In order to use collections to manage class objects, you must do the
following:
- Create an instance of the class
- Set the properties and methods of the class
- Add the class to a public collection
- Unload the instance of the class
You might expect that unloading the instance of the class results in the
class being closed and terminated. However, the class object persists
because you add it to a collection, which then owns the reference to the
class. This is a very powerful technique that allows you to control object
references through a collection; the class object does not terminate until
you remove it from the collection.
The following example creates a class object and a form object, and then
manages both objects from a collection in a standard module.
Create a Class Module
- Create a new database called ClassTest.mdb.
- On the Insert menu, click Class Module.
- Save the class module as clsTest.
- Type the following lines in the Declarations section:
Private This_ClassID As String
Private This_frm As New Form_frmTest
- Select Class in the Object box of the Module window. "Initialize" is
automatically selected in the Procedure box.
- Type the following procedure:
Private Sub Class_Initialize()
On Local Error GoTo Class_Initialize_Err
Dim Msg As String
This_frm.Visible = True
This_ClassID = "Initialized"
This_frm.Caption = This_ClassID
MsgBox "Class Initialized", vbInformation, "Class Example"
Class_Initialize_End:
Exit Sub
Class_Initialize_Err:
Msg = "Error #: " & Format$(Err.Number) & vbCrLf
Msg = Msg & Err.Description
Err.Raise vbObjectError, "clsTest.Initialize (Private)", Msg
Resume Class_Initialize_End
End Sub
- On the Insert menu, click Procedure.
- In the Insert Procedure dialog box, type ClassID in the Name box and
click Property in the Type box. Then type the following procedures
Public Property Get ClassID() As Variant
ClassID = This_ClassID
End Property
Public Property Let ClassID(ByVal vNewValue As Variant)
This_ClassID = vNewValue
This_frm.ClassID = This_ClassID
This_frm.Caption = This_ClassID
End Property
- Save and close the clsTest class module.
Create a Form
- Create the following form not based on any table or query in Design
view:
Form: frmTest
-----------------
Caption: TestForm
- With the form still open in Design view, click Code on the View menu.
- Type the following line in the Declarations section of the form's class
module:
Dim This_ClassID As String
- Add the following event procedure to the form's Unload property:
Private Sub Form_Unload(Cancel As Integer)
col.Remove This_ClassID
End Sub
- On the Insert menu, click Procedure.
- In the Insert Procedure dialog box, type ClassID in the Name box and
click Property in the Type box. Then type the following procedures. Note
that the Get ClassID() function and vNewValue variable in this example
are dimensioned as String instead of the default, which is Variant:
Public Property Get ClassID() As String
ClassID = This_ClassID
End Property
Public Property Let ClassID(ByVal vNewValue As String)
This_ClassID = vNewValue
End Property
- Save and close the frmTest form.
Create a Standard Module
- Create a new standard module and save it as Module1.
- Type the following line in the Declarations section:
Public col As New Collection
- Type the following procedure:
Function CreateClassTest() As String
' Create an instance of the clsTest class module, which creates
' an instance of the frmTest form.
Dim cls As New clsTest
' Create a unique identifier string and set it to the upper index
' of the Public col Collection plus one.
Dim varClassId As String
varClassId = "Key_" & CStr(col.Count + 1)
' Set the clsTest class module's ClassID property to the value of
' varClassId, which in turn sets the frmTest.ClassId property to
' the same value. This is so the form has a method to track its
' relationship to the collection.
cls.ClassID = varClassId
' Add the instance of the class object to the collection passing
' varClassId as the Key argument.
col.Add cls, varClassId
MsgBox "Created New Collection Item: " & varClassId, _
vbInformation, "Class Example"
' Unload the cls object variable.
Set cls = Nothing
' Return the varClassId.
CreateClassTest = varClassId
End Function
- Close and save the module.
Test the Example
When you call the CreateClassTest() function multiple times, it opens
multiple instances of the frmTest form, each of which is unique and capable
of managing itself and its participation in the public collection. Each
form is aware of its Key position in the collection, and each one removes
itself from the collection when you close the form.
The following sample procedure creates three instances of the clsTest
class:
- Create a standard module and type the following procedure:
Function CreateThreeItems() As Boolean
Dim strKeys(1 To 3) As String
Dim i As Integer
For i = LBound(strKeys) To UBound(strKeys)
strKeys(i) = CreateClassTest()
Next i
For i = LBound(strKeys) To UBound(strKeys)
MsgBox col.Item(strKeys(i)).ClassID, vbInformation, _
"Class Test"
Next i
End Function
- To test this function, type the following line in the Immediate window,
and then press ENTER:
Note that messages boxes are displayed each time the clsTest class
module initializes, when each of three instances of the frmTest form is
created, and again after all three instances of the form are open.