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
刚开始学习设计模式在.NET中的应用，希望对大家在学习过程中有所帮助！
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