Decorator

Intent

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

Class diagram of the original GoF pattern

Improvement in MixJuice

Solved Problem(s)

Extensibility problem

It is impossible to add new method to Component and Decorator without modifying the source code. (This problem is mentioned at p.179,line.3-5 in the GoF book.)

Used programming technique(s)

Method addition or abstract method addition to Component and Decorator.

Consequences

• (+) New methods can be added to Component and Decorator without modifying the source code.
• (-) Addition of both a new method and a new ConcreteDecorator requires a complementary module; however, if default behavior of the method can be implemented in Decorator, e.g. just forward the operation to the next decorator, no complementary module is required.

Structure and pseudo code

module original {
  define abstract class Component {
    define abstract void operation();
  }
  define class ConcreteComponent extends Component {
    void operation(){...}
  }
  define abstract class Decorator extends Component {
    Component component;
    void operation(){ component.operation(); }
  }
  define class ConcreteDecorator extends Decorator {
    void operation(){...}
  }
}
module extension extends original {
  class Component {
    define abstract void newOperation();
  }
  class ConcreteComponent {
    void newOperation(){...}
  }
  class Decorator {
    void newOperation(){ component.newOperation(); }
  }
}

Another solution in MixJuice

Solved Problem(s)

Extensibility problem

It is impossible to add new method to Component and Decorator without modifying the source code. (This problem is mentioned at p.179,line.3-5 in the GoF book.)

Complexity problem

A decorator and its component aren't identical. (This problem is mentioned at p.178,line.29-32 in the GoF book.) Lots of little objects. (This problem is mentioned at p.178,line.33-37 in the GoF book.)

Used programming technique(s)

Class extension for Component.

Consequences

• (+) Object identity problem no longer occurs.
• (+) Class structure is not complicated.
• (+) New methods can be added to Component without modifying the source code.
• (-) It is impossible to attach additional responsibilities to an object dynamically.
• (-) This solution is applicable only if just one behavior of Component is used in a program.

Structure and pseudo code

module original {
  define class Component {
    define void operation();
  }
}
module extension1 extends original {
  class Component {
    void operation(){...}
  }
}
module extension2 extends original {
  class Component {
    void operation(){...}
  }
}

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