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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
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 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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