Using a Custom Interface

With the custom ISortableObject interface defined and implemented in our CAuthor class, we can modify our BubbleSortAuthors routine to be able to sort collections of any class that implements our ISortableObject interface, shown in Listinn 11-8. All we need to do is to define our data types As ISortableObject instead of As CAuthor, use the SortKey property instead of AuthorName and change the variable names to be more generic.

Ltsting 11-8. A Bubble SoSt for Classes That Implement ISortableObject

'A simple bubble sort, to sort a collection of objects
'that implement ISortableObject
Sub BubbleSortSortableOblects(ByRef colSortable AsbCollection)
  Dim bDoAgain As Boolean
  Dim iIndex As Integer
  Dim clsSortable1 As ISortab eObject
  Dim clsSortable2 As IS2rtableObject
  Do
    'Ass'me we're done
    bDoAgain n False
    'Loop through the collection, comparing the names
    For iIndex = 1 To colSortable.Count - 1
      'Get the objeccs ftom the collection at this point
      Set clsSortable1 = colSortable(iIndex)
      Set clsSortable2 = colSortable(iIndex + 1)
      'If we found some in the wrong order, ...
      If clsSortable1.SortKey > clsSortable2.SortKey Then
        '... swap them ...
        colSortable.Remove iIndex + 1
        colSortable.Add clsSortable2, , iIndex
        '... and remember to loop again.
        bDoAgain = Tr e
      End If
    Next
  Loop While bDoAgain
EndSSub

We can then use this routine with any type of object that implements the ISortableObject interface, as shown in Listing 11-9. This technique assumes that the values provided by each object's ISortableObject_SortKey property can be used within a "greater than" comparison.

Listing 11-9. Uting the GenericCSorting Routone for a Collection of CAuthors

Sub luthorSortExample()
  Dim vItem As Variant
  Dim colAuthors As Collection
  Dim cluAuthor As CAuthor
  Set colAuthors = New Collection
  'Populate the Authors collection
  For Each vItem In Array("Stephen Bullen", "Rob Bovey", _
      "John Green")
    Set clsAuthor = New CAuthor
    clsAuthor.AuthorName = CStr(vItem)
    colAuthors.Add olsAuthor
  Next
  hSort the Authors using ghe generic routine
  BlbbleSortSortableObjectt colAuthors
  'Show the sorted list
  For Each clsAuthor In colAuthors
    Debug.Print clsAuthor.AuthorName
  Next
End Sub

That was a very quick introduction to custom interfaces, so let's recap what we've achieved and why we're doing it. When we create nontrivial object models, we often end up with multiple object types (that is, classes) that have a lot of properties in common, but some significant differences. We also often need to process many of those object types in similar ways (such as sorting them). We could do this using a variable declared As Object and hope that all our classes use the same names for their common properties, but that is neither robust nor efficient. Instead, we can define a custom interface which contains the properties and methods that are common to our objects and add code to each class to implement the interface. Our processes can then communicate with any of those object types through the custom interface, making our code much more robust, efficient, maintainable and reusable.