Add visitor pattern.

CHANGELOG.md

@@ -1,3 +1,6 @@
+## 0.22.0
+- Add visitor pattern: "Shape Xml Export".
+
 ## 0.21.0
 - Add strategy pattern: "Reservation cargo spaces".
 

README.md

@@ -19,8 +19,8 @@ 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)] [[Subscriber Flutter Widget](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/observer/subscriber_flutter_widget)]
     - [ ] **State**
     - [ ] **Template Method**
-    - [ ] **Visitor**
-    - [X] **Strategy** [[Reservation cargo spaces](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/strategy/reservation_cargo_spaces)]
+    - [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)]
 - [ ] **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)] [[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)] 

patterns/visitor/shapes_exporter/README.md

@@ -0,0 +1,78 @@
+# 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: Shape XML Exporter.
+In this example, the Visitor pattern adds XML export support to the class hierarchy of geometric 
+shapes [More info](https://refactoring.guru/design-patterns/visitor#pseudocode).
+
+This example rewrite from original source code [java example](https://github.com/RefactoringGuru/design-patterns-java/tree/main/src/refactoring_guru/visitor/example)
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/167304227-04237030-879e-4d7f-be32-4c815a3e1cbf.png)
+
+### Client code:
+```dart
+  final compoundShape = CompoundShape(
+    x: 30,
+    y: 45,
+    children: [
+      Rectangle(x: 10, y: 10, width: 100, height: 100),
+      Circle(xCenter: 300, yCenter: 20, radius: 35),
+      Dot(x: 60, y: 60),
+      CompoundShape(
+        x: 5,
+        y: 5,
+        children: [
+          Dot(x: 15, y: 15),
+          Dot(x: 20, y: 20),
+        ],
+      ),
+    ],
+  );
+
+  final xml = XMLExportVisitor().export(compoundShape);
+  print(xml);
+```
+
+### Output:
+```xml
+<?xml version="1.0" encoding="utf-8"?>
+<compound>
+  <x>30</x>
+  <y>45</y>
+  <children>
+    <rectangle>
+      <x>10</x>
+      <y>10</y>
+      <width>100</width>
+      <height>100</height>
+    </rectangle>
+    <circle>
+      <xCenter>300</xCenter>
+      <yCenter>20</yCenter>
+      <radius>35</radius>
+    </circle>
+    <dot>
+      <x>60</x>
+      <y>60</y>
+    </dot>
+    <compound>
+      <x>5</x>
+      <y>5</y>
+      <children>
+        <dot>
+          <x>15</x>
+          <y>15</y>
+        </dot>
+        <dot>
+          <x>20</x>
+          <y>20</y>
+        </dot>
+      </children>
+    </compound>
+  </children>
+</compound>
+```

patterns/visitor/shapes_exporter/main.dart

@@ -0,0 +1,28 @@
+import 'shapes/circle.dart';
+import 'shapes/compound_shape.dart';
+import 'shapes/dot.dart';
+import 'shapes/rectangle.dart';
+import 'visitor/xml_export_visitor.dart';
+
+void main() {
+  final compoundShape = CompoundShape(
+    x: 30,
+    y: 45,
+    children: [
+      Rectangle(x: 10, y: 10, width: 100, height: 100),
+      Circle(xCenter: 300, yCenter: 20, radius: 35),
+      Dot(x: 60, y: 60),
+      CompoundShape(
+        x: 5,
+        y: 5,
+        children: [
+          Dot(x: 15, y: 15),
+          Dot(x: 20, y: 20),
+        ],
+      ),
+    ],
+  );
+
+  final xml = XMLExportVisitor().export(compoundShape);
+  print(xml);
+}

patterns/visitor/shapes_exporter/shapes/circle.dart

@@ -0,0 +1,24 @@
+import '../visitor/visitor.dart';
+import 'shape.dart';
+
+class Circle implements Shape {
+  final int xCenter;
+  final int yCenter;
+  final int radius;
+
+  Circle({
+    required this.xCenter,
+    required this.yCenter,
+    required this.radius,
+  });
+
+  @override
+  void accept(Visitor visitor) {
+    visitor.visitCircle(this);
+  }
+
+  @override
+  void draw() {
+    // ...
+  }
+}

patterns/visitor/shapes_exporter/shapes/compound_shape.dart

@@ -0,0 +1,24 @@
+import '../visitor/visitor.dart';
+import 'shape.dart';
+
+class CompoundShape implements Shape {
+  final int x;
+  final int y;
+  final List<Shape> children;
+
+  CompoundShape({
+    required this.x,
+    required this.y,
+    required this.children,
+  });
+
+  @override
+  void accept(Visitor visitor) {
+    visitor.visitCompoundShape(this);
+  }
+
+  @override
+  void draw() {
+    // ...
+  }
+}

patterns/visitor/shapes_exporter/shapes/dot.dart

@@ -0,0 +1,22 @@
+import '../visitor/visitor.dart';
+import 'shape.dart';
+
+class Dot implements Shape {
+  final int x;
+  final int y;
+
+  Dot({
+    required this.x,
+    required this.y,
+  });
+
+  @override
+  void accept(Visitor visitor) {
+    visitor.visitDot(this);
+  }
+
+  @override
+  void draw() {
+    // ...
+  }
+}

patterns/visitor/shapes_exporter/shapes/rectangle.dart

@@ -0,0 +1,26 @@
+import '../visitor/visitor.dart';
+import 'shape.dart';
+
+class Rectangle implements Shape {
+  final int x;
+  final int y;
+  final int width;
+  final int height;
+
+  Rectangle({
+    required this.x,
+    required this.y,
+    required this.width,
+    required this.height,
+  });
+
+  @override
+  void accept(Visitor visitor) {
+    visitor.visitRectangle(this);
+  }
+
+  @override
+  void draw() {
+    // ...
+  }
+}

patterns/visitor/shapes_exporter/shapes/shape.dart

@@ -0,0 +1,7 @@
+import '../visitor/visitor.dart';
+
+abstract class Shape {
+  void accept(Visitor visitor);
+
+  void draw();
+}

patterns/visitor/shapes_exporter/visitor/visitor.dart

@@ -0,0 +1,14 @@
+import '../shapes/circle.dart';
+import '../shapes/compound_shape.dart';
+import '../shapes/dot.dart';
+import '../shapes/rectangle.dart';
+
+abstract class Visitor {
+  void visitCompoundShape(CompoundShape compound);
+
+  void visitDot(Dot dot);
+
+  void visitCircle(Circle circle);
+
+  void visitRectangle(Rectangle rectangle);
+}

patterns/visitor/shapes_exporter/visitor/xml_export_visitor.dart

@@ -0,0 +1,73 @@
+import '../shapes/compound_shape.dart';
+import '../shapes/dot.dart';
+import '../shapes/rectangle.dart';
+import '../shapes/shape.dart';
+import 'visitor.dart';
+
+class XMLExportVisitor implements Visitor {
+  final buff = StringBuffer();
+
+  String export(Shape shape) {
+    buff.clear();
+    _write('<?xml version="1.0" encoding="utf-8">?');
+    shape.accept(this);
+    return buff.toString().trim();
+  }
+
+  @override
+  void visitCompoundShape(CompoundShape compound) {
+    _write('<compound>', openTag: true);
+    _write('<x>${compound.x}</x>');
+    _write('<y>${compound.y}</y>');
+    _write('<children>', openTag: true);
+
+    for (final shape in compound.children) {
+      shape.accept(this);
+    }
+
+    _write('</children>', closeTag: true);
+    _write('</compound>', closeTag: true);
+  }
+
+  @override
+  void visitCircle(circle) {
+    _write('<circle>', openTag: true);
+    _write('<xCenter>${circle.xCenter}</xCenter>');
+    _write('<yCenter>${circle.yCenter}</yCenter>');
+    _write('<radius>${circle.radius}</radius>');
+    _write('</circle>', closeTag: true);
+  }
+
+  @override
+  void visitDot(Dot dot) {
+    _write('<dot>', openTag: true);
+    _write('<x>${dot.x}</x>');
+    _write('<y>${dot.y}</y>');
+    _write('</dot>', closeTag: true);
+  }
+
+  @override
+  void visitRectangle(Rectangle rectangle) {
+    _write('<rectangle>', openTag: true);
+    _write('<x>${rectangle.x}</x>');
+    _write('<y>${rectangle.y}</y>');
+    _write('<width>${rectangle.width}</width>');
+    _write('<height>${rectangle.height}</height>');
+    _write('</rectangle>', closeTag: true);
+  }
+
+  int _openTags = 0;
+
+  void _write(String str, {openTag = false, closeTag = false}) {
+    if (closeTag) {
+      _openTags--;
+    }
+
+    final tab = '  ' * _openTags;
+    buff.writeln(tab + str);
+
+    if (openTag) {
+      _openTags++;
+    }
+  }
+}

pubspec.yaml

@@ -1,6 +1,6 @@
 name: design_patterns_dart
 description: Dart examples for all classic GoF design patterns.
-version: 0.21.0
+version: 0.22.0
 homepage: https://refactoring.guru/design-patterns
 repository: https://github.com/RefactoringGuru/design-patterns-dart
 issue_tracker: https://github.com/RefactoringGuru/design-patterns-dart/issue