Merge pull request #61 from ilopX/three_state

Add conceptual state pattern: Three state

Author: ilopX

CHANGELOG.md

@@ -1,3 +1,6 @@
+## 0.25.0
+- Add conceptual state pattern: Three State.
+
 ## 0.24.1
 - Add flutter icon to project links.
 

README.md

@@ -17,7 +17,7 @@ It contains **Dart** examples for all classic **GoF** design patterns.
     - [ ] **Mediator**
     - [x] **Memento** - [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/memento/conceptual)] [[![16x16](https://user-images.githubusercontent.com/8049534/171852337-57db0faf-1f5e-489a-a79a-22ed4f47b4ed.png) Memento Editor](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/memento/memento_editor)] 
     - [x] **Observer** - [[Open-Close Editor Events](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/observer/open_close_editor_events)] [[AppObserver](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/observer/app_observer)] [[![16x16](https://user-images.githubusercontent.com/8049534/171852337-57db0faf-1f5e-489a-a79a-22ed4f47b4ed.png) Subscriber Flutter Widget](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/observer/subscriber_flutter_widget)]
-    - [x] **State** - [[![16x16](https://user-images.githubusercontent.com/8049534/171852337-57db0faf-1f5e-489a-a79a-22ed4f47b4ed.png) State Manipulator](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/state/manipulator_state)]
+    - [x] **State** - [[Three State](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/state/three_state)] [[![16x16](https://user-images.githubusercontent.com/8049534/171852337-57db0faf-1f5e-489a-a79a-22ed4f47b4ed.png) State Manipulator](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/state/manipulator_state)]
     - [ ] **Template Method**
     - [X] **Visitor**  [[Shape XML Exporter](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/visitor/shapes_exporter)]
     - [X] **Strategy** [[Reservation Cargo Spaces](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/strategy/reservation_cargo_spaces)]

patterns/state/three_state/README.md

@@ -0,0 +1,22 @@
+# State Pattern
+State is a behavioral design pattern that lets an object alter its behavior when its internal state 
+changes. It appears as if the object changed its class.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/state).
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/172000870-a96c6dd2-5b6a-4a64-b3a9-4edb75dd8076.png)
+
+### Client code:
+```dart
+void main() {
+  final switcher = Switcher(
+    initState: One(),
+  );
+
+  switcher.call(); // call(1): One
+  switcher.call(); // call(2): Two
+  switcher.call(); // call(3): Three
+  switcher.call(); // call:(4) One
+}
+```

patterns/state/three_state/main.dart

@@ -0,0 +1,13 @@
+import 'pattern/switcher.dart';
+import 'states/one.dart';
+
+void main() {
+  final switcher = Switcher(
+    initState: One(),
+  );
+
+  switcher.call(); // call(1): One
+  switcher.call(); // call(2): Two
+  switcher.call(); // call(3): Three
+  switcher.call(); // call:(4) One
+}

patterns/state/three_state/pattern/state.dart

@@ -0,0 +1,9 @@
+part of switcher;
+
+abstract class State {
+  Switcher get context => _context;
+
+  void call();
+
+  late Switcher _context;
+}

patterns/state/three_state/pattern/switcher.dart

@@ -0,0 +1,24 @@
+library switcher;
+
+part 'state.dart';
+
+class Switcher {
+  Switcher({required State initState}) {
+    changeState(initState);
+  }
+
+  int get calls => _calls;
+
+  void call() {
+    _calls++;
+    _state.call();
+  }
+
+  void changeState(State newState) {
+    _state = newState;
+    _state._context = this;
+  }
+
+  late State _state;
+  int _calls = 0;
+}

patterns/state/three_state/states/one.dart

@@ -0,0 +1,10 @@
+import '../pattern/switcher.dart';
+import 'two.dart';
+
+class One extends State {
+  @override
+  void call() {
+    print('call(${context.calls}): One');
+    context.changeState(Two());
+  }
+}

patterns/state/three_state/states/three.dart

@@ -0,0 +1,10 @@
+import '../pattern/switcher.dart';
+import 'one.dart';
+
+class Three extends State {
+  @override
+  void call() {
+    print('call(${context.calls}): Three');
+    context.changeState(One());
+  }
+}

patterns/state/three_state/states/two.dart

@@ -0,0 +1,10 @@
+import '../pattern/switcher.dart';
+import 'three.dart';
+
+class Two extends State {
+  @override
+  void call() {
+    print('call(${context.calls}): Two');
+    context.changeState(Three());
+  }
+}

pubspec.yaml

@@ -1,6 +1,6 @@
 name: design_patterns_dart
 description: Dart examples for all classic GoF design patterns.
-version: 0.24.1
+version: 0.25.0
 homepage: https://refactoring.guru/design-patterns
 repository: https://github.com/RefactoringGuru/design-patterns-dart
 issue_tracker: https://github.com/RefactoringGuru/design-patterns-dart/issue