Imports System.Windows.Controls.Primitives
Imports System.Windows.Controls
Imports System.Windows
Imports System.Windows.Media
Imports System
Imports System.Diagnostics
Imports System.Collections.Specialized
Imports System.Windows.Data

Public Class VirtualizingTilePanel
   Inherits VirtualizingPanel
   Implements IScrollInfo

   Public Sub New()
      ' For use in the IScrollInfo implementation
      Me.RenderTransform = _trans
   End Sub

   Public Shared ReadOnly ChildSizeProperty As DependencyProperty = DependencyProperty.RegisterAttached("ChildSize", GetType(Double), GetType(VirtualizingTilePanel), New FrameworkPropertyMetadata(200.0, FrameworkPropertyMetadataOptions.AffectsMeasure Or FrameworkPropertyMetadataOptions.AffectsArrange))

   Public Property ChildSize As Double
      Get
         Return CDbl(GetValue(ChildSizeProperty))
      End Get
      Set(ByVal value As Double)
         SetValue(ChildSizeProperty, value)
      End Set
   End Property

   Protected Overrides Function MeasureOverride(ByVal availableSize As Size) As Size
      UpdateScrollInfo(availableSize)
      Dim firstVisibleItemIndex, lastVisibleItemIndex As Integer
      GetVisibleRange(firstVisibleItemIndex, lastVisibleItemIndex)
      Dim children As UIElementCollection = Me.InternalChildren
      Dim generator As IItemContainerGenerator = Me.ItemContainerGenerator
      Dim startPos As GeneratorPosition = generator.GeneratorPositionFromIndex(firstVisibleItemIndex)
      Dim childIndex As Integer = If((startPos.Offset = 0), startPos.Index, startPos.Index + 1)
      Using generator.StartAt(startPos, GeneratorDirection.Forward, True)
         Dim itemIndex As Integer = firstVisibleItemIndex
         While itemIndex <= lastVisibleItemIndex
            Dim newlyRealized As Boolean
            Dim child As UIElement = TryCast(generator.GenerateNext(newlyRealized), UIElement)
            If newlyRealized Then
               If childIndex >= children.Count Then
                  MyBase.AddInternalChild(child)
               Else
                  MyBase.InsertInternalChild(childIndex, child)
               End If
               generator.PrepareItemContainer(child)
            Else
               Debug.Assert(child Is children(childIndex), "Wrong child was generated")
            End If

            child.Measure(GetChildSize())
            System.Threading.Interlocked.Increment(itemIndex)
            System.Threading.Interlocked.Increment(childIndex)
         End While
      End Using

      CleanUpItems(firstVisibleItemIndex, lastVisibleItemIndex)
      Return availableSize
   End Function

   Protected Overrides Function ArrangeOverride(ByVal finalSize As Size) As Size
      Dim generator As IItemContainerGenerator = Me.ItemContainerGenerator
      UpdateScrollInfo(finalSize)
      For i As Integer = 0 To Me.Children.Count - 1
         Dim child As UIElement = Me.Children(i)
         Dim itemIndex As Integer = generator.IndexFromGeneratorPosition(New GeneratorPosition(i, 0))
         ArrangeChild(itemIndex, child, finalSize)
      Next

      Return finalSize
   End Function

   Private Sub CleanUpItems(ByVal minDesiredGenerated As Integer, ByVal maxDesiredGenerated As Integer)
      Dim children As UIElementCollection = Me.InternalChildren
      Dim generator As IItemContainerGenerator = Me.ItemContainerGenerator
      Dim i As Integer = children.Count - 1
      While i >= 0
         Dim childGeneratorPos As GeneratorPosition = New GeneratorPosition(i, 0)
         Dim itemIndex As Integer = generator.IndexFromGeneratorPosition(childGeneratorPos)
         If itemIndex < minDesiredGenerated OrElse itemIndex > maxDesiredGenerated Then
            generator.Remove(childGeneratorPos, 1)
            RemoveInternalChildRange(i, 1)
         End If
         i -= 1
      End While
   End Sub

   Protected Overrides Sub OnItemsChanged(ByVal sender As Object, ByVal args As ItemsChangedEventArgs)
      Select Case args.Action
         Case NotifyCollectionChangedAction.Remove, NotifyCollectionChangedAction.Replace, NotifyCollectionChangedAction.Move
            RemoveInternalChildRange(args.Position.Index, args.ItemUICount)
      End Select
   End Sub

   Private Function CalculateExtent(ByVal availableSize As Size, ByVal itemCount As Integer) As Size
      Dim childrenPerRow As Integer = CalculateChildrenPerRow(availableSize)
      Return New Size(childrenPerRow * Me.ChildSize, Me.ChildSize * Math.Ceiling(CDbl(itemCount) / childrenPerRow))
   End Function

   Private Sub GetVisibleRange(ByRef firstVisibleItemIndex As Integer, ByRef lastVisibleItemIndex As Integer)
      Dim childrenPerRow As Integer = CalculateChildrenPerRow(_extent)
      firstVisibleItemIndex = CInt(Math.Floor(_offset.Y / Me.ChildSize)) * childrenPerRow
      lastVisibleItemIndex = CInt(Math.Ceiling((_offset.Y + _viewport.Height) / Me.ChildSize)) * childrenPerRow - 1
      Dim itemsControl As ItemsControl = itemsControl.GetItemsOwner(Me)
      Dim itemCount As Integer = If(itemsControl.HasItems, itemsControl.Items.Count, 0)
      If lastVisibleItemIndex >= itemCount Then lastVisibleItemIndex = itemCount - 1
   End Sub

   Private Function GetChildSize() As Size
      Return New Size(Me.ChildSize, Me.ChildSize)
   End Function

   Private Sub ArrangeChild(ByVal itemIndex As Integer, ByVal child As UIElement, ByVal finalSize As Size)
      Dim childrenPerRow As Integer = CalculateChildrenPerRow(finalSize)
      Dim row As Integer = itemIndex \ childrenPerRow    ' IMPORTANTE L'operatore è di divisione intera, quindi \ NON / che invece arrotonda all'intero pari più vicino!!!!
      Dim column As Integer = itemIndex Mod childrenPerRow
      child.Arrange(New Rect(column * Me.ChildSize, row * Me.ChildSize, Me.ChildSize, Me.ChildSize))
   End Sub

   Private Function CalculateChildrenPerRow(ByVal availableSize As Size) As Integer
      Dim childrenPerRow As Integer
      If availableSize.Width = Double.PositiveInfinity Then childrenPerRow = Me.Children.Count Else childrenPerRow = Math.Max(1, CInt(Math.Floor(availableSize.Width / Me.ChildSize)))
      Return childrenPerRow
   End Function

   Private Sub UpdateScrollInfo(ByVal availableSize As Size)
      Dim itemsControl As ItemsControl = itemsControl.GetItemsOwner(Me)
      Dim itemCount As Integer = If(itemsControl.HasItems, itemsControl.Items.Count, 0)
      Dim extent As Size = CalculateExtent(availableSize, itemCount)
      If extent <> _extent Then
         _extent = extent
         If _owner IsNot Nothing Then _owner.InvalidateScrollInfo()
      End If

      If availableSize <> _viewport Then
         _viewport = availableSize
         If _owner IsNot Nothing Then _owner.InvalidateScrollInfo()
      End If
   End Sub

   Public Property ScrollOwner As ScrollViewer Implements IScrollInfo.ScrollOwner
      Get
         Return _owner
      End Get

      Set(ByVal value As ScrollViewer)
         _owner = value
      End Set
   End Property

   Public Property CanHorizontallyScroll As Boolean Implements IScrollInfo.CanHorizontallyScroll
      Get
         Return _canHScroll
      End Get

      Set(ByVal value As Boolean)
         _canHScroll = value
      End Set
   End Property

   Public Property CanVerticallyScroll As Boolean Implements IScrollInfo.CanVerticallyScroll
      Get
         Return _canVScroll
      End Get

      Set(ByVal value As Boolean)
         _canVScroll = value
      End Set
   End Property

   Public ReadOnly Property HorizontalOffset As Double Implements IScrollInfo.HorizontalOffset
      Get
         Return _offset.X
      End Get
   End Property

   Public ReadOnly Property VerticalOffset As Double Implements IScrollInfo.VerticalOffset
      Get
         Return _offset.Y
      End Get
   End Property

   Public ReadOnly Property ExtentHeight As Double Implements IScrollInfo.ExtentHeight
      Get
         Return _extent.Height
      End Get
   End Property

   Public ReadOnly Property ExtentWidth As Double Implements IScrollInfo.ExtentWidth
      Get
         Return _extent.Width
      End Get
   End Property

   Public ReadOnly Property ViewportHeight As Double Implements IScrollInfo.ViewportHeight
      Get
         Return _viewport.Height
      End Get
   End Property

   Public ReadOnly Property ViewportWidth As Double Implements IScrollInfo.ViewportWidth
      Get
         Return _viewport.Width
      End Get
   End Property

   Public Sub LineUp() Implements IScrollInfo.LineUp
      SetVerticalOffset(Me.VerticalOffset - 10)
   End Sub

   Public Sub LineDown() Implements IScrollInfo.LineDown
      SetVerticalOffset(Me.VerticalOffset + 10)
   End Sub

   Public Sub PageUp() Implements IScrollInfo.PageUp
      SetVerticalOffset(Me.VerticalOffset - _viewport.Height)
   End Sub

   Public Sub PageDown() Implements IScrollInfo.PageDown
      SetVerticalOffset(Me.VerticalOffset + _viewport.Height)
   End Sub

   Public Sub MouseWheelUp() Implements IScrollInfo.MouseWheelUp
      SetVerticalOffset(Me.VerticalOffset - 50)
   End Sub

   Public Sub MouseWheelDown() Implements IScrollInfo.MouseWheelDown
      SetVerticalOffset(Me.VerticalOffset + 50)
   End Sub

   Public Sub LineLeft() Implements IScrollInfo.LineLeft
      Throw New InvalidOperationException()
   End Sub

   Public Sub LineRight() Implements IScrollInfo.LineRight
      Throw New InvalidOperationException()
   End Sub

   Public Function MakeVisible(ByVal visual As Visual, ByVal rectangle As Rect) As Rect Implements IScrollInfo.MakeVisible
      Return New Rect()
   End Function

   Public Sub MouseWheelLeft() Implements IScrollInfo.MouseWheelLeft
      Throw New InvalidOperationException()
   End Sub

   Public Sub MouseWheelRight() Implements IScrollInfo.MouseWheelRight
      Throw New InvalidOperationException()
   End Sub

   Public Sub PageLeft() Implements IScrollInfo.PageLeft
      Throw New InvalidOperationException()
   End Sub

   Public Sub PageRight() Implements IScrollInfo.PageRight
      Throw New InvalidOperationException()
   End Sub

   Public Sub SetHorizontalOffset(ByVal offset As Double) Implements IScrollInfo.SetHorizontalOffset
      Throw New InvalidOperationException()
   End Sub

   Public Sub SetVerticalOffset(ByVal offset As Double) Implements IScrollInfo.SetVerticalOffset
      If offset < 0 OrElse _viewport.Height >= _extent.Height Then
         offset = 0
      Else
         If offset + _viewport.Height >= _extent.Height Then
            offset = _extent.Height - _viewport.Height
         End If
      End If

      _offset.Y = offset
      If _owner IsNot Nothing Then _owner.InvalidateScrollInfo()
      _trans.Y = -offset
      InvalidateMeasure()
   End Sub

   Private _trans As TranslateTransform = New TranslateTransform()

   Private _owner As ScrollViewer

   Private _canHScroll As Boolean = False

   Private _canVScroll As Boolean = False

   Private _extent As Size = New Size(0, 0)

   Private _viewport As Size = New Size(0, 0)

   Private _offset As Point

End Class