笔记：Gof设计模式--Decorator

1、意图

  Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

2、适应性

  Use the Decorator pattern when

  •  to add responsibilities to individual objects dynamically and transparently, that is, without affecting other objects.
  •  for responsibilities that can be withdrawn.
  •  when extension by subclassing is impractical. Sometimes a large number of independent extensions are possible and would produce an explosion of subclasses to support every combination. Or a class definition may be hidden or otherwise unavailable for subclassing.

3、结构

 

4、示例代码

The following code shows how to implement user interface decorators in C++. We'll assume there's a Component class called VisualComponent And subclass Decorator.

  class VisualComponent {     public:         VisualComponent();   virtual void Draw();         virtual void Resize();         // ...     }; 

 class Decorator : public VisualComponent {     public:         Decorator(VisualComponent*);              virtual void Draw();         virtual void Resize();         // ...     private:         VisualComponent* _component;     }; 

For each operation in VisualComponent's interface, Decorator defines a default implementation that passes the request on to _component:

 void Decorator::Draw () {         _component->Draw();     }  void Decorator::Resize () {         _component->Resize();     } 

the class BorderDecorator adds a border to its enclosing component.

class BorderDecorator : public Decorator {     public:         BorderDecorator(VisualComponent*, int borderWidth);  virtual void Draw();     private:         void DrawBorder(int);     private:         int _width;     };          void BorderDecorator::Draw () {         Decorator::Draw();         DrawBorder(_width);     }  

we want a bordered and scrollable TextView. So we decorate it accordingly before putting it in the window. We'll assume our Window class provides a SetContents operation for this purpose:

  void Window::SetContents (VisualComponent* contents) {         // ...     }   window->SetContents(         new BorderDecorator(        new ScrollDecorator(textView), 1         )     );