Properties



Properties are a special mix of member variable and member procedure. They provide a way to set or retrieve values of an object, through normal looking assignments or member accesses, but also let the object perform actions if it needs to update itself.

Basic properties
Declaring and using setter and getter properties.
Indexed properties
Properties with an additional parameter.

Basic properties
A property is declared similar to a member procedure, except that the Property keyword is used instead of Sub or Function. For example, let's consider a window class for a windowing system or GUI library.

Type Window
Private:
    As String title_
End Type

Dim As Window w


In order to set the window's title, a setter property can be added:

Type Window
    Declare Property title(ByRef s As String)
Private:
    As String title_
End Type

Property Window.title(ByRef s As String)
    This.title_ = s
End Property

Dim As Window w
w.title = "My Window"


It is very similar to a member Sub, as it takes a parameter and updates the object to the new state based on the parameter. However, the syntax for sending this parameter is a basic assignment, not a function call. By assigning the new value to the title property, the property procedure will automatically be called with the given new value, and can update the window to reflect the change. It is up to the object how to represent the property state internally.

By design, properties can only be assigned one value at a time, and as a result the property procedure can not have more than one parameter.

After setting the window title, it should also be possible to retrieve it. Here is how to add a getter property:

Type Window
    '' setter
    Declare Property title(ByRef s As String)
    '' getter
    Declare Property title() As String
Private:
    As String title_
End Type

'' setter
Property Window.title(ByRef s As String)
    This.title_ = s
End Property

'' getter
Property Window.title() As String
    Return This.title_
End Property

Dim As Window w
w.title = "My Window"
Print w.title


The getter is very similar to a Function. It is supposed to return the current value of the property, and it allows the current value to be calculated from other internal values, if needed. Note that both setter and getter use the same identifier, indicating they handle the same property.

Just like method overloading, it is possible to specify multiple setters, provided they have different parameter types:

Type Window
    Declare Property title(ByRef s As String)
    Declare Property title(ByVal i As Integer)
    Declare Property title() As String
Private:
    As String title_
End Type

Property Window.title(ByRef s As String)
    This.title_ = s
End Property

Property Window.title(ByVal i As Integer)
    This.title_ = "Number: " & i
End Property

Property Window.title() As String
    Return This.title_
End Property

Dim As Window w
w.title = "My Window"
Print w.title
w.title = 5
Print w.title


In comparison to this example of properties, here is similar code that does not use properties:

Type Window
    Declare Sub set_title(ByRef s As String)
    Declare Sub set_title(ByVal i As Integer)
    Declare Function get_title() As String
Private:
    As String title
End Type

Sub Window.set_title(ByRef s As String)
    This.title = s
End Sub

Sub Window.set_title(ByVal i As Integer)
    This.title = "Number: " & i
End Sub

Function Window.get_title() As String
    Return This.title
End Function

Dim As Window w
w.set_title("My Window")
Print w.get_title()
w.set_title(5)
Print w.get_title()


The code is basically the same, only the syntax is different. Properties are specifically designed to combine the setter/getter concept and the language's normal way of literally assigning and accessing values to a class' member variables. It is up to the programmers to decide which way they prefer.

Here is an example demonstrating a text user interface window class allowing to set position and title using properties:

Namespace tui
    Type Point
        Dim As Integer x, y
    End Type

    Type char
        Dim As UByte value
        Dim As UByte Color
    End Type

    Type Window
        '' public
        Declare Constructor _
            ( _
                x As Integer = 1, y As Integer = 1, _
                w As Integer = 20, h As Integer = 5, _
                title As ZString Ptr = 0 _
            )
       
        Declare Destructor

        Declare Sub show

        '' title property
        Declare Property title As String
        Declare Property title( new_title As String )

        '' position properties
        Declare Property x As Integer
        Declare Property x( new_x As Integer )

        Declare Property y As Integer
        Declare Property y( new_y As Integer )

    Private:
        Declare Sub redraw
        Declare Sub remove
        Declare Sub drawtitle

        Dim As String p_title
        Dim As Point Pos
        Dim As Point siz
    End Type

    Constructor Window _
        ( _
            x_ As Integer, y_ As Integer, _
            w_ As Integer, h_ As Integer, _
            title_ As ZString Ptr _
        )

        Pos.x = x_
        Pos.y = y_
        siz.x = w_
        siz.y = h_

        If( title_ = 0 ) Then
            title_ = @"untitled"
        End If

        p_title = *title_
    End Constructor

    Destructor Window
        Color 7, 0
        Cls
    End Destructor

    Property Window.title As String
        title = p_title
    End Property

    Property Window.title( new_title As String )
        p_title = new_title
        drawtitle
    End Property

    Property Window.x As Integer
        Return Pos.x
    End Property

    Property Window.x( new_x As Integer )
        remove
        Pos.x = new_x
        redraw
    End Property

    Property Window.y As Integer
        Property = Pos.y
    End Property

    Property Window.y( new_y As Integer )
        remove
        Pos.y = new_y
        redraw
    End Property

    Sub Window.show
        redraw
    End Sub

    Sub Window.drawtitle
        Locate Pos.y, Pos.x
        Color 15, 1
        Print Space( siz.x );
        Locate Pos.y, Pos.x + (siz.x \ 2) - (Len( p_title ) \ 2)
        Print p_title;
    End Sub

    Sub Window.remove
        Color 0, 0
        Var sp = Space( siz.x )
        For i As Integer = Pos.y To Pos.y + siz.y - 1
            Locate i, Pos.x
            Print sp;
        Next
    End Sub

    Sub Window.redraw
        drawtitle
        Color 8, 7
        Var sp = Space( siz.x )
        For i As Integer = Pos.y + 1 To Pos.y + siz.y - 1
            Locate i, Pos.x
            Print sp;
        Next
    End Sub
End Namespace

Dim win As tui.Window = tui.Window( 3, 5, 50, 15 )

win.show
Sleep 500

win.title = "Window 1"
Sleep 250
win.x = win.x + 10
Sleep 250

win.title = "Window 2"
Sleep 250
win.y = win.y - 2
Sleep 250

Locate 25, 1
Color 7, 0
Print "Press any key...";

Sleep


Note how updating the window's position or title automatically causes the window to be redrawn.

Indexed properties
Properties can have an additional parameter that is called an index (currently only one additional parameter is allowed). The index is specified in parentheses behind the property's name, as if the property was an array (with only one dimension). For example:

Type IntArray
    '' setters
    Declare Property value(index As Integer, v As Integer)
    Declare Property value(index As String, v As Integer)
    Declare Property value(index As Integer, v As String)
    Declare Property value(index As String, v As String)

    '' getters
    Declare Property value(index As Integer) As Integer
    Declare Property value(index As String) As Integer

Private:
    Dim As Integer data_(0 To 9)
End Type

Property IntArray.value(index As Integer) As Integer
    Return This.data_(index)
End Property

Property IntArray.value(index As String) As Integer
    Return This.data_(CInt(index))
End Property

Property IntArray.value(index As Integer, v As Integer)
    This.data_(index) = v
End Property

Property IntArray.value(index As String, v As Integer)
    This.data_(CInt(index)) = v
End Property

Property IntArray.value(index As Integer, v As String)
    This.data_(index) = CInt(v)
End Property

Property IntArray.value(index As String, v As String)
    This.data_(CInt(index)) = CInt(v)
End Property

Dim a As IntArray

a.value(0) = 1234
a.value("1") = 5678
a.value(2) = "-1234"
a.value("3") = "-5678"

Print a.value(0)
Print a.value("1")
Print a.value(2)
Print a.value("3")

Sleep


This simulates an integer array that can be assigned strings, and even be indexed with strings. See KeyPgProperty for another example.

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