Undo a Visitor Pattern
Visitor is a classical
object-oriented design pattern. Undoing a visitor – the topic of this page – is
to remove all traces of a Visitor pattern.
In a Visitor pattern, all
methods of a program with the same signature (method name, parameter types in
order) are moved into a single class, called a "Visitor". The
original methods remain, except their bodies delegate to their corresponding
moved method. The ¡°Undo¡± Visitor pattern moves all these methods back to their
originating classes. The figure below summarizes the transformation:
Please
reference a simple tutorial about UML
Class Diagram if you¡¯re not familiar with it.
Below is our "Mechanics
Section" that explains how to undo a visitor. The same procedure is
used for undoing visitors for other methods.
1. The starting program has this class diagram.
2. For each visit method in DrawVisitor: move it back to its original
class. The first parameter of every visit method (e.g., Graphic of visit(Graphic)) indicates the original class. You
may assume that only visit
methods exist in DrawVisitor.
3. Each class (Picture, Square, Triangle, and Graphic) has an accept and a visit method with the same arguments and
return type. (This is to be expected, as the accept method delegates to the visit method). Replace the body of each accept method with the body of the visit method, and delete the visit method. Equivalently, inline the visit method into the accept method, eliminating the visit method.
4. Remove the DrawVisitor type parameter from all accept methods (and relatives*).
*Relatives mean all methods
that have the same signature (name and parameter types in order) in the same
class/interface hierarchy. If you
rename or alter the arguments to any accept() method, you
have to rename or alter the arguments in ALL of the accept() methods. We use the term relatives to refer the set of methods that should be
updated simultaneously.
5. Rename all accept relatives to ¡°draw¡±, the original method name.
6. Delete the visitor class and you¡¯re done!
