Objects, Properties, and Callbacks in VBA, Lecture notes of Compiler Design

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CSCI-GA.3033.

Scripting Languages

Objects in VBA

Properties, Call-backs

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Outline

• Classes and properties

• Visual programming and callbacks

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Documentation of OO in VBA

• MSDN library →Development tools and languages →Visual Studio 6. →Visual Basic 6. →Product documentation →Using Visual Basic →Programmer’s guide →Part 2: What can you do with Visual Basic →Programming with objects Reference

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Defining Properties

• Syntax
  • Property Get id ([ arg *]) … End Property
  • Property Let id ([ arg *,] arg ) … End Property
  • Property Set id ([ arg *,] arg ) … End Property
• Remarks
  • To return value from Get, assign to id
  • Let assigns regular value, Set assigns object
  • Extra arguments, if any, are for indexing
  • Each routine can be Private or Public
  • Public variable is just shortcut for routines

VBA

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Subtyping Example

VBA

Dim someFruit As Fruit If … Then Set someFruit = New Apple Else Set someFruit = New Banana End If ' compiler knows that the method exists, ' even if it doesn’t know which version ' will get called someFruit.prepare("slice")

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Inheritance in VBA

VBA

Public weight As Double Public Function pluck() As String 'empty routine End Function Public Sub prepare(how As String) 'empty routine End Sub Implements Fruit Public color As String Private wght As Double Private Property Let Fruit_weight(ByVal RHS As Double) wght = RHS End Property Private Property Get Fruit_weight() As Double Fruit_weight = wght End Property Private Function Fruit_pluck() As String Fruit_pluck = color & " apple" End Function Private Sub Fruit_prepare(how As String) Debug.Print how & "d " & Fruit_pluck() End Sub Class module “Fruit” (Interface) Class module “Apple” (subclass) Would be abstract in other languages “ Implements ” in subclass identifies superclass Override private with mangled name

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Missing OO Features in VBA

• Implementation inheritance

• Constructors

• Can write Class_Initialize() method
• Or write subroutine in separate module to

allocate new instance and initialize it

• Static fields and static methods

⇒ OO features changed in VB.NET

Soap-box

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Outline

• Classes and properties

• Visual programming and callbacks

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Hungarian Notation

• Convention: start

identifier with

indication of type

Concepts chk Check box cmd Command button frm User form fra Frame lst List box cmb Combo box mnu Menu opt Option button lbl Label txt Text box

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Dialog Example

VBA

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Writing Event Handlers

Sub paintLemonStar(nLines As Integer) Dim n As Integer, i As Integer n = nLines - 1 For i = 0 To n Dim l As Shape Set l = ActiveWindow.Selection.SlideRange.Shapes.AddLine( _ BeginX:=300, BeginY:=200 + i * 100 / n, _ EndX :=400, EndY :=300 - i * 100 / n) l.Line.ForeColor.RGB = RGB(i * 255 / n, i * 255 / n, 0) Next i End Sub Private Sub cmdPaint_Click() paintLemonStar CInt(txtNumberOfLines.Text) Hide End Sub Private Sub cmdCancel_Click() End End Sub

VBA

Retrieve user input Callbacks with mangled names Auxiliary subroutine

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Code Sheets

• Right-click→View code
  • Just like other modules
  • Contains event handlers as well as regular subroutines
• Double-click on control
  • Goes to handler, creating it if necessary
  • Caution: handler name not changed automatically with control name!
• Dynamic dialog: assign properties at runtime
  • E.g., Visible=True reveals hidden controls
  • UserForm_Initialize sets default, e.g. for list box

VBA

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Common Controls and Events

• Controls: Label, TextBox, ComboBox, ListBox, CheckBox, OptionButton, ToggleButton, Frame, CommandButton, TabStrip, MultiPage, ScrollBar, SpinButton, Image
• Events: Change, Click, DblClick, Enter, Exit, Initialize, KeyPress, SpinDown, and many more
• In editor for user form code sheet:
  • Left drop-down list: control objects on this form
  • Right drop-down list: events on that object Reference

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Callback Mechanisms

VBA form Subroutine in form with mangled name VBA class WithEvent / RaiseEvent statements Java Pass object on which to call method Perl, Python, JavaScript Pass anonymous function (lambda) C, C++ Pass function pointer C++ Pass object on which to call “()” operator SmallTalk Pass code block PHP Pass name of function as string Concepts