Add conceptual visitor pattern

CHANGELOG.md

@@ -1,3 +1,6 @@
+## 0.29.0
+- Add conceptual visitor pattern.
+
 ## 0.28.0
 - Add conceptual flyweight pattern.
 

README.md

@@ -19,15 +19,15 @@ It contains **Dart** examples for all classic **GoF** design patterns.
     - [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** - [[Conceptual](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)]
     - [x] **Template Method** - [[Data Miner](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/template_method/data_miner)] 
-    - [X] **Visitor**  [[Shape XML Exporter](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/visitor/shapes_exporter)]
+    - [X] **Visitor** - [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/visitor/conceptual)] [[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)]
 - [ ] **Structural**
     - [x] **Adapter** - [[Text Graphics](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/adapter/text_graphics)] [[Square Round conflict](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/adapter/square_round_conflict)] [[![16x16](https://user-images.githubusercontent.com/8049534/171852337-57db0faf-1f5e-489a-a79a-22ed4f47b4ed.png) Flutter Adapter](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/adapter/flutter_adapter)] 
     - [x] **Bridge** - [[Remote Device Control](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/bridge/devices_remote_control)] [[Clock](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/bridge/clock)] 
     - [x] **Composite** - [[Image Editor](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/composite/image_editor)] [[Products and Boxes](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/composite/products_and_boxes)] 
     - [x] **Decorator** - [[Data Source Decoder](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/decorator/data_source_decoder)]
     - [ ] **Facade**
-    - [x] **Flyweight** [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/flyweight/conceptual)]
+    - [x] **Flyweight** - [[Conceptual](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/flyweight/conceptual)]
     - [ ] **Proxy**
 
 ## Requirements

patterns/visitor/conceptual/README.md

@@ -0,0 +1,82 @@
+# Visitor pattern
+Visitor is a behavioral design pattern that lets you separate algorithms from the objects on which 
+they operate.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/visitor).
+
+### About example.
+
+## Client code:
+### Before:
+```dart
+void main() {
+  final list = createElements();
+  list.forEach(operation1);
+}
+
+Iterable<Object> createElements() {
+  return [
+    One(),
+    Two(),
+    Three(),
+  ];
+}
+
+void operation1(Object obj) {
+  if (obj is One) {
+    print('operation1: one (param1 = ${obj.param1})');
+  } else if (obj is Two) {
+    print('operation1: two (param2 = ${obj.param2})');
+  } else if (obj is Three) {
+    print('operation1: two (param3 = ${obj.param3})');
+  }
+}
+```
+
+### After:
+```dart
+void main() {
+  final list = createElements();
+
+  for (final e in elements) {
+    e.accept(visitor);
+  }
+}
+
+Iterable<Object> createElements() {
+  return [
+    One(),
+    Two(),
+    Three(),
+  ];
+}
+
+class ConcreteVisitor1 implements Visitor {
+  @override
+  void visitOne(One one) {
+    print('operation1: one (param1 = ${one.param1})');
+  }
+
+  @override
+  void visitTwo(Two two) {
+    print('operation1: two (param2 = ${two.param2})');
+  }
+
+  @override
+  void visitThree(Three three) {
+    print('operation1: three (param3 = ${three.param3})');
+  }
+}
+```
+#### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/174583542-5f57463c-148b-4113-acd3-2814ec017ecc.png)
+
+### Output:
+```
+operation1: one (param1 = 1)
+operation1: two (param2 = 2)
+operation1: three (param3 = 3)
+operation2: one (param1 = 1)
+operation2: two (param2 = 2)
+operation2: three (param3 = 3)
+```

patterns/visitor/conceptual/after/elements/one.dart

@@ -0,0 +1,11 @@
+import '../pattern/element.dart';
+import '../pattern/visitor.dart';
+
+class One implements Element {
+  final String param1 = '1';
+
+  @override
+  void accept(Visitor visitor) {
+    visitor.visitOne(this);
+  }
+}

patterns/visitor/conceptual/after/elements/three.dart

@@ -0,0 +1,11 @@
+import '../pattern/element.dart';
+import '../pattern/visitor.dart';
+
+class Three implements Element {
+  final String param3 = '3';
+
+  @override
+  void accept(Visitor visitor) {
+    visitor.visitThree(this);
+  }
+}

patterns/visitor/conceptual/after/elements/two.dart

@@ -0,0 +1,11 @@
+import '../pattern/element.dart';
+import '../pattern/visitor.dart';
+
+class Two implements Element {
+  final String param2 = '2';
+
+  @override
+  void accept(Visitor visitor) {
+    visitor.visitTwo(this);
+  }
+}

patterns/visitor/conceptual/after/main.dart

@@ -0,0 +1,27 @@
+import 'elements/one.dart';
+import 'elements/three.dart';
+import 'elements/two.dart';
+import 'operations/concrete_visitor1.dart';
+import 'operations/concrete_visitor2.dart';
+import 'pattern/element.dart';
+import 'pattern/visitor.dart';
+
+void main() {
+  final list = createElements();
+  operation(list, ConcreteVisitor1());
+  operation(list, ConcreteVisitor2());
+}
+
+Iterable<Element> createElements() {
+  return [
+    One(),
+    Two(),
+    Three(),
+  ];
+}
+
+void operation(Iterable<Element> elements, Visitor visitor) {
+  for (final e in elements) {
+    e.accept(visitor);
+  }
+}

patterns/visitor/conceptual/after/operations/concrete_visitor1.dart

@@ -0,0 +1,21 @@
+import '../elements/one.dart';
+import '../elements/three.dart';
+import '../elements/two.dart';
+import '../pattern/visitor.dart';
+
+class ConcreteVisitor1 implements Visitor {
+  @override
+  void visitOne(One one) {
+    print('operation1: one (param1 = ${one.param1})');
+  }
+
+  @override
+  void visitTwo(Two two) {
+    print('operation1: two (param2 = ${two.param2})');
+  }
+
+  @override
+  void visitThree(Three three) {
+    print('operation1: three (param3 = ${three.param3})');
+  }
+}

patterns/visitor/conceptual/after/operations/concrete_visitor2.dart

@@ -0,0 +1,21 @@
+import '../elements/one.dart';
+import '../elements/three.dart';
+import '../elements/two.dart';
+import '../pattern/visitor.dart';
+
+class ConcreteVisitor2 implements Visitor {
+  @override
+  void visitOne(One one) {
+    print('operation2: one (param1 = ${one.param1})');
+  }
+
+  @override
+  void visitTwo(Two two) {
+    print('operation2: two (param2 = ${two.param2})');
+  }
+
+  @override
+  void visitThree(Three three) {
+    print('operation2: three (param3 = ${three.param3})');
+  }
+}

patterns/visitor/conceptual/after/pattern/element.dart

@@ -0,0 +1,5 @@
+import 'visitor.dart';
+
+abstract class Element {
+  void accept(Visitor visitor);
+}

patterns/visitor/conceptual/after/pattern/visitor.dart

@@ -0,0 +1,11 @@
+import '../elements/one.dart';
+import '../elements/three.dart';
+import '../elements/two.dart';
+
+abstract class Visitor {
+  void visitOne(One one);
+
+  void visitTwo(Two two);
+
+  void visitThree(Three three);
+}

patterns/visitor/conceptual/before/elements/elements.dart

@@ -0,0 +1,11 @@
+class One {
+  final String param1 = '1';
+}
+
+class Two {
+  final String param2 = '2';
+}
+
+class Three {
+  final String param3 = '3';
+}

patterns/visitor/conceptual/before/main.dart

@@ -0,0 +1,36 @@
+import 'elements/elements.dart';
+
+void main() {
+  final list = createElements();
+
+  list.forEach(operation1);
+  list.forEach(operation2);
+}
+
+Iterable<Object> createElements() {
+  return [
+    One(),
+    Two(),
+    Three(),
+  ];
+}
+
+void operation1(Object obj) {
+  if (obj is One) {
+    print('operation1: one (param1 = ${obj.param1})');
+  } else if (obj is Two) {
+    print('operation1: two (param2 = ${obj.param2})');
+  } else if (obj is Three) {
+    print('operation1: two (param3 = ${obj.param3})');
+  }
+}
+
+void operation2(Object obj) {
+  if (obj is One) {
+    print('operation2: one (param1 = ${obj.param1})');
+  } else if (obj is Two) {
+    print('operation2: two (param2 = ${obj.param2})');
+  } else if (obj is Three) {
+    print('operation2: two (param3 = ${obj.param3})');
+  }
+}

pubspec.yaml

@@ -1,6 +1,6 @@
 name: design_patterns_dart
 description: Dart examples for all classic GoF design patterns.
-version: 0.28.0
+version: 0.29.0
 homepage: https://refactoring.guru/design-patterns
 repository: https://github.com/RefactoringGuru/design-patterns-dart
 issue_tracker: https://github.com/RefactoringGuru/design-patterns-dart/issue