Add "Observer" pattern.

Author: ilopX

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

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+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

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+abstract class Event {
+
+}