Prime Programming Proficiency, Part 3: Lines-of-code Counter, Page 2

Implementing the IVisitor Interface

The Visitor pattern can be implemented using abstract classes or interfaces. I chose interfaces. The basic idea is that you define an interface with methods named Visit. For each kind of thing you want to Visit, you define an overloaded Visit method. For example, I want to the Visit SolutionElement, ItemElement, and ProjectElement types; hence, I declare a Visit method for each of these types in IVisitor. In addition, the Visitor is visiting each of these types and grabbing information from them. It will be up to the Visitor to store that information. In our example, we store project, item, and line counts. Therefore, our visitors also will need to collect and make accessible this information.

Listing 3 shows the definition of IVisitor and the implementation of an IVisitor, the Visitor class.

Listing 3: The IVisitor interface and Visitor class.

' IVisitor.vb
Imports EnvDTE
Imports System.Diagnostics

Public Interface IVisitor
   Sub Visit(ByVal element As SolutionElement)
   Sub Visit(ByVal element As ItemElement)
   Sub Visit(ByVal element As ProjectElement)
   Property ProjectCount() As Long
   Property ItemCount()    As Long
   Property LineCount()    As Long
End Interface


' Visitor.vb
Imports EnvDTE
Imports System.Diagnostics

Public Class Visitor
   Implements IVisitor

   Private FProjectCount As Long = 0
   Private FItemCount    As Long = 0
   Private FLineCount    As Long = 0

   Public Property ProjectCount() As Long Implements _
          IVisitor.ProjectCount
      Get
         Return FProjectCount
      End Get
      Set(ByVal Value As Long)
         FProjectCount = Value
      End Set
   End Property

   Public Property ItemCount() As Long Implements _
          IVisitor.ItemCount
      Get
         Return FItemCount
      End Get
      Set(ByVal Value As Long)
         FItemCount = Value
      End Set
   End Property

   Public Property LineCount() As Long Implements _
          IVisitor.LineCount
      Get
         Return FLineCount
      End Get
      Set(ByVal Value As Long)
         FLineCount = Value
      End Set
   End Property

   Public Sub VisitProject(ByVal element As ProjectElement) _
          Implements IVisitor.Visit
      element.Iterate()
   End Sub

   Public Sub VisitProjectItem(ByVal element As ItemElement) _
          Implements IVisitor.Visit
      element.Iterate()
   End Sub

   Public Sub VisitProjects(ByVal element As SolutionElement) _
          Implements IVisitor.Visit
      element.Iterate()
   End Sub
End Class

I hope you appreciate just how simple the code is. Individual pieces of code should be pretty simple. This supports the idea of divid et imperum, or divide and conquer. To divide and conquer a problem means to subdivide it into smaller, easier-to-implement pieces.

As you will see shortly, each of the prefixSolution classes implement an Iterate method that supports traversing that node's solution and project directory-based file system. For example, a ProjectItem might be a folder that contains other folders or files that require traversing that ProjectItem's elements.