设计工厂方法

THE FACTORY METHOD

我们已经学会了简单工厂模式(Simple Factory Pattern)，工厂（factory）思想贯穿于整个面向对象编程（OOP）以及其他一些设计模式的始终。如：生成器(Builder)模式。其间，一个单一的类扮演类似交通警察的角色，决定哪一个单一层次上的子类将被实例化。

工厂方法模式（Factory Method pattern）是对工厂（factory）思想进行了巧妙的延伸，它使得将超类的实例化延迟到它的每一个子类。这个模式没有具体的指出延迟到哪一个子类，而是定义一个抽象类创建对象，让其子类决定创建哪一个对象。

下面是一个简单的例子，在一个游泳比赛中如何确定游泳运动员的泳道。按照运动员的成绩排列决赛事的分组，速度越快所分的小组的决赛的次序越靠后，反之，速度越慢就越先比赛，而且在每一个小组中成绩越好、速度越快的选手也就越靠近中间的泳道。这被称为straight seeding。

当游泳运动员在锦标赛比赛过程中，他们通常要游两次。 通过在预赛中相互竞争，前12名或者16名游泳运动员将继续在决赛中继续彼此竞争。 为了预赛工作更公平， 采用circle seeded，这样使得速度最快的3名选手分别处于最迅速3个小组的中心泳道。在剩下的选手中再选出速度最好的3名选手，等等。

我们要实现这个选拔模式并且使用工厂方法。

首先，设计抽象事件类：

Public Class Events

Protected numLanes As Integer

Protected swmmers As Swimmers

'-----

Public Sub New(ByVal Filename As String, ByVal lanes As Integer)

MyBase.New()

Dim s As String

Dim sw As Swimmer

Dim fl As vbFile

fl = New vbFile(Filename) '打开一个文本文件

fl.OpenForRead()

numLanes = lanes '保存泳道数量信息

swmmers = New Swimmers

'读取游泳选手信息

s = fl.readLine

While Not fl.fEOF

sw = New Swimmer(s) '建立对象

swmmers.Add(sw) 'add to list

s = fl.readLine

End While

fl.closeFile()

End Sub

'-----

Public Function getSwimmers() As ArrayList

getSwimmers = swmmers

End Function

'-----

Public Overridable Function isPrelim() As Boolean

End Function

'-----

Public Overridable Function isFinal() As Boolean

End Function

'-----

Public Overridable Function isTimedFinal() As Boolean

End Function

'-----

Public Overridable Function getSeeding() As Seeding

End Function



End Class



因为所有的派生类都要从文本文件读取数据，所以，我们把Events类作为基类。其中所定义的方法均为虚方法，可以通过继承Events类来实现具体的类（PrelimEvent类、TimedFinalEvent类），这两个类之间唯一的不同就是返回的选拔的类别不同。我们也定义了一个包含以下方法的抽象选拔类：



Public MustInherit Class Seeding

Protected numLanes As Integer

Protected laneOrder As ArrayList

Protected numHeats As Integer

Private asw() As Swimmer

Protected sw As Swimmers

'-----

Public Function getSeeding() As Swimmers

getSeeding = sw

End Function

'-----

Public Function getHeat() As Integer

End Function

'-----

Public Function getCount() As Integer

getCount = sw.Count

End Function

'-----

Public MustOverride Sub seed()

'-----

Public Function getSwimmers() As ArrayList

getSwimmers = sw

End Function

'-----

Public Function getHeats() As Integer

Return numHeats

End Function

'-----

Public Function odd(ByVal n As Integer) As Boolean

odd = (n \ 2) * 2 <> n

End Function

'-----

Public Function calcLaneOrder(ByVal lns As Integer) As ArrayList

numLanes = lns

Dim lanes(numLanes) As Integer

Dim i As Integer

Dim mid, incr, ln As Integer

mid = (numLanes \ 2)

If (odd(numLanes)) Then

mid = mid + 1

End If

incr = 1

ln = mid

For i = 0 To numLanes - 1

lanes(i) = ln

ln = mid + incr

incr = -incr

If (incr > 0) Then

incr = incr + 1

End If

Next i



laneOrder = New ArrayList

For i = 0 To numLanes - 1

laneOrder.Add(lanes(i))

Next i

calcLaneOrder = laneOrder

End Function



Public Sub New(ByVal swmrs As Swimmers, ByVal lanes As Integer)

MyBase.New()

sw = swmrs

numLanes = lanes

End Sub

'-------------------

Public Function sort(ByVal sw As Swimmers) As Swimmers



Dim i, j, max As Integer

Dim tmp As Swimmer



Try

max = sw.Count

Dim asw(max) As Swimmer



For i = 0 To max - 1



asw(i) = CType(sw.Item(i), Swimmer)

Next i

For i = 0 To max - 1

For j = i To max - 1

If asw(i).getTime > asw(j).getTime Then

tmp = asw(i)

asw(j) = asw(i)

asw(i) = tmp

End If

Next j

Next i



sw = New Swimmers

For i = 0 To max - 1

sw.Add(asw(i))

Next i



sort = sw

Catch e As Exception

Console.WriteLine("Caught " + i.ToString + " " + j.ToString + " " + max.ToString + " " + e.ToString())

Console.WriteLine(e.StackTrace)

End Try

End Function

End Class