VBScript is a very popular automation language.
Microsoft VBScript Engine, which is available for free, is one of the reasons, another - on the surface VBScript is extremely simple. But I admit that switching to VBScript from C was hard at times. It required unlearning approach.
And yet VBScript is powerful enough so that a seasoned programmer used to Object-Oriented paradigm can use the same approach with a little bit of creativity.
Class Constructor
Each VBScript class has a constructor which is
automatically called when a new object instance is created.
[sourcecode language="vb"]
Private Sub Class_Initialize()
' put your custom initialization code
End Sub
[/sourcecode]
Class Destructor
Each VBScript class has also a destructor which is
automatically called when life span of the object comes to an end.
[sourcecode language="vb"]
Private Sub Class_Terminate()
' put your custom initialization code
End Sub
[/sourcecode]
Virtual Properties
Special methods
Property Get,
Property Let, and
Property Set allow using object properties that do not actually exist.
[sourcecode language="vb" highlight="39, 43"]
Class Point2D
'' Properties
Private p_X
Private p_Y
'----------------------
'' Methods
'Constructor - called automatically
Private Sub Class_Initialize()
X = 0
Y = 0
End Sub
'----------------------
'Destructor - called automatically
Private Sub Class_Terminate()
End Sub
'----------------------
'A pair of methods to access private property X
Property Get X
X = p_X
End Property
Property Let X(ByVal in_X)
p_X = in_X
End Property
'----------------------
'A pair of methods to access private property Y
Property Get Y
Y = p_Y
End Property
Property Let Y(ByVal in_Y)
p_Y = in_Y
End Property
'----------------------
'A pair of methods to access virtual properties:
'Point's Polar coordinates
Property Get Polar_R
Polar_R = Sqr(p_X*p_X + p_Y*p_Y)
End Property
Property Get Polar_Phi
Polar_Phi = Atn(p_Y/p_X)
End Property
'----------------------
End Class
[/sourcecode]
Inheritance through Delegation
VBScript does not allow declaring a class through derivation. However, a programmer can declare a property (or multiple properties) and initialize it (them) with reference(s) of ancestor object(s). That will give the desired access to the properties and methods of ancestor objects.
[sourcecode language="vb" highlight="22, 26, 31, 35"]
Class ScreenPoint
'' Properties
'Ancestor Point2D
Private P2D
'Point color
Private Color
'----------------------
'' Methods
'Constructor - called automatically
Private Sub Class_Initialize()
Set P2D = new Point2D
End Sub
'----------------------
'Destructor - called automatically
Private Sub Class_Terminate()
Set P2D = Nothing
End Sub
'----------------------
'A pair of methods to access private property X
Property Get X
X = P2D.X
End Property
Property Let X(ByVal in_X)
P2D.X = in_X
End Property
'----------------------
'A pair of methods to access private property Y
Property Get Y
Y = P2D.Y
End Property
Property Let Y(ByVal in_Y)
P2D.Y = in_Y
End Property
'----------------------
End Class
[/sourcecode]
Dynamic Code and Callbacks
To demonstrate this feature, I first prompt you to read about
Eval function and
ExecuteGlobal statement in VBScript.
[sourcecode language="vb" highlight="17"]
Public Function CTZ()
sGlobalValue = "ExecuteGlobal successful"
End Function
Public sGlobalValue
Public Function TestExecuteGlobal()
Dim sCallName, sLocalValue
Dim intRC, boolRC
sGlobalValue = "Testing ExecuteGlobal"
sCallName = "CTZ()"
boolRC = True
On Error Resume Next
ExecuteGlobal sCallName
intRC = Err.Number
On Error GoTo 0
If intRC <> 0 Then boolRC = False
If Not boolRC Then
sGlobalValue = "ExecuteGlobal failed"
End If
MessageBox(sGlobalValue)
End Function
[/sourcecode]
Thus, you can add functions and any definitions (including class definitions) dynamically, during run-time - which allows referring undefined methods and objects!
There are also other magic tricks, as overloading and access to external class libraries. Hopefully this post has been a useful introduction to ignite your interest to automation tricks with VBScript.
