diff --git a/mobx/CHANGELOG.md b/mobx/CHANGELOG.md
index 5ed353fba..78be08618 100644
--- a/mobx/CHANGELOG.md
+++ b/mobx/CHANGELOG.md
@@ -1,7 +1,8 @@
-## 2.1.2 - 2.1.3
+## 2.1.2 - 2.1.3+1
 
 - Fix tests in dart 2.19 - [@amondnet](https://github.com/amondnet)
 - Dart formatting fixes
+- Refactor `atom.dart` to make code cleaner by [@fzyzcjy](https://github.com/fzyzcjy) in [#854](https://github.com/mobxjs/mobx.dart/pull/854)
 
 ## 2.1.1
 
diff --git a/mobx/lib/src/core/atom.dart b/mobx/lib/src/core/atom.dart
index f9b579772..56005c7d3 100644
--- a/mobx/lib/src/core/atom.dart
+++ b/mobx/lib/src/core/atom.dart
@@ -96,11 +96,7 @@ class Atom {
       _addListener(_ListenerKind.onBecomeUnobserved, fn);
 
   void Function() _addListener(_ListenerKind kind, void Function() fn) {
-    if (_observationListeners[kind] == null) {
-      _observationListeners[kind] = {}..add(fn);
-    } else {
-      _observationListeners[kind]!.add(fn);
-    }
+    (_observationListeners[kind] ??= {}).add(fn);
 
     return () {
       final listeners = _observationListeners[kind];
diff --git a/mobx/lib/version.dart b/mobx/lib/version.dart
index 34cc06f1b..dde8ee573 100644
--- a/mobx/lib/version.dart
+++ b/mobx/lib/version.dart
@@ -1,4 +1,4 @@
 // Generated via set_version.dart. !!!DO NOT MODIFY BY HAND!!!
 
 /// The current version as per `pubspec.yaml`.
-const version = '2.1.3';
+const version = '2.1.3+1';
diff --git a/mobx/pubspec.yaml b/mobx/pubspec.yaml
index fb1ad30cf..e81d01082 100644
--- a/mobx/pubspec.yaml
+++ b/mobx/pubspec.yaml
@@ -1,5 +1,5 @@
 name: mobx
-version: 2.1.3
+version: 2.1.3+1
 description: "MobX is a library for reactively managing the state of your applications. Use the power of observables, actions, and reactions to supercharge your Dart and Flutter apps."
 
 homepage: https://github.com/mobxjs/mobx.dart