diff --git a/.gitignore b/.gitignore
index d111b3a..c282350 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 .dart_tool/
 .idea/
 build/
+windows/
 
 .packages
 pubspec.lock
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 203262a..52b67b9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,6 @@
+## 0.15.0
+- Add second "Observer" example. This example was created to be used in a more complex example.
+
 ## 0.14.0
 - Add "Memento" conceptual pattern
 
diff --git a/README.md b/README.md
index 8f0052c..c7cbbda 100644
--- a/README.md
+++ b/README.md
@@ -15,8 +15,8 @@ It contains **Dart** examples for all classic **GoF** design patterns.
     - [ ] Interpreter
     - [ ] [**Iterator**](https://refactoring.guru/design-patterns/iterator)
     - [ ] [**Mediator**](https://refactoring.guru/design-patterns/mediator)
-    - [x] [**Memento**](https://refactoring.guru/design-patterns/memento) [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/master/patterns/memento/conceptual)]
-    - [x] [**Observer**](https://refactoring.guru/design-patterns/observer) - [[Open-Close Editor Events](https://github.com/RefactoringGuru/design-patterns-dart/tree/master/patterns/observer/open_close_editor_events)]
+    - [x] [**Memento**](https://refactoring.guru/design-patterns/memento) - [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/master/patterns/memento/conceptual)]
+    - [x] [**Observer**](https://refactoring.guru/design-patterns/observer) - [[Open-Close Editor Events](https://github.com/RefactoringGuru/design-patterns-dart/tree/master/patterns/observer/open_close_editor_events)] [[AppObserver](https://github.com/RefactoringGuru/design-patterns-dart/tree/master/patterns/observer/app_observer)]
     - [ ] [**State**](https://refactoring.guru/design-patterns/state)
     - [ ] [**Template Method**](https://refactoring.guru/design-patterns/template-method)
     - [ ] [**Visitor**](https://refactoring.guru/design-patterns/visitor)
diff --git a/bin/main.dart b/bin/main.dart
new file mode 100644
index 0000000..3865bb7
--- /dev/null
+++ b/bin/main.dart
@@ -0,0 +1,20 @@
+import 'package:flutter/material.dart';
+
+void main() {
+  runApp(const MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  const MyApp({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Refactoring Guru: Flutter launcher',
+      theme: ThemeData(
+          primarySwatch: Colors.pink
+      ),
+      home: Container(),
+    );
+  }
+}
diff --git a/patterns/observer/app_observer/README.md b/patterns/observer/app_observer/README.md
new file mode 100644
index 0000000..4e509eb
--- /dev/null
+++ b/patterns/observer/app_observer/README.md
@@ -0,0 +1,56 @@
+# Observer pattern
+Observer is a behavioral design pattern that lets you define a subscription mechanism to notify 
+multiple objects about any events that happen to the object they’re observing.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/observer).
+
+## AppObserver example 
+This example was created to be used in a more complex example.
+A complex example will be implemented later.
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/152049751-b111e02a-1d33-4796-810c-b7ed069cecdc.png)
+
+### Sequence
+![image](https://user-images.githubusercontent.com/8049534/152049996-72131655-402d-4b92-b5d0-10e3f2dd0e79.png)
+
+### Client code:
+```dart
+void main() {
+  final observer = AppObserver();
+
+  observer.subscribe<FirstEvent>((e) {
+    print('First');
+  });
+
+
+  observer.subscribe((SecondEvent e) {
+    print('Second');
+  });
+
+  final saveThirdEvent = observer.subscribe((ThirdEvent e) {
+    print('Third');
+  });
+
+  observer.notify(FirstEvent());
+  observer.notify(SecondEvent());
+  observer.notify(ThirdEvent());
+
+  print('---unsubscribe "ThirdEvent"---');
+  observer.unsubscribe(saveThirdEvent);
+
+  observer.notify(FirstEvent());
+  observer.notify(SecondEvent());
+  observer.notify(ThirdEvent());
+}
+```
+
+**Output:**
+```
+First
+Second
+Third
+---unsubscribe "ThirdEvent"---
+First
+Second
+```
diff --git a/patterns/observer/app_observer/main.dart b/patterns/observer/app_observer/main.dart
new file mode 100644
index 0000000..620fb21
--- /dev/null
+++ b/patterns/observer/app_observer/main.dart
@@ -0,0 +1,36 @@
+import 'observer/app_observer.dart';
+import 'observer/event.dart';
+
+class FirstEvent extends Event {}
+
+class SecondEvent extends Event {}
+
+class ThirdEvent extends Event {}
+
+void main() {
+  final observer = AppObserver();
+
+  observer.subscribe<FirstEvent>((e) {
+    print('First');
+  });
+
+
+  observer.subscribe((SecondEvent e) {
+    print('Second');
+  });
+
+  final saveThirdEvent = observer.subscribe((ThirdEvent e) {
+    print('Third');
+  });
+
+  observer.notify(FirstEvent());
+  observer.notify(SecondEvent());
+  observer.notify(ThirdEvent());
+
+  print('---unsubscribe "ThirdEvent"---');
+  observer.unsubscribe(saveThirdEvent);
+
+  observer.notify(FirstEvent());
+  observer.notify(SecondEvent());
+  observer.notify(ThirdEvent());
+}
diff --git a/patterns/observer/app_observer/observer/app_observer.dart b/patterns/observer/app_observer/observer/app_observer.dart
new file mode 100644
index 0000000..a6fa01a
--- /dev/null
+++ b/patterns/observer/app_observer/observer/app_observer.dart
@@ -0,0 +1,46 @@
+import 'event.dart';
+
+typedef EventFunction<T> = void Function(T e);
+
+class AppObserver {
+  final _subscribers = <int, List<dynamic>>{};
+
+  EventFunction<T> subscribe<T>(EventFunction<T> func) {
+    assert(
+      Class<T>() is Class<Event>,
+      '\n\nThe callback argument must implement the "Event" class.\n'
+      'Correct use: \n'
+      '\tobserver.subscribe((MyEvent e) {}); \n'
+      'Mistaken usage: \n'
+      '\tobserver.subscribe((String e) {});\n'
+      '\tobserver.subscribe((e) {});\n',
+    );
+
+    _subscribers.putIfAbsent(T.hashCode, () => []).add(func);
+    return func;
+  }
+
+  void unsubscribe<T extends Event>(EventFunction<T> func) {
+    final isDelete = _subscribers[T.hashCode]?.remove(func) ?? false;
+
+    if (isDelete) {
+      return;
+    }
+
+    throw Exception('Subscriber not found.');
+  }
+
+  void notify<T extends Event>(T event) {
+    final subscribers = _subscribers[T.hashCode];
+
+    if (subscribers == null) {
+      return;
+    }
+
+    for (var sub in subscribers) {
+      sub.call(event);
+    }
+  }
+}
+
+class Class<T> {}
diff --git a/patterns/observer/app_observer/observer/event.dart b/patterns/observer/app_observer/observer/event.dart
new file mode 100644
index 0000000..dbd3bf8
--- /dev/null
+++ b/patterns/observer/app_observer/observer/event.dart
@@ -0,0 +1,3 @@
+abstract class Event {
+
+}
diff --git a/pubspec.yaml b/pubspec.yaml
index 26fbabd..c516e54 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,6 +1,6 @@
 name: design_patterns_dart
 description: Dart examples for all classic GoF design patterns.
-version: 0.14.0
+version: 0.15.0
 homepage: https://refactoring.guru/design-patterns
 repository: https://github.com/RefactoringGuru/design-patterns-dart
 issue_tracker: https://github.com/RefactoringGuru/design-patterns-dart/issue
@@ -10,6 +10,10 @@ environment:
 
 dependencies:
   collection: ^1.15.0
+  flutter:
+    sdk: flutter
 
 dev_dependencies:
   lints: ^1.0.0
+
+
diff --git a/web/favicon.png b/web/favicon.png
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diff --git a/web/icons/Icon-maskable-192.png b/web/icons/Icon-maskable-192.png
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diff --git a/web/index.html b/web/index.html
new file mode 100644
index 0000000..0e76fb2
--- /dev/null
+++ b/web/index.html
@@ -0,0 +1,104 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <!--
+    If you are serving your web app in a path other than the root, change the
+    href value below to reflect the base path you are serving from.
+
+    The path provided below has to start and end with a slash "/" in order for
+    it to work correctly.
+
+    For more details:
+    * https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base
+
+    This is a placeholder for base href that will be replaced by the value of
+    the `--base-href` argument provided to `flutter build`.
+  -->
+  <base href="$FLUTTER_BASE_HREF">
+
+  <meta charset="UTF-8">
+  <meta content="IE=Edge" http-equiv="X-UA-Compatible">
+  <meta name="description" content="A new Flutter project.">
+
+  <!-- iOS meta tags & icons -->
+  <meta name="apple-mobile-web-app-capable" content="yes">
+  <meta name="apple-mobile-web-app-status-bar-style" content="black">
+  <meta name="apple-mobile-web-app-title" content="flutter_launcher">
+  <link rel="apple-touch-icon" href="icons/Icon-192.png">
+
+  <!-- Favicon -->
+  <link rel="icon" type="image/png" href="favicon.png"/>
+
+  <title>flutter_launcher</title>
+  <link rel="manifest" href="manifest.json">
+</head>
+<body>
+  <!-- This script installs service_worker.js to provide PWA functionality to
+       application. For more information, see:
+       https://developers.google.com/web/fundamentals/primers/service-workers -->
+  <script>
+    var serviceWorkerVersion = null;
+    var scriptLoaded = false;
+    function loadMainDartJs() {
+      if (scriptLoaded) {
+        return;
+      }
+      scriptLoaded = true;
+      var scriptTag = document.createElement('script');
+      scriptTag.src = 'main.dart.js';
+      scriptTag.type = 'application/javascript';
+      document.body.append(scriptTag);
+    }
+
+    if ('serviceWorker' in navigator) {
+      // Service workers are supported. Use them.
+      window.addEventListener('load', function () {
+        // Wait for registration to finish before dropping the <script> tag.
+        // Otherwise, the browser will load the script multiple times,
+        // potentially different versions.
+        var serviceWorkerUrl = 'flutter_service_worker.js?v=' + serviceWorkerVersion;
+        navigator.serviceWorker.register(serviceWorkerUrl)
+          .then((reg) => {
+            function waitForActivation(serviceWorker) {
+              serviceWorker.addEventListener('statechange', () => {
+                if (serviceWorker.state == 'activated') {
+                  console.log('Installed new service worker.');
+                  loadMainDartJs();
+                }
+              });
+            }
+            if (!reg.active && (reg.installing || reg.waiting)) {
+              // No active web worker and we have installed or are installing
+              // one for the first time. Simply wait for it to activate.
+              waitForActivation(reg.installing || reg.waiting);
+            } else if (!reg.active.scriptURL.endsWith(serviceWorkerVersion)) {
+              // When the app updates the serviceWorkerVersion changes, so we
+              // need to ask the service worker to update.
+              console.log('New service worker available.');
+              reg.update();
+              waitForActivation(reg.installing);
+            } else {
+              // Existing service worker is still good.
+              console.log('Loading app from service worker.');
+              loadMainDartJs();
+            }
+          });
+
+        // If service worker doesn't succeed in a reasonable amount of time,
+        // fallback to plaint <script> tag.
+        setTimeout(() => {
+          if (!scriptLoaded) {
+            console.warn(
+              'Failed to load app from service worker. Falling back to plain <script> tag.',
+            );
+            loadMainDartJs();
+          }
+        }, 4000);
+      });
+    } else {
+      // Service workers not supported. Just drop the <script> tag.
+      loadMainDartJs();
+    }
+  </script>
+</body>
+</html>
diff --git a/web/manifest.json b/web/manifest.json
new file mode 100644
index 0000000..7bab0e5
--- /dev/null
+++ b/web/manifest.json
@@ -0,0 +1,35 @@
+{
+    "name": "flutter_launcher",
+    "short_name": "flutter_launcher",
+    "start_url": ".",
+    "display": "standalone",
+    "background_color": "#0175C2",
+    "theme_color": "#0175C2",
+    "description": "A new Flutter project.",
+    "orientation": "portrait-primary",
+    "prefer_related_applications": false,
+    "icons": [
+        {
+            "src": "icons/Icon-192.png",
+            "sizes": "192x192",
+            "type": "image/png"
+        },
+        {
+            "src": "icons/Icon-512.png",
+            "sizes": "512x512",
+            "type": "image/png"
+        },
+        {
+            "src": "icons/Icon-maskable-192.png",
+            "sizes": "192x192",
+            "type": "image/png",
+            "purpose": "maskable"
+        },
+        {
+            "src": "icons/Icon-maskable-512.png",
+            "sizes": "512x512",
+            "type": "image/png",
+            "purpose": "maskable"
+        }
+    ]
+}