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VB.Net中文教程(6) 母子对象关系

作者：未知来源：月光软件站 加入时间：2005-2-28　月光软件站

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主题：母子对象关系
副题： Whole-Part关系

?????? 内容 ??????
v 1. 特殊Whole-Part关系
---- 母子对象关系
v 2. 母子对象谁先诞生﹖
v 3. 类别继承与母子对象

1. 特殊Whole-Part关系
---- 母子对象关系

大家已经熟悉父子类别关系﹐也就是「继承」关系了。于此说明另一种常见关系── 母子对象。一般称之为「组合/部分」关系。日常生活中﹐处处可见到这种母子对象。例如﹐客厅内有沙发、桌子、椅子等等。客厅是母对象﹐沙发、桌子、椅子是子对象。再如计算机屏幕上的窗口内有按钮、选择表、对话盒等等。窗口是母对象﹐按钮、选择表是子对象。于此将说明如何建立母子对象关系。有了关系﹐母子就能互相沟通了。母子对象之间﹐如何沟通呢﹖也就是说﹐母对象如何呼叫子对象的程序呢﹖反过来﹐子对象如何呼叫母对象的程序呢﹖欲呼叫对方的程序﹐必先与对方建立关系才行。因之﹐如何建立母子对象关系﹐是顶重要之课题﹗
请先看个例子﹐有两个类别──Room和Desk。若Room代表房间﹐Desk代表房间内的桌子﹐则它们会诞生母子对象﹕

通常﹐您会依房间的大小来决定桌子的大小。因之﹐Desk对象应跟Room对象沟通﹐取得房间的大小﹐才决定自己的大小。若有个Room之参考﹐则Desk对象就能跟Room对象沟通了。于是﹐可设计下述VB程序：

'ex01.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Class Room
Protected rSize As Double
Shared motherObject As Room

    Public Sub New()
        motherObject = Me
    End Sub
    Shared Function GetMother() As Room
        GetMother = motherObject
    End Function
    Public Function GetSize() As Double
        GetSize = rSize
    End Function
End Class

Class Desk
    Protected dSize As Double
    Public Sub New()
        dSize = Room.GetMother().GetSize() * 0.18
    End Sub
    Public Function GetSize() As Double
        GetSize = dSize
    End Function
End Class
'----------------------------------------------------
Class MyRoom
    Inherits Room
    Private rd As Desk

    Public Sub New()
        rSize = 100
        rd = New Desk()
    End Sub
    Public Sub Show()
        MessageBox.Show("Room Size: " + str(rSize))
        MessageBox.Show("Desk Size: " + str(rd.GetSize()))
    End Sub
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
     .....
#End Region
    Protected Sub Form1_Click( ByVal sender As Object,
                            ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        r.Show()
    End Sub
End Class

此程序输出：
Room Size: 100
Desk Size: 18

Desk类别内之指令──
Room.GetMother().GetSize()

就是Desk对象与Room对象之沟通了。MyRoom类别有个rd参考﹐指向子对象﹐如下﹕

母对象经由rd参考来跟子对象沟通。反过来﹐子对象经由Room.GetMother()参考跟母对象沟通。
2. 母子对象谁先诞生﹖

    在VB里﹐有时候子对象比母对象早诞生完成。就像建造房子﹐建到半途﹐改而建造桌子﹐桌子建好了﹐再继续把房子建完成。由于这不太合乎人们的日常生活习惯﹐令人感到困惑﹗在计算机软件上﹐也有类似的冲突。例如﹐在Windows 下﹐母窗口建好了﹐才建立窗内的按钮、选择表等子对象。但VB有时并非如此﹗因之﹐如何化解这冲突﹐是个重要的话题。兹拿上小节的例子来说明吧﹗该例子中﹐其诞生对象之过程为﹕
Step 1: 先诞生母对象。

Step 2: 接着建立子对象。

这过程合乎人们的习惯﹐是美好的。其原因是﹕MyRoom类别在其建构者New()程序里呼叫Desk的New()诞生Desk子对象。假若您写下述程序﹐就出问题了﹗

'ex02.bas
'Some error here!
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Class Room
Protected rSize As Double
Shared motherObject As Room

Class Desk
    Protected dSize As Double
    Public Sub New()
        dSize = Room.GetMother().GetSize() * 0.18
    End Sub
    Public Function GetSize() As Double
        GetSize = dSize
    End Function
End Class
'----------------------------------------------------
Class MyRoom
    Inherits Room
    Private rd As New Desk()

    Public Sub New()
        rSize = 100
    End Sub
    Public Sub Show()
        MessageBox.Show("Room Size: " + str(rSize))
        MessageBox.Show("Desk Size: " + str(rd.GetSize()))
    End Sub
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
     .....
#End Region
    Protected Sub Form1_Click( ByVal sender As Object,
                            ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        r.Show()
    End Sub
End Class

其对象之建造过程为﹕
Step 1: 指令──
Dim r As New MyRoom()
呼叫MyRoom()建构者New()。它再呼叫Room()建构者New()将Room部分建好。

Step 2: 执行指令──
                Private rd As New Desk()
       呼叫Desk建构者New()﹐建好Desk子对象。此时会执行指令──
                dSize = Room.GetMother().GetSize() * 0.18

Step 3: MyRoom建构者New() 继续将母对象建完成。
        此时会执行指令── rSize=100

其中，GetSize()程序会取出rSize 值﹐但那时还未执行rSize=100 指令﹐那来
的rSize 值呢﹖所以出问题了。此程序可能输出如下错误结果﹕
                   Room Size: 100
                   Desk Size: 0

如何解决上述问题呢﹖常见方法是﹕

把会出问题的指令﹐从建构者程序中提出来﹐放到另一程序里。

例如下述程序：

'ex03.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Class Room
    Protected rSize As Double
    Shared motherObject As Room

    Shared Function GetMother() As Room
        GetMother = motherObject
    End Function
    Public Overridable Sub Create()
        motherObject = Me
    End Sub
    Public Function GetSize() As Double
        GetSize = rSize
    End Function
End Class

Class Desk
    Protected dSize As Double
    Public Sub Create()
        dSize = Room.GetMother().GetSize() * 0.18
    End Sub
    Public Function GetSize() As Double
        GetSize = dSize
    End Function
End Class
'----------------------------------------------------
Class MyRoom
    Inherits Room
    Private rd As New Desk()

    Public Sub New()
        Me.Create()
    End Sub
    Public Overrides Sub Create()
        MyBase.Create()
        rSize = 100
        rd.Create()
    End Sub
    Public Sub Show()
        MessageBox.Show("Room Size: " + str(rSize))
        MessageBox.Show("Desk Size: " + str(rd.GetSize()))
    End Sub
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    .......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        r.Show()
    End Sub
End Class

此程序输出：
Room Size: 100
Desk Size: 18

像指令── dSize = Room.GetMother().GetSize() * 0.18﹐已挪到新设的Create()程序里。待母对象完全建好了﹐才会呼叫这Create()程序﹐GetSize() 就能取得正确值了。MyRoom的New()呼叫Create()程序时﹐母子对象皆已建造完成了。Create()内部依人们的习惯来设定对象之值，例如建立母子对象之关系。

如此就不会出问题了。
New()与Create()分离之后，MyRoom类别里的指令：

Class MyRoom
    Inherits Room
    Private rd As New Desk()

    Public Sub New()
        Me.Create()
    End Sub
........

也能写为：
Class MyRoom
    Inherits Room
    Private rd As Desk

    Public Sub New()
        rd = New Desk()
        Me.Create()
    End Sub
........

只要确保Desk类别的指令──
dSize = Room.GetMother().GetSize() * 0.18

是在MyRoom类别的指令──
rSize = 100

之后执行即可了。
上述的子对象是透过Shared 程序来取得母对象的参考值﹐然后才跟母对象沟通。如果不透过Shared程序，也可以采取下述方法：

'ex04.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Class Room
    Protected rSize As Double

    Public Overridable Sub Create()
    End Sub
    Public Function GetSize() As Double
        GetSize = rSize
    End Function
End Class

Class Desk
    Protected dSize As Double
    Protected myMother As Room

    Public Sub Create(ByVal mo As Room)
        myMother = mo
        dSize = myMother.GetSize() * 0.18
    End Sub
    Public Function GetSize() As Double
        GetSize = dSize
    End Function
End Class
'----------------------------------------------------
Class MyRoom
    Inherits Room
    Private rd As Desk

    Public Sub New()
        rd = New Desk()
        Me.Create()
    End Sub
    Public Overrides Sub Create()
        rSize = 100
        rd.Create(Me)
    End Sub
    Public Sub Show()
        MessageBox.Show("Room Size: " + str(rSize))
        MessageBox.Show("Desk Size: " + str(rd.GetSize()))
    End Sub
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    ......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object,
                            ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        r.Show()
    End Sub
End Class

此程序输出：
Room Size: 100
Desk Size: 18

Desk之对象含个参考 myMother ﹐指向其母对象。这项关系是在母子对象皆建好时﹐才由Create()程序所建立的。于是﹐建立出母子对象之关系﹕

综上所述，当MyRoom类别使用如下指令──
Private rd As New Desk()

时，才必须把New()与Create()分离。如果使用如下指令──

Private rd As Desk
Public Sub New()
rd = New Desk()
.....
End Sub

就不必分离了，原因是：New()与Create()的执行顺序是一致的，例如两者可合并如下的VB程序：

'ex05.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Class Room
    Protected rSize As Double

    Public Function GetSize() As Double
        GetSize = rSize
    End Function
End Class

Class Desk
    Protected dSize As Double
    Protected myMother As Room

    Public Sub New(ByVal mo As Room)
        myMother = mo
        dSize = myMother.GetSize() * 0.18
    End Sub
    Public Function GetSize() As Double
        GetSize = dSize
    End Function
End Class
'----------------------------------------------------
Class MyRoom
    Inherits Room
    Private rd As Desk

    Public Sub New()
        rSize = 100
        rd = New Desk(Me)
    End Sub
    Public Sub Show()
        MessageBox.Show("Room Size: " + str(rSize))
        MessageBox.Show("Desk Size: " + str(rd.GetSize()))
    End Sub
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub

#Region " Windows Form Designer generated code "
     ......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object,
                            ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        r.Show()
    End Sub
End Class

此程序输出：
Room Size: 100
Desk Size: 18

3. 特类别继承与母子对象

您已很熟悉父子类别关系了﹐这继承关系的另一面是母子对象关系。例如﹐

'ex06.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Class Room
    Private mother As MyRoom

    Public Sub New(ByVal mo As MyRoom)
        mother = mo
    End Sub
    Public Function GetID() As String
        GetID = mother.yourID()
    End Function
End Class

Class MyRoom
    Inherits Room

    Public Sub New()
        MyBase.New(Me)
    End Sub
    Public Function yourID() As String
        yourID = "VIP888"
    End Function
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    ......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        MessageBox.Show(r.GetID())
    End Sub
End Class

此程序输出﹕
VIP888

MyRoom()建构者New()将Me值给 Room的mother参考。于是mother参考到母对象r。Room类别是MyRoom之父类别﹐但Room之对象却是MyRoom之子对象。

如果将上述Room的指令：

Public Function GetID() As String
GetID = mother.yourID()
End Function

更改为：

Public Function GetID() As String
GetID = Me.yourID() 'Error!!
End Function

就错了。因为Room类别里没有定义yourID()程序。事实上，VB的继承机制里已经有母对象的参考值了，VB的Overridable机制就是基于它而呼叫到子类别（母对象）的程序的。例如上述程序相当于：

'ex07.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Class Room
    Public Function GetID() As String
        GetID = Me.yourID()
    End Function
    Public Overridable Function yourID() As String
    End Function
End Class

Class MyRoom
    Inherits Room

    Public Sub New()
        MyBase.New()
    End Sub
    Public Overrides Function yourID() As String
        yourID = "VIP888"
    End Function
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    .......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        MessageBox.Show(r.GetID())
    End Sub
End Class

此程序输出﹕
VIP888

上述ex07.bas比ex06.bas好的地方是：
ex06.bas程序的Room类别里面用到MyRoom的名称。而ex07.bas程序的Room类别里面并没用到MyRoom的名称，因此Room类别可以先设计，MyRoom类别能后来才设计，这是继承机制的目的之一。不过，如果您一定不想用继承与Overridable概念的话，可使用VB的Interface机制来改善ex06.bas程序，如下：

'ex08.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Interface IMyRoom
Function yourID() As String
End Interface

Class Room
    Private mother As IMyRoom

    Public Sub New(ByVal mo As IMyRoom)
        mother = mo
    End Sub
    Public Function GetID() As String
        GetID = "RoomID: " + mother.yourID()
    End Function
End Class

Class MyRoom
    Implements IMyRoom
    Private base As Room

    Public Sub New()
        base = New Room(Me)
    End Sub
    Public Function yourID() As String Implements IMyRoom.yourID
        yourID = "VIP888"
    End Function
    Public Function GetID() As String
        GetID = base.GetID()
    End Function
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    .......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        MessageBox.Show(r.GetID())
    End Sub
End Class

此程序输出﹕
RoomID: VIP888

一般使用委托(Delegation)来代替继承时，常用的手法。然而上述ex08.bas程序的MyRoom类别里面用到了Room名称，如果您不希望如此，可定义一个IRoom接口，供MyRoom类别使用，如下程序：

'ex09.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'----------------------------------------------------
Interface IMyRoom
Function yourID() As String
End Interface

Interface IRoom
Function GetID() As String
Sub Connect(ByVal m As IMyRoom)
End Interface

Class Room
    Implements IRoom
    Private motherObject As IMyRoom

    Public Function GetID() As String Implements IRoom.GetID
        GetID = motherObject.yourID() + " ***"
    End Function
    Public Sub Connect(ByVal m As IMyRoom) Implements IRoom.Connect
        motherObject = m
    End Sub
End Class

Class MyRoom
    Implements IMyRoom
    Private base As IRoom

    Public Sub Connect(ByVal r As IRoom)
        base = r
        r.Connect(Me)
    End Sub
    Public Function yourID() As String Implements IMyRoom.yourID
        yourID = "Dog888"
    End Function
    Public Function GetID() As String
        GetID = base.GetID()
    End Function
End Class
'----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form

    Public Sub New()
        MyBase.New()
        Form1 = Me
        'This call is required by the Win Form Designer.
        InitializeComponent()
        'TODO: Add any initialization after the InitializeComponent() call
    End Sub
    'Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    .......
#End Region

    Protected Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim my As New MyRoom()
        Dim base As New Room()
        my.Connect(base)
        MessageBox.Show(my.GetID())
    End Sub
End Class

此程序输出﹕
RoomID: VIP888

Room类别里面没用到MyRoom名称，而且MyRoom类别里没有用到Room名，因此两个类别可独立设计。这是分布式组件软件，如MTS（Microsoft transaction server)等系统里的常用手法。n

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