Classes are Objects which allow you to group a set of related functionality into one group.The advantage of using classes instead of just subroutines is that classes create a level of abstraction that allow you to write cleaner code. One key indication that you should switch to classes is if you're constantly adding parameters to your functions and subroutines.
A Class can contain:
· Subs and Functions – generally called Class Methods
Properties – you can Get and Let these (or Set them if they are Objects)
The Get procedure is used to return a value out of the class, and the Let procedure is to put a value into the class. Note that the return data type of the Get property procedure must be the same data type as the (last) parameter to the Let property procedure. Otherwise, you'll get a compiler error.
Events – can be fired
A Constructor and a Destructor – called Class_Initialize and Class_Terminate.They are called automatically when a Class object is instantiated and destroyed, respectively. You don‘t need to call these Methods; they are called automatically by VBA
Public Member Variables – can be accessed from outside the class – generally considered bad Object Oriented Design – use Properties instead
Private Member Variables – can only be accessed inside the class
Sample Code for an Excel VBA Classes:
In the VBA editor, go to Insert > Class Module. In the Properties window (bottom left of the screen by default), change the name of the module to WorkLogItem. Add the following code to the class:
Option Explicit
Private pTaskID As Long
Private pPersonName As String
Private pHoursWorked As Double
Public Property Get TaskID() As Long
TaskID = pTaskID
End Property
Public Property Let TaskID(lTaskID As Long)
pTaskID = lTaskID
End Property
Public Property Get PersonName() As String
PersonName = pPersonName
End Property
Public Property Let PersonName(lPersonName As String)
pPersonName = lPersonName
End Property
Public Property Get HoursWorked() As Double
HoursWorked = pHoursWorked
End Property
Public Property Let HoursWorked(lHoursWorked As Double)
pHoursWorked = lHoursWorked
End Property
Let's keep moving with this example. Instead of storing the objects in array, we'll try using acollection.
Next, add a new class module and call it ProcessWorkLog. Put the following code in there:
Option Explicit
Private pWorkLogItems As Collection
Public Property Get WorkLogItems() As Collection
Set WorkLogItems = pWorkLogItems
End Property
Public Property Set WorkLogItems(lWorkLogItem As Collection)
Set pWorkLogItems = lWorkLogItem
End Property
Function GetHoursWorked(strPersonName As String) As Double
On Error GoTo Handle_Errors
Dim wli As WorkLogItem
Dim doubleTotal As Double
doubleTotal = 0
For Each wli In WorkLogItems
If strPersonName = wli.PersonName Then
doubleTotal = doubleTotal + wli.HoursWorked
End If
Next wli
Exit_Here:
GetHoursWorked = doubleTotal
Exit Function
Handle_Errors:
'You will probably want to catch the error that will '
'occur if WorkLogItems has not been set '
Resume Exit_Here
End Function
The above class is going to be used to "do something" with a colleciton of WorkLogItem. Initially, we just set it up to count the total number of hours worked. Let's test the code we wrote. Create a new Module (not a class module this time; just a "regular" module). Paste the following code in the module:
Option Explicit
Function PopulateArray() As Collection
Dim clnWlis As Collection
Dim wli As WorkLogItem
'Put some data in the collection'
Set clnWlis = New Collection
Set wli = New WorkLogItem
wli.TaskID = 1
wli.PersonName = "Fred"
wli.HoursWorked = 4.5
clnWlis.Add wli
Set wli = New WorkLogItem
wli.TaskID = 2
wli.PersonName = "Sally"
wli.HoursWorked = 3
clnWlis.Add wli
Set wli = New WorkLogItem
wli.TaskID = 3
wli.PersonName = "Fred"
wli.HoursWorked = 2.5
clnWlis.Add wli
Set PopulateArray = clnWlis
End Function
Sub TestGetHoursWorked()
Dim pwl As ProcessWorkLog
Dim arrWli() As WorkLogItem
Set pwl = New ProcessWorkLog
Set pwl.WorkLogItems = PopulateArray()
Debug.Print pwl.GetHoursWorked("Fred")
End Sub
Why is this helpful?
Let's suppose your data changes and you want to add a new method. Suppose your WorkLogItemnow includes a field for HoursOnBreak and you want to add a new method to calculate that.
All you need to do is add a property to WorkLogItem like so:
Private pHoursOnBreak As Double
Public Property Get HoursOnBreak() As Double
HoursOnBreak = pHoursOnBreak
End Property
Public Property Let HoursOnBreak(lHoursOnBreak As Double)
pHoursOnBreak = lHoursOnBreak
End Property
