属性 (Properties)

来源: https://www.freebasic.net/wiki/wikka.php?wakka=ProPgProperties
最后更新: 2021-10-30

属性是成员变量与成员过程的特殊结合体。

它们提供了一种通过普通赋值或成员访问语法来设置或获取对象值的方式，同时也允许对象在需要更新自身时执行相应操作。

基本属性声明和使用 setter 与 getter 属性。

索引属性带附加参数的属性。

基本属性

属性的声明方式类似于成员过程，只是使用 Property 关键字代替 Sub 或 Function。例如，考虑一个窗口系统或 GUI 库的窗口类。

start GeSHi

Type Window
Private:
    As String title_
End Type

Dim As Window w

end GeSHi

为了设置窗口标题，可以添加一个 setter 属性：

start GeSHi

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"

end GeSHi

它非常类似于成员 Sub，它接受一个参数并根据参数将对象更新到新状态。但是，发送此参数的语法是基本赋值，而不是函数调用。通过将新值赋给 title 属性，属性过程将自动以给定的新值被调用，并可以更新窗口以反映更改。对象如何在内部表示属性状态由对象自己决定。

根据设计，属性一次只能被赋一个值，因此属性过程不能有多个参数。

设置窗口标题后，还应该能够检索它。以下是添加 getter 属性的方法：

start GeSHi

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

end GeSHi

getter 非常类似于 Function。它应该返回属性的当前值，并允许在需要时从其他内部值计算当前值。注意 setter 和 getter 使用相同的标识符，表明它们处理同一个属性。

就像方法重载一样，可以指定多个 setter，只要它们具有不同的参数类型：

start GeSHi

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

end GeSHi

与此属性示例对比，以下是不使用属性的类似代码：

start GeSHi

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()

end GeSHi

代码基本相同，只是语法不同。属性专门设计为将 setter/getter 概念与语言中直接赋值和访问类成员变量的正常方式相结合。由程序员决定哪种方式更合适。

以下是一个演示文本用户界面窗口类的示例，允许使用属性设置位置和标题：

start GeSHi

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 属性
        Declare Property title As String
        Declare Property title( new_title As String )

        '' 位置属性
        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

end GeSHi

注意更新窗口位置或标题如何自动导致窗口重新绘制。

索引属性

属性可以有一个称为 index（索引）的附加参数（目前只允许一个附加参数）。索引在属性名后面的括号中指定，就像属性是一个数组（只有一个维度）一样。例如：

start GeSHi

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

end GeSHi

这模拟了一个可以被赋予字符串的整数数组，甚至可以用字符串作为索引。参见 KeyPgProperty 获取另一个示例。

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属性 (Properties)

基本属性

索引属性

属性 (Properties) ​

基本属性 ​

索引属性 ​

属性 (Properties)

基本属性

索引属性