Collection objects have three built-in methods and one built-in property:
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The Count property, which returns the number of items in the collection
The Add method, which adds a new item to the collection and
returns a pointer to the new item
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The Remove method, which removes an item, given its index or
unique key
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The Item method, which returns a pointer to an existing item,
given its index or unique key
As mentioned in the preceding section, you don't call these features directly from outside the
dependent collection class. Instead, you need to write Public wrapper functions that, of course,
become methods of the dependent collection class. These methods then mediate between controller
calls to the collection and the collection itself.
The following sections list the wrapper methods you must write for each of these features.
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The Collection's Count Property and Class
Wrapper Method
The Collection's Add Method and Class Wrapper
Method
-
The Collection's Delete Method and Class
Wrapper Method
-
The Collection's Item Method and Class Wrapper
Method
-
Initializing the Collection in the Parent Class
The Collection's Count Property and Class Wrapper Method
You need to write a Count method in the dependent collection class to wrap the collection's
Count property. This method just calls the Private Collection object's Count property and returns
its value, as in Listing 12.7.
LISTING 12.7
A WRAPPER METHOD FOR THE COLLECTION'S COUNT PROPERTY
Public Function
Count() As Long
Count = colFiles.Count
End Function
The Collection's Add Method and Class Wrapper Method
You need to write an Add method in the dependent collection class to wrap the collection's
Add method. You will implement this as a Function returning an object variable pointing to the newly
added item, as in Listing 12.8. Notice that you pass the wrapper Add method any properties that
you want to assign to the new object you will create. You initialize a new object variable whose
type is an object of the dependent class. You then assign the properties that were passed into
the wrapper and call the built-in Add method of the Collection object to add the newly created
object to the Collection and give it a key value. Finally, you set the return value of your wrapper
method to point to the newly added item.
LISTING 12.8
A Class ADD METHOD TO WRAP THE COLLECTION'S ADD METHOD
Public Function Add (ByVal Name
As String, _
ByVal Path As String)
_
As File
'Initialize new object variable
Dim filNew As New File
'assign parameters to its
'properties
FilNew.Name = Name
FilNew.Path = Path
'Add it to the collection
colFiles.Add filNew, Name
'Let the return value of this
'function point to it
Set Add = filNew
End Function
The Collection's Delete Method and Class Wrapper Method
You need to write a Remove method in the dependent collection class to wrap the collection's
Remove method. You will implement this as a Sub procedure that takes an argument indicating the
index in the collection of the object to be deleted, as in Listing 12.9. Notice that the parameter
is declared as a Variant rather than as an Integer or a Long as you might expect for an Index value.
The reason for this is that controlling code can use either a numeric index or a String-type key
value to look up an item in the collection.
LISTING 12.9
A WRAPPER FUNCTION FOR THE COLLECTION'S REMOVE METHOD
Public Sub Remove (ByVal Index As Variant)
colFiles.Remove Index
End Sub
The Collection's Item Method and Class Wrapper Method
The Item method you will write wraps around the collection's builtin Item method, as
you can see in Listing 12.10. The method will return a pointer to a particular item in the collection.
To do so, it must take an Index parameter, which can be either an Integer array index of a unique
key of type String. As with the Remove method, this parameter must be declared as a Variant to
accommodate both these possibilities.
LISTING 12.10
AN ITEM WRAPPER METHOD FOR THE COLLECTION'S BUILT- IN ITEM METHOD
Public Function Item _
(ByVal Index As Variant) _
As Variant
Set Item = colFiles.Item(Index)
End Function
Initializing the Collection in the Parent Class
The Parent class shows us just the tip of the collection's iceberg. It doesn't have
to do much to implement this custom object class collection, but it does need to refer to the collection,
because it provides the gateway to the collection by being the only externally creatable class.
You need to put a statement in the Parent class's General Declarations that initializes a pointer
to the dependent collection class. If the dependent collection class were named Files, for example,
you would put a line in the Parent class's General Declarations that would read as follows:
Public Files As New Files
