Merge pull request #21 from ilopX/add-observer-example-for-further-use-in-more-complex-examples

Add example "Observer" for later use in more complex examples.

Author: neochief

.gitignore

@@ -1,6 +1,7 @@
 .dart_tool/
 .idea/
 build/
+windows/
 
 .packages
 pubspec.lock

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
 

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)

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(),
+    );
+  }
+}

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
+```

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());
+}

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> {}

patterns/observer/app_observer/observer/event.dart

@@ -0,0 +1,3 @@
+abstract class Event {
+
+}

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
+
+