Add "Observer" pattern. Official example from java.

CHANGELOG.md

@@ -1,3 +1,6 @@
+## 0.13.0
+- Add "Observer" pattern from official book, rewritten from Java example.
+
 ## 0.12.19
 - Refactoring: reformatting and minor changes
 

README.md

@@ -16,7 +16,7 @@ It contains **Dart** examples for all classic **GoF** design patterns.
     - [ ] [**Iterator**](https://refactoring.guru/design-patterns/iterator)
     - [ ] [**Mediator**](https://refactoring.guru/design-patterns/mediator)
     - [ ] [**Memento**](https://refactoring.guru/design-patterns/memento)
-    - [ ] [**Observer**](https://refactoring.guru/design-patterns/observer)
+    - [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)]
     - [ ] [**State**](https://refactoring.guru/design-patterns/state)
     - [ ] [**Template Method**](https://refactoring.guru/design-patterns/template-method)
     - [ ] [**Visitor**](https://refactoring.guru/design-patterns/visitor)

patterns/observer/open_close_editor_events/README.md

@@ -0,0 +1,44 @@
+# 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.
+
+## Editor example 
+In this example, the Observer pattern lets the text editor object notify other service objects about 
+changes in its state.
+
+More detailed explanation on [RefactoringGuru](https://refactoring.guru/design-patterns/observer?#pseudocode).
+
+### Origin source code:
+This example rewrite from [java example](https://github.com/RefactoringGuru/design-patterns-java/tree/master/src/refactoring_guru/observer/example).
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/150955865-7fbb29f3-ed48-4317-a356-cbb9cc79ed11.png)
+
+### Client code:
+```dart
+void main() {
+  final editor = Editor();
+  editor.events
+    ..subscribe(
+      'open',
+      LogOpenListener('log.txt'),
+    )
+    ..subscribe(
+      'save',
+      EmailNotificationListener('admin@example.com'),
+    );
+
+  try {
+    editor.openFile('test.txt');
+    editor.saveFile();
+  } catch (e) {
+    print(e);
+  }
+}
+```
+
+**Output:**
+```
+Save to log "log.txt": Someone has performed "open" operation with the following file: "test.txt"
+Email to "admin@example.com": Someone has performed "save" operation with the following file: "test.txt"
+```

patterns/observer/open_close_editor_events/editor/editor.dart

@@ -0,0 +1,22 @@
+import 'dart:io';
+
+import '../event_manager/event_manager.dart';
+
+class Editor {
+  final events = EventManager(['open', 'save']);
+
+  File? _file;
+
+  void openFile(String filePath) {
+    _file = File(filePath);
+    events.notify("open", _file!);
+  }
+
+  void saveFile() {
+    if (_file == null) {
+      throw Exception('Please open a file first.');
+    }
+
+    events.notify('save', _file!);
+  }
+}

patterns/observer/open_close_editor_events/event_manager/event_manager.dart

@@ -0,0 +1,38 @@
+import 'dart:io';
+
+import '../listeners/event_listener.dart';
+
+class EventManager {
+  final _listeners = <String, List<EventListener>>{};
+
+  EventManager(List<String> operations) {
+    for (final operation in operations) {
+      _listeners[operation] = [];
+    }
+  }
+
+  void subscribe(String eventType, EventListener listener) {
+    _usersBy(eventType).add(listener);
+  }
+
+  void unsubscribe(String eventType, EventListener listener) {
+    _usersBy(eventType).remove(listener);
+  }
+
+  void notify(String eventType, File file) {
+    final users = _usersBy(eventType);
+    for (final listener in users) {
+      listener.update(eventType, file);
+    }
+  }
+
+  List<EventListener> _usersBy(String eventType) {
+    final users = _listeners[eventType];
+
+    if (users == null) {
+      throw UnsupportedError('Event type "$eventType" do not support.');
+    }
+
+    return users;
+  }
+}

patterns/observer/open_close_editor_events/listeners/email_notification_listener.dart

@@ -0,0 +1,16 @@
+import 'dart:io';
+
+import 'event_listener.dart';
+
+class EmailNotificationListener implements EventListener {
+  String email;
+
+  EmailNotificationListener(this.email);
+
+  @override
+  void update(String eventType, File file) {
+    print('Email to "$email": '
+        'Someone has performed "$eventType" '
+        'operation with the following file: "${file.path}"');
+  }
+}

patterns/observer/open_close_editor_events/listeners/event_listener.dart

@@ -0,0 +1,5 @@
+import 'dart:io';
+
+abstract class EventListener {
+  void update(String eventType, File file);
+}

patterns/observer/open_close_editor_events/listeners/log_open_listener.dart

@@ -0,0 +1,16 @@
+import 'dart:io';
+
+import 'event_listener.dart';
+
+class LogOpenListener implements EventListener {
+  File logFile;
+
+  LogOpenListener(String logFileName) : logFile = File(logFileName);
+
+  @override
+  void update(String eventType, File file) {
+    print('Save to log "${logFile.path}": '
+        'Someone has performed "$eventType" '
+        'operation with the following file: "${file.path}"');
+  }
+}

patterns/observer/open_close_editor_events/main.dart

@@ -0,0 +1,23 @@
+import 'editor/editor.dart';
+import 'listeners/email_notification_listener.dart';
+import 'listeners/log_open_listener.dart';
+
+void main() {
+  final editor = Editor();
+  editor.events
+    ..subscribe(
+      'open',
+      LogOpenListener('log.txt'),
+    )
+    ..subscribe(
+      'save',
+      EmailNotificationListener('admin@example.com'),
+    );
+
+  try {
+    editor.openFile('test.txt');
+    editor.saveFile();
+  } catch (e) {
+    print(e);
+  }
+}

pubspec.yaml

@@ -1,6 +1,6 @@
 name: design_patterns_dart
 description: Dart examples for all classic GoF design patterns.
-version: 0.12.19
+version: 0.13.0
 homepage: https://refactoring.guru/design-patterns
 repository: https://github.com/RefactoringGuru/design-patterns-dart
 issue_tracker: https://github.com/RefactoringGuru/design-patterns-dart/issue