Design Mode Study Notes 24-Visitor Mode

Motivation : Indicates an operation that acts on each element in an object structure. It can define new operations that act on these elements without changing the classes of each element.

Scenario: Graphic System, assuming its image type is stable, and its operation changes are available.

Structure

Code

Namespace Designpattern. Visitor
{
Public   Abstract   Class Shape
{< br> Public abstract void draw ();
// Second Distribution
Public abstract void Accept (shapevisitor visitor);
}

Public   Class Rectangle: Shape
{
Public   Override   Void Draw ()
{
}

Public   Override   Void Accept (shapevisitor visitor)
{
Visitor. Visit (This);
}
}

Public   Class Circle: Shape
{
Public   Override   Void Draw ()
{
}

Public   Override   Void Accept (shapevisitor visitor)
{
Visitor. Visit (This);
}
}

Public   Class Line: Shape
{
Public   Override   Void Draw ()
{
}

Public   Override   Void Accept (shapevisitor visitor)
{
Visitor. Visit (This);
}
}
}

Namespace Designpattern. Visitor
{
Public   Abstract   Class Shapevisitor
{
Public   Abstract   Void Visit (rectangle );
Public   Abstract   Void Visit (circle );
Public   Abstract   Void Visit (line );
}

Public   Class Scalevisitor: shapevisitor
{
Public   Override   Void Visit (rectangle)
{
}

Public   Override   Void Visit (circle)
{
}

Public   Override   Void Visit (line)
{
}
}

Public   Class Movevisitor: shapevisitor
{
Public   Override   Void Visit (rectangle)
{
}

Public   Override   Void Visit (circle)
{
}

Public   Override   Void Visit (line)
{
}
}
}

Namespace Designpattern. Visitor
{
Public   Class Application
{
Shapevisitor visitor;

Public Application (shapevisitor visitor)
{
This. Visitor=Visitor;
}

Public   Void Process (shape)
{
Shape. Accept (visitor );//First Distribution
}
}
}

Key Points :
1. In this mode, the layer structure of the element class is not changed through the so-called dual-distribution, during runtime, new operations are dynamically added for each class in the class hierarchy transparently.
2. The so-called dual-distribution model contains two polymorphism distributions. The first is the polymorphism of the accept method, and the second is the polymorphism of the visit method.
3. The biggest drawback of this mode is that the extension class hierarchy (adding new element subclass) will change the visitor class. Therefore, this mode applies to "element class hierarchies are stable, but operations are subject to frequent changes ".