	return n.ni.Topic()
+}
+
+// SetTopic sets optional object notification
+// topic.
+func (n *NotificationInfo) SetTopic(topic string) {
+	n.ni.SetTopic(topic)
+}
+
+// NotificationInfo returns notification info
+// read from the object structure.
+// Returns any error that appeared during notification
+// information parsing.
+func (o *rwObject) NotificationInfo() (*NotificationInfo, error) {
+	ni, err := object.GetNotificationInfo((*object.Object)(o))
+	if err != nil {
+		return nil, err
+	}
+
+	return &NotificationInfo{
+		ni: *ni,
+	}, nil
+}
+
+func (o *rwObject) setNotification(ni NotificationInfo) {
+	object.WriteNotificationInfo((*object.Object)(o), ni.ni)
+}
+
 // SplitID return split identity of split object. If object is not split
 // returns nil.
 func (o *rwObject) SplitID() *SplitID {