added shape_decorator example

Author: AdelDaniel

structural_design_pattern/decorator/shape_decorator.dart

@@ -0,0 +1,84 @@
+//!--
+//! in this example we will decorate a shape with some color
+//! without alter (Change) shape class.
+//!--
+// Source: https://www.tutorialspoint.com/design_pattern/decorator_pattern.htm
+
+// We're going to create a Shape interface and concrete classes implementing the Shape interface. We will then create an abstract decorator class ShapeDecorator implementing the Shape interface and having Shape object as its instance variable.
+// RedShapeDecorator is concrete class implementing ShapeDecorator.
+// DecoratorPatternDemo, our demo class will use RedShapeDecorator to decorate Shape objects.
+// Decorator Pattern UML Diagram
+
+// Step 1
+// Create an interface.
+abstract class IShape {
+  void draw();
+}
+
+// Step 2
+// Create concrete classes implementing the same interface.
+class Rectangle implements IShape {
+  @override
+  void draw() => print("Shape: Rectangle");
+}
+
+class Circle implements IShape {
+  @override
+  void draw() => print("Shape: Circle");
+}
+
+// Step 3
+// Create abstract decorator class implementing the Shape interface.
+abstract class ShapeDecorator implements IShape {
+  IShape decoratedShape;
+  ShapeDecorator(IShape decoratedShape) : this.decoratedShape = decoratedShape;
+  @override
+  void draw() => decoratedShape.draw();
+}
+
+// Step 4
+// Create concrete decorator class extending the ShapeDecorator class.
+class RedShapeDecorator extends ShapeDecorator {
+  RedShapeDecorator(IShape decoratedShape) : super(decoratedShape);
+
+  @override
+  void draw() {
+    decoratedShape.draw();
+    _setRedBorder(decoratedShape);
+  }
+
+  void _setRedBorder(IShape decoratedShape) {
+    print("Border Color: Red");
+  }
+}
+
+// Step 5
+// Use the main() to decorate Shape objects.
+void main(List<String> args) {
+  IShape circle = Circle();
+  IShape redCircle = RedShapeDecorator(Circle());
+  IShape redRectangle = RedShapeDecorator(Rectangle());
+
+  print("Circle with normal border");
+  circle.draw();
+
+  print("\nCircle of red border");
+  redCircle.draw();
+
+  print("\nRectangle of red border");
+  redRectangle.draw();
+}
+
+// Step 6
+// Verify the output.
+
+// Circle with normal border
+// Shape: Circle
+
+// Circle of red border
+// Shape: Circle
+// Border Color: Red
+
+// Rectangle of red border
+// Shape: Rectangle
+// Border Color: Red