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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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