Impl Singleton pattern.

Author: ilopX

patterns/singleton/conceptual_dart_style/main.dart

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+import 'pattern/singleton.dart';
+
+void main() {
+  // dart style
+  Singleton().doSome();
+  Singleton().doSome();
+
+  // standard style
+  Singleton.instance.doSome();
+}

patterns/singleton/conceptual_dart_style/pattern/singleton.dart

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+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()');
+  }
+}