Add singleton pattern.

CHANGELOG.md

@@ -1,3 +1,6 @@
+## 0.34.0
+- Add conceptual single pattern.
+
 ## 0.33.0
 - Add conceptual builder pattern.
 

README.md

@@ -8,7 +8,7 @@ It contains **Dart** examples for all classic **GoF** design patterns.
     - [x] **Builder** - [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/builder/conceptual)] [[Color Text Format](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/builder/color_text_format)]
     - [x] **Factory Method** [[Conceptual Platform Dialog](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/factory_method/conceptual_platform_dialog)]
     - [x] **Prototype** - [[Shapes](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/prototype/shapes)] 
-    - [ ] **Singleton**
+    - [x] **Singleton** - [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/singleton/conceptual)]
 - [ ]  **Behavioral**
     - [x] **Chain of Responsibility** - [[Server Middleware](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/chain_of_responsibility/server_middleware)]
     - [x] **Command** - [[Text Editor](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/command/text_editor)] 

patterns/singleton/conceptual/README.md

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+# Singleton Pattern
+Singleton is a creational design pattern that lets you ensure that a class has only one instance, 
+while providing a global access point to this instance.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/singleton).
+
+### Diagram:
+![Singleton Pattern Diagram](https://user-images.githubusercontent.com/8049534/182938119-78a21534-5751-4dea-afa3-8acaec46eed9.png)
+
+### Client code:
+```dart
+void main() {
+  // dart style
+  Singleton().doSome();
+  Singleton().doSome();
+
+  // standard style
+  Singleton.instance.doSome();
+}
+```
+
+### Output:
+```
+Create singleton once.
+doSome()
+doSome()
+doSome()
+```

patterns/singleton/conceptual/main.dart

@@ -0,0 +1,10 @@
+import 'pattern/singleton.dart';
+
+void main() {
+  // dart style
+  Singleton().doSome();
+  Singleton().doSome();
+
+  // standard style
+  Singleton.instance.doSome();
+}

patterns/singleton/conceptual/pattern/singleton.dart

@@ -0,0 +1,23 @@
+abstract class Singleton {
+  factory Singleton() {
+    if (_instance == null) {
+      print('Create singleton once.');
+      _instance = ConcreteSingleton();
+    }
+
+    return _instance!;
+  }
+
+  static Singleton get instance => Singleton();
+
+  void doSome();
+
+  static Singleton? _instance;
+}
+
+class ConcreteSingleton implements Singleton {
+  @override
+  void doSome() {
+    print('doSome()');
+  }
+}

pubspec.yaml

@@ -1,6 +1,6 @@
 name: design_patterns_dart
 description: Dart examples for all classic GoF design patterns.
-version: 0.33.0
+version: 0.34.0
 homepage: https://refactoring.guru/design-patterns
 repository: https://github.com/RefactoringGuru/design-patterns-dart
 issue_tracker: https://github.com/RefactoringGuru/design-patterns-dart/issue