C # learning notes for object-oriented design patterns (9)-decorator decoration patterns (Structural Patterns)

In some cases, we may "over-use inheritance to extend the object's functions". This extension method lacks flexibility due to the static (compile-time) characteristics introduced by inheritance as a type; as the number of child classes increases (the number of extended functions), the combination of various child classes (the combination of extended functions) will lead to the expansion of more child classes (more inheritance ).

How can we enable "extension of object functions" to be implemented dynamically (during runtime) as needed? At the same time, avoid the subclass expansion problem caused by "more extended functions? So that any "function scaling changes" will cause the lowest impact?

Intention (Intent) 

Dynamically add some additional responsibilities to an object. The decorator mode is more flexible than the subclass generation function.

 

 

  Public     Abstract     Class  Tank
{
 Public     Abstract     Void  Shot ();
  Public     Abstract     Void  Run ();
}

  Public     Abstract     Class Decorator: Tank  //  Interface inheritance  
  {
  Private  Tank tank;  //  Has-A Object combination  
  
  Public  Decorator (tank)
{
  This  . Tank  = Tank;
}

  Public     Override     Void  Shot ()
{
Tank. Shot ();
}

  Public     Override     Void  Run ()
{
Tank. Run ();
}
} 

 

  ///     <Summary>  
  ///  Decoration Class
  ///     </Summary>  
  Public     Class  Decoratora: decorator
{
 Public  Decoratora (tank)
:  Base  (Tank)
{
}

  Public     Override     Void  Shot ()
{
  //  Extended infrared function
  //  Do shot...  
   Base  . Shot ();
}

  Public     Override     Void  Run ()
{
  //  Extended infrared function
  //  Do run...  
    Base  . Run ();
}
}

 ///     <Summary>  
  ///  Decoration Class B
  ///     </Summary>  
  Public     Class  Decoratorb: decorator
{
  Public  Decoratorb (tank)
: Base  (Tank)
{
}

  Public     Override     Void  Shot ()
{
  //  Amphibious Functions
  //  Do shot...  
    Base  . Shot ();
}

 Public     Override     Void  Run ()
{
  //  Amphibious Functions
  //  Do run...  
    Base  . Run ();
}
}

  ///     <Summary> 
  ///  Decoration Class C
  ///     </Summary>  
  Public     Class  Decoratorc: decorator
{
  Public  Decoratorc (tank)
:  Base  (Tank)
{
}

  Public    Override     Void  Shot ()
{
  //  Satellite Positioning Function Extension
  //  Do shot...  
    Base  . Shot ();
}

  Public     Override    Void  Run ()
{
  //  Satellite Positioning Function Extension
  //  Do run...  
    Base  . Run ();
}
} 

 

  Class  App
{
  Public    Static     Void  Main ()
{
Tank  =     New  T50 ();
Decoratora da  =     New  Decoratora (tank );  //  Extended infrared function  
  Decoratora DB =     New  Decoratora (DA );  //  Amphibious Functions  
  Decoratora DC  =     New  Decoratora (db );  //  Satellite Positioning Function Extension  
  }
} 

 

Key points:

By using a combination rather than an inheritance method, the decorator mode enables the ability to dynamically expand object functions at runtime, and can expand multiple functions as needed. This avoids the "poor flexibility" and "Multi-subclass Derivative Problems" caused by inheritance ".

The decorator class representsIs-a componentThat is, the decorator class inherits the interfaces of the component class. However, the implementation is as follows:Has-a componentThat is, the decorator class uses another component class.

The decorator mode does not solve the problem of "Multi-subclass-derived multi-inheritance, the main point of the decorator mode application is to solve the problem of "the extension function of the Subject Class in multiple directions", which is the meaning of "decoration.

Applicability 

The decoration mode should be used in the following circumstances:

1. You need to extend the functions of a class or add additional responsibilities to a class.

2. You need to dynamically add functions to an object. These functions can be dynamically revoked.

3. A large number of functions are generated by the arrangement and combination of some basic functions to make the inheritance relationship unrealistic.