Professional Excel Development [Electronic resources] : The Definitive Guide to Developing Applications Using Microsoft® Excel and VBA® نسخه متنی

Implementing a Custom Interface

After we've defined our interface, we have to add it to all the classes that we want to use it with. We do this by adding the Implements keyword at the top of the class module:

Implements IsortableObject

Figure 11-1 shows that as soon as we add that line to the class, the interface name appears in the object dropdown at the top-left of the code pane, just like an object on a userform.

Figure 11-1. The Interface Appears in the Object Drop-Down

When the interface name is selected in that drop-down, the right-hand drop-down lists the methods and properties defined for that interface, as shown in Figure 11-2.

Figure 11-2. With the Interface Selected, the Properties and Methods Appear in the Right-Hand Drop-Down

Clicking one of the methods adds an outline procedure to the code module, just as it does for a userform object's event. We just need to add code to that routine to return the value to use when sorting this type of object. The complete, sortable CAuthor class is shown in Listing 11-7, where the code to implement the ISortableObject interface has been highlighted.

Listing 11-7. The Sortable CAuthor Class

'Name:          CAuthor
'Description:   Class to represent a book's author
'Allow this type of object to be sortable
'by the generic routine
Implements ISortableObject
Dim msAuthName As String
Public Property Let AuthorName(sNew As String)
msAuthName = sNew
End Property
Public Property Get AuthorName() As String
AuthorName = msAuthName
End Property
'Return the value to be used when sorting this object
Private Property Get ISortableObject_SortKey() As Variant
ISortableObject_SortKey = AuthorName
End Property

Note that the name of the ISortableObject_SortKey routine is the concatenation of the interface name and the property name and that it is a Private property of the CAuthor class, so won't appear on the CAuthor interface.