Iterator in VB.NET

   1:      ' Iterator Design Pattern.

   2:      ' See description in //www.vb-net.com/ProgramTheory/Iterator.htm

   3:      Class MainApp

   4:          ' Entry point into console application.

   5:          Public Shared Sub Main()

   6:              Dim a As New ConcreteAggregate()

   7:              a(0) = "Item A"

   8:              a(1) = "Item B"

   9:              a(2) = "Item C"

  10:              a(3) = "Item D"

  11:              ' Create Iterator and provide aggregate

  12:              Dim i As Iterator = a.CreateIterator()

  13:              Console.WriteLine("Iterating over collection:")

  14:              Dim item As Object = i.First()

  15:              While item IsNot Nothing

  16:                  Console.WriteLine(item)

  17:                  item = i.[Next]()

  18:              End While

  19:              ' Wait for user

  20:              Console.ReadKey()

  21:          End Sub

  22:      End Class

  23:   

  24:      ' The 'Aggregate' abstract class

  25:      MustInherit Class Aggregate

  26:          Public MustOverride Function CreateIterator() As Iterator

  27:      End Class

  28:   

  29:      ' The 'ConcreteAggregate' class

  30:      Class ConcreteAggregate

  31:          Inherits Aggregate

  32:          Private _items As New ArrayList()

  33:          Public Overrides Function CreateIterator() As Iterator

  34:              Return New ConcreteIterator(Me)

  35:          End Function

  36:          ' Gets item count

  37:          Public ReadOnly Property Count() As Integer

  38:              Get

  39:                  Return _items.Count

  40:              End Get

  41:          End Property

  42:          ' Indexer

  43:          Default Public Property Item(index As Integer) As Object

  44:              Get

  45:                  Return _items(index)

  46:              End Get

  47:              Set(value As Object)

  48:                  _items.Insert(index, value)

  49:              End Set

  50:          End Property

  51:      End Class

  52:   

  53:      ' The 'Iterator' abstract class

  54:      MustInherit Class Iterator

  55:          Public MustOverride Function First() As Object

  56:          Public MustOverride Function [Next]() As Object

  57:          Public MustOverride Function IsDone() As Boolean

  58:          Public MustOverride Function CurrentItem() As Object

  59:      End Class

  60:   

  61:      ' The 'ConcreteIterator' class

  62:      Class ConcreteIterator

  63:          Inherits Iterator

  64:          Private _aggregate As ConcreteAggregate

  65:          Private _current As Integer = 0

  66:          ' Constructor

  67:          Public Sub New(aggregate As ConcreteAggregate)

  68:              Me._aggregate = aggregate

  69:          End Sub

  70:          ' Gets first iteration item

  71:          Public Overrides Function First() As Object

  72:              Return _aggregate(0)

  73:          End Function

  74:          ' Gets next iteration item

  75:          Public Overrides Function [Next]() As Object

  76:              Dim ret As Object = Nothing

  77:              If _current < _aggregate.Count - 1 Then

  78:                  _current = _current - 1

  79:                  ret = _aggregate(_current)

  80:              End If

  81:              Return ret

  82:          End Function

  83:          ' Gets current iteration item

  84:          Public Overrides Function CurrentItem() As Object

  85:              Return _aggregate(_current)

  86:          End Function

  87:          ' Gets whether iterations are complete

  88:          Public Overrides Function IsDone() As Boolean

  89:              Return _current >= _aggregate.Count

  90:          End Function

  91:      End Class