如果读者想在本篇文章中看到观察者模式的巧妙应用，大概有些不可能了。因为这里我只是想把自己的一种思路展现给大家，然后和大家探讨。

背景：

       在敲机房收费系统的过程中，都是别人说可能会用到什么模式，然后自己针对这些模式进行思考，然后去模仿书中的例子去用。

       这不，师姐说上下机这里能用观察者模式，然后我就琢磨了一番，只是收获不是很丰硕，貌似也和师姐讲课中的设想不一样。

       我在琢磨使用观察者模式时，已经把系统中和上下机有关的功能实现了。只是在写代码的过程中发现，每次下机，都需要先把正在上机的卡的上机信息查出来（上机时间），然后再更新上下机记录。而且，咱们的系统中有三个地方可以实现下机：主窗体上的下机，选中学生下机，全部学生下机。它们中都有重复的过程。

想法：

        每次单击上机按钮时，就实例化一个Observer类，把该卡的上机信息（卡号，卡类型（固定/临时），上机时间）保存到该对象中，然后在另一个Subject类中有一个泛型集合，只要上机成功，就在List中添加一个Observer类，这样Subject中保存所有的上机的卡对象。

       点击下机按钮时或选中学生下机时，就更改Subject的状态，并调用Subject的Notify方法，通知要下机的观察者更新。       

类图：

代码：

• 主题Subject

Public Class OffLineSubject
    '添加一个观察者的泛型集合
    Private onlineObservers As New List(Of OnLineObserver)

    Private Shared offlineSubject As OffLineSubject
    Private Sub New()

    End Sub
    ''' <summary>
    ''' 单例模式的应用，保证程序中只有一个OffLineSubject的实例
    ''' </summary>
    ''' <returns></returns>
    ''' <remarks></remarks>
    Public Shared Function GetSubjectInstance() As OffLineSubject
        If offlineSubject Is Nothing Then
            offlineSubject = New OffLineSubject
        End If
        Return offlineSubject
    End Function


    '下机日期
    Private _offDate As Date
    Public Property OffDate() As Date
        Get
            Return _offDate
        End Get
        Set(ByVal value As Date)
            _offDate = value
        End Set
    End Property
    '下机时间
    Private _offTime As TimeSpan
    Public Property OffTime() As TimeSpan
        Get
            Return _offTime
        End Get
        Set(ByVal value As TimeSpan)
            _offTime = value
        End Set
    End Property
    '下机方式
    Private _offWay As String
    Public Property OffWay() As String
        Get
            Return _offWay
        End Get
        Set(ByVal value As String)
            _offWay = value
        End Set
    End Property
    '增加上机观察者
    Sub Attach(ByVal onlineObserver As OnLineObserver)
        onlineObservers.Add(onlineObserver)

    End Sub
    '减少上机观察者
    Sub Detach(ByVal onlineObserver As OnLineObserver)
        onlineObservers.Remove(onlineObserver)

    End Sub
    '通知上机观察者，根据上机的卡的卡号来确定具体通知那个上机观察者
    Sub Notify(ByVal key As String)
        Dim i As Integer
        For i = 0 To onlineObservers.Count - 1
            If onlineObservers.Item(i).CardID = key Then
                onlineObservers.Item(i).Update()
            End If
        Next
    End Sub

End Class

• 观察者Observer

Public Class OnLineObserver


    '添加一个通知者的引用
    Private offSubject As OffLineSubject

    Sub New(ByVal offSubject As OffLineSubject)
        Me.offSubject = offSubject
    End Sub
    Private _cardID As String
    Public Property CardID() As String
        Get
            Return _cardID
        End Get
        Set(ByVal value As String)
            _cardID = value
        End Set
    End Property

    Private _cardType As String
    Public Property CardType() As String
        Get
            Return _cardType
        End Get
        Set(ByVal value As String)
            _cardType = value
        End Set
    End Property

    Private _onTime As TimeSpan
    Public Property OnTime() As TimeSpan
        Get
            Return _onTime
        End Get
        Set(ByVal value As TimeSpan)
            _onTime = value
        End Set
    End Property

    Sub Update()
        Dim enOnOffLineRecord As New Entity.OnOffLineRecordEntity
        Dim bllOnOffLineRecord As New BLL.OnOffLineRecordBLL
        '获得通知者的一些属性（下机方式，下机日期和下机时间）
        enOnOffLineRecord.OffWay = offSubject.OffWay
        enOnOffLineRecord.OffDate = offSubject.OffDate
        enOnOffLineRecord.OffTime = offSubject.OffTime
        '调用策略模式计算消费金额,略
        '……
        '……
        '更新上下机记录
        bllOnOffLineRecord.UpdateOnOffLineRecordBLL(enOnOffLineRecord)

    End Sub

End Class

• Function OnLine（Byval enCardInfo as Entity.CardInfoEntity）as Boolean

Dim onObserver As OnLineObserver
        Dim systemDate As DateTime = DateTime.Now
        Dim offSubject As OffLineSubject
        '获取唯一的主题通知者实例
        offSubject = OffLineSubject.GetSubjectInstance
        onObserver = New OnLineObserver(offSubject)
        '将上机信息封装
        onObserver.CardID = enCardInfo.CardID
        onObserver.CardType = enCardInfo.CardType
        onObserver.OnTime = systemDate.TimeOfDay
        '在通知者中添加一个观察者
        offSubject.Attach(onObserver)

• Function OffLine(Byval enCardInfo as Entity.CardInfoEntity) as Boolean

Dim offSubject As OffLineSubject
        offSubject = OffLineSubject.GetSubjectInstance
        offSubject.OffWay = "正常下机"
        'offSubject .OffDate =""
        'offSubject .OffTime =""
        '通知下机
        offSubject.Notify(enCardInfo.CardID)