Add "Observer" pattern. Official example from java.

Author: ilopX

patterns/observer/open_close_editor_events/README.md

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+# 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

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+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

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+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

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+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

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

patterns/observer/open_close_editor_events/listeners/log_open_listener.dart

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+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

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+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);
+  }
+}