WPF 虚拟化 VirtualizingWrapPanel 和 VirtualLizingTilePanel
一、 UI 上两个扩展
public class VirtualizingWrapPanel : VirtualizingPanel, IScrollInfo
{
#region Fields
UIElementCollection _children;
ItemsControl _itemsControl;
IItemContainerGenerator _generator;
private Point _offset = new Point(0, 0);
private Size _extent = new Size(0, 0);
private Size _viewport = new Size(0, 0);
private int firstIndex = 0;
private Size childSize;
private Size _pixelMeasuredViewport = new Size(0, 0);
Dictionary<UIElement, Rect> _realizedChildLayout = new Dictionary<UIElement, Rect>();
WrapPanelAbstraction _abstractPanel;
#endregion
#region Properties
private Size ChildSlotSize
{
get
{
return new Size(ItemWidth, ItemHeight);
}
}
#endregion
#region Dependency Properties
[TypeConverter(typeof(LengthConverter))]
public double ItemHeight
{
get
{
return (double)base.GetValue(ItemHeightProperty);
}
set
{
base.SetValue(ItemHeightProperty, value);
}
}
[TypeConverter(typeof(LengthConverter))]
public double ItemWidth
{
get
{
return (double)base.GetValue(ItemWidthProperty);
}
set
{
base.SetValue(ItemWidthProperty, value);
}
}
public Orientation Orientation
{
get { return (Orientation)GetValue(OrientationProperty); }
set { SetValue(OrientationProperty, value); }
}
public static readonly DependencyProperty ItemHeightProperty = DependencyProperty.Register("ItemHeight", typeof(double), typeof(VirtualizingWrapPanel), new FrameworkPropertyMetadata(double.PositiveInfinity));
public static readonly DependencyProperty ItemWidthProperty = DependencyProperty.Register("ItemWidth", typeof(double), typeof(VirtualizingWrapPanel), new FrameworkPropertyMetadata(double.PositiveInfinity));
public static readonly DependencyProperty OrientationProperty = StackPanel.OrientationProperty.AddOwner(typeof(VirtualizingWrapPanel), new FrameworkPropertyMetadata(Orientation.Horizontal));
#endregion
#region Methods
public void SetFirstRowViewItemIndex(int index)
{
SetVerticalOffset((index) / Math.Floor((_viewport.Width) / childSize.Width));
SetHorizontalOffset((index) / Math.Floor((_viewport.Height) / childSize.Height));
}
private void Resizing(object sender, EventArgs e)
{
if (_viewport.Width != 0)
{
int firstIndexCache = firstIndex;
_abstractPanel = null;
MeasureOverride(_viewport);
SetFirstRowViewItemIndex(firstIndex);
firstIndex = firstIndexCache;
}
}
public int GetFirstVisibleSection()
{
int section;
var maxSection = _abstractPanel.Max(x => x.Section);
if (Orientation == Orientation.Horizontal)
{
section = (int)_offset.Y;
}
else
{
section = (int)_offset.X;
}
if (section > maxSection)
section = maxSection;
return section;
}
public int GetFirstVisibleIndex()
{
int section = GetFirstVisibleSection();
var item = _abstractPanel.Where(x => x.Section == section).FirstOrDefault();
if (item != null)
return item._index;
return 0;
}
private void CleanUpItems(int minDesiredGenerated, int maxDesiredGenerated)
{
for (int i = _children.Count - 1; i >= 0; i--)
{
GeneratorPosition childGeneratorPos = new GeneratorPosition(i, 0);
int itemIndex = _generator.IndexFromGeneratorPosition(childGeneratorPos);
if (itemIndex < minDesiredGenerated || itemIndex > maxDesiredGenerated)
{
_generator.Remove(childGeneratorPos, 1);
RemoveInternalChildRange(i, 1);
}
}
}
private void ComputeExtentAndViewport(Size pixelMeasuredViewportSize, int visibleSections)
{
if (Orientation == Orientation.Horizontal)
{
_viewport.Height = visibleSections;
_viewport.Width = pixelMeasuredViewportSize.Width;
}
else
{
_viewport.Width = visibleSections;
_viewport.Height = pixelMeasuredViewportSize.Height;
}
if (Orientation == Orientation.Horizontal)
{
_extent.Height = _abstractPanel.SectionCount + ViewportHeight - 1;
}
else
{
_extent.Width = _abstractPanel.SectionCount + ViewportWidth - 1;
}
_owner.InvalidateScrollInfo();
}
private void ResetScrollInfo()
{
_offset.X = 0;
_offset.Y = 0;
}
private int GetNextSectionClosestIndex(int itemIndex)
{
var abstractItem = _abstractPanel[itemIndex];
if (abstractItem.Section < _abstractPanel.SectionCount - 1)
{
var ret = _abstractPanel.
Where(x => x.Section == abstractItem.Section + 1).
OrderBy(x => Math.Abs(x.SectionIndex - abstractItem.SectionIndex)).
First();
return ret._index;
}
else
return itemIndex;
}
private int GetLastSectionClosestIndex(int itemIndex)
{
var abstractItem = _abstractPanel[itemIndex];
if (abstractItem.Section > 0)
{
var ret = _abstractPanel.
Where(x => x.Section == abstractItem.Section - 1).
OrderBy(x => Math.Abs(x.SectionIndex - abstractItem.SectionIndex)).
First();
return ret._index;
}
else
return itemIndex;
}
private void NavigateDown()
{
var gen = _generator.GetItemContainerGeneratorForPanel(this);
UIElement selected = (UIElement)Keyboard.FocusedElement;
int itemIndex = gen.IndexFromContainer(selected);
int depth = 0;
while (itemIndex == -1)
{
selected = (UIElement)VisualTreeHelper.GetParent(selected);
itemIndex = gen.IndexFromContainer(selected);
depth++;
}
DependencyObject next = null;
if (Orientation == Orientation.Horizontal)
{
int nextIndex = GetNextSectionClosestIndex(itemIndex);
next = gen.ContainerFromIndex(nextIndex);
while (next == null)
{
SetVerticalOffset(VerticalOffset + 1);
UpdateLayout();
next = gen.ContainerFromIndex(nextIndex);
}
}
else
{
if (itemIndex == _abstractPanel._itemCount - 1)
return;
next = gen.ContainerFromIndex(itemIndex + 1);
while (next == null)
{
SetHorizontalOffset(HorizontalOffset + 1);
UpdateLayout();
next = gen.ContainerFromIndex(itemIndex + 1);
}
}
while (depth != 0)
{
next = VisualTreeHelper.GetChild(next, 0);
depth--;
}
(next as UIElement).Focus();
}
private void NavigateLeft()
{
var gen = _generator.GetItemContainerGeneratorForPanel(this);
UIElement selected = (UIElement)Keyboard.FocusedElement;
int itemIndex = gen.IndexFromContainer(selected);
int depth = 0;
while (itemIndex == -1)
{
selected = (UIElement)VisualTreeHelper.GetParent(selected);
itemIndex = gen.IndexFromContainer(selected);
depth++;
}
DependencyObject next = null;
if (Orientation == Orientation.Vertical)
{
int nextIndex = GetLastSectionClosestIndex(itemIndex);
next = gen.ContainerFromIndex(nextIndex);
while (next == null)
{
SetHorizontalOffset(HorizontalOffset - 1);
UpdateLayout();
next = gen.ContainerFromIndex(nextIndex);
}
}
else
{
if (itemIndex == 0)
return;
next = gen.ContainerFromIndex(itemIndex - 1);
while (next == null)
{
SetVerticalOffset(VerticalOffset - 1);
UpdateLayout();
next = gen.ContainerFromIndex(itemIndex - 1);
}
}
while (depth != 0)
{
next = VisualTreeHelper.GetChild(next, 0);
depth--;
}
(next as UIElement).Focus();
}
private void NavigateRight()
{
var gen = _generator.GetItemContainerGeneratorForPanel(this);
UIElement selected = (UIElement)Keyboard.FocusedElement;
int itemIndex = gen.IndexFromContainer(selected);
int depth = 0;
while (itemIndex == -1)
{
selected = (UIElement)VisualTreeHelper.GetParent(selected);
itemIndex = gen.IndexFromContainer(selected);
depth++;
}
DependencyObject next = null;
if (Orientation == Orientation.Vertical)
{
int nextIndex = GetNextSectionClosestIndex(itemIndex);
next = gen.ContainerFromIndex(nextIndex);
while (next == null)
{
SetHorizontalOffset(HorizontalOffset + 1);
UpdateLayout();
next = gen.ContainerFromIndex(nextIndex);
}
}
else
{
if (itemIndex == _abstractPanel._itemCount - 1)
return;
next = gen.ContainerFromIndex(itemIndex + 1);
while (next == null)
{
SetVerticalOffset(VerticalOffset + 1);
UpdateLayout();
next = gen.ContainerFromIndex(itemIndex + 1);
}
}
while (depth != 0)
{
next = VisualTreeHelper.GetChild(next, 0);
depth--;
}
(next as UIElement).Focus();
}
private void NavigateUp()
{
var gen = _generator.GetItemContainerGeneratorForPanel(this);
UIElement selected = (UIElement)Keyboard.FocusedElement;
int itemIndex = gen.IndexFromContainer(selected);
int depth = 0;
while (itemIndex == -1)
{
selected = (UIElement)VisualTreeHelper.GetParent(selected);
itemIndex = gen.IndexFromContainer(selected);
depth++;
}
DependencyObject next = null;
if (Orientation == Orientation.Horizontal)
{
int nextIndex = GetLastSectionClosestIndex(itemIndex);
next = gen.ContainerFromIndex(nextIndex);
while (next == null)
{
SetVerticalOffset(VerticalOffset - 1);
UpdateLayout();
next = gen.ContainerFromIndex(nextIndex);
}
}
else
{
if (itemIndex == 0)
return;
next = gen.ContainerFromIndex(itemIndex - 1);
while (next == null)
{
SetHorizontalOffset(HorizontalOffset - 1);
UpdateLayout();
next = gen.ContainerFromIndex(itemIndex - 1);
}
}
while (depth != 0)
{
next = VisualTreeHelper.GetChild(next, 0);
depth--;
}
(next as UIElement).Focus();
}
#endregion
#region Override
protected override void OnKeyDown(KeyEventArgs e)
{
switch (e.Key)
{
case Key.Down:
NavigateDown();
e.Handled = true;
break;
case Key.Left:
NavigateLeft();
e.Handled = true;
break;
case Key.Right:
NavigateRight();
e.Handled = true;
break;
case Key.Up:
NavigateUp();
e.Handled = true;
break;
default:
base.OnKeyDown(e);
break;
}
}
protected override void OnItemsChanged(object sender, ItemsChangedEventArgs args)
{
base.OnItemsChanged(sender, args);
_abstractPanel = null;
ResetScrollInfo();
}
protected override void OnInitialized(EventArgs e)
{
this.SizeChanged += new SizeChangedEventHandler(this.Resizing);
base.OnInitialized(e);
_itemsControl = ItemsControl.GetItemsOwner(this);
_children = InternalChildren;
_generator = ItemContainerGenerator;
}
protected override Size MeasureOverride(Size availableSize)
{
if (_itemsControl == null || _itemsControl.Items.Count == 0)
return availableSize;
if (_abstractPanel == null)
_abstractPanel = new WrapPanelAbstraction(_itemsControl.Items.Count);
_pixelMeasuredViewport = availableSize;
_realizedChildLayout.Clear();
Size realizedFrameSize = availableSize;
int itemCount = _itemsControl.Items.Count;
int firstVisibleIndex = GetFirstVisibleIndex();
GeneratorPosition startPos = _generator.GeneratorPositionFromIndex(firstVisibleIndex);
int childIndex = (startPos.Offset == 0) ? startPos.Index : startPos.Index + 1;
int current = firstVisibleIndex;
int visibleSections = 1;
using (_generator.StartAt(startPos, GeneratorDirection.Forward, true))
{
bool stop = false;
bool isHorizontal = Orientation == Orientation.Horizontal;
double currentX = 0;
double currentY = 0;
double maxItemSize = 0;
int currentSection = GetFirstVisibleSection();
while (current < itemCount)
{
bool newlyRealized;
// Get or create the child
UIElement child = _generator.GenerateNext(out newlyRealized) as UIElement;
if (newlyRealized)
{
// Figure out if we need to insert the child at the end or somewhere in the middle
if (childIndex >= _children.Count)
{
base.AddInternalChild(child);
}
else
{
base.InsertInternalChild(childIndex, child);
}
_generator.PrepareItemContainer(child);
child.Measure(ChildSlotSize);
}
else
{
// The child has already been created, let's be sure it's in the right spot
Debug.Assert(child == _children[childIndex], "Wrong child was generated");
}
childSize = child.DesiredSize;
Rect childRect = new Rect(new Point(currentX, currentY), childSize);
if (isHorizontal)
{
maxItemSize = Math.Max(maxItemSize, childRect.Height);
if (childRect.Right > realizedFrameSize.Width) //wrap to a new line
{
currentY = currentY + maxItemSize;
currentX = 0;
maxItemSize = childRect.Height;
childRect.X = currentX;
childRect.Y = currentY;
currentSection++;
visibleSections++;
}
if (currentY > realizedFrameSize.Height)
stop = true;
currentX = childRect.Right;
}
else
{
maxItemSize = Math.Max(maxItemSize, childRect.Width);
if (childRect.Bottom > realizedFrameSize.Height) //wrap to a new column
{
currentX = currentX + maxItemSize;
currentY = 0;
maxItemSize = childRect.Width;
childRect.X = currentX;
childRect.Y = currentY;
currentSection++;
visibleSections++;
}
if (currentX > realizedFrameSize.Width)
stop = true;
currentY = childRect.Bottom;
}
_realizedChildLayout.Add(child, childRect);
_abstractPanel.SetItemSection(current, currentSection);
if (stop)
break;
current++;
childIndex++;
}
}
CleanUpItems(firstVisibleIndex, current - 1);
ComputeExtentAndViewport(availableSize, visibleSections);
return availableSize;
}
protected override Size ArrangeOverride(Size finalSize)
{
if (_children != null)
{
foreach (UIElement child in _children)
{
var layoutInfo = _realizedChildLayout[child];
child.Arrange(layoutInfo);
}
}
return finalSize;
}
#endregion
#region IScrollInfo Members
private bool _canHScroll = false;
public bool CanHorizontallyScroll
{
get { return _canHScroll; }
set { _canHScroll = value; }
}
private bool _canVScroll = false;
public bool CanVerticallyScroll
{
get { return _canVScroll; }
set { _canVScroll = value; }
}
public double ExtentHeight
{
get { return _extent.Height; }
}
public double ExtentWidth
{
get { return _extent.Width; }
}
public double HorizontalOffset
{
get { return _offset.X; }
}
public double VerticalOffset
{
get { return _offset.Y; }
}
public void LineDown()
{
if (Orientation == Orientation.Vertical)
SetVerticalOffset(VerticalOffset + 20);
else
SetVerticalOffset(VerticalOffset + 1);
}
public void LineLeft()
{
if (Orientation == Orientation.Horizontal)
SetHorizontalOffset(HorizontalOffset - 20);
else
SetHorizontalOffset(HorizontalOffset - 1);
}
public void LineRight()
{
if (Orientation == Orientation.Horizontal)
SetHorizontalOffset(HorizontalOffset + 20);
else
SetHorizontalOffset(HorizontalOffset + 1);
}
public void LineUp()
{
if (Orientation == Orientation.Vertical)
SetVerticalOffset(VerticalOffset - 20);
else
SetVerticalOffset(VerticalOffset - 1);
}
public Rect MakeVisible(Visual visual, Rect rectangle)
{
var gen = (ItemContainerGenerator)_generator.GetItemContainerGeneratorForPanel(this);
var element = (UIElement)visual;
int itemIndex = gen.IndexFromContainer(element);
while (itemIndex == -1)
{
element = (UIElement)VisualTreeHelper.GetParent(element);
itemIndex = gen.IndexFromContainer(element);
}
int section = _abstractPanel[itemIndex].Section;
Rect elementRect = _realizedChildLayout[element];
if (Orientation == Orientation.Horizontal)
{
double viewportHeight = _pixelMeasuredViewport.Height;
if (elementRect.Bottom > viewportHeight)
_offset.Y += 1;
else if (elementRect.Top < 0)
_offset.Y -= 1;
}
else
{
double viewportWidth = _pixelMeasuredViewport.Width;
if (elementRect.Right > viewportWidth)
_offset.X += 1;
else if (elementRect.Left < 0)
_offset.X -= 1;
}
InvalidateMeasure();
return elementRect;
}
public void MouseWheelDown()
{
PageDown();
}
public void MouseWheelLeft()
{
PageLeft();
}
public void MouseWheelRight()
{
PageRight();
}
public void MouseWheelUp()
{
PageUp();
}
public void PageDown()
{
SetVerticalOffset(VerticalOffset + _viewport.Height * 0.8);
}
public void PageLeft()
{
SetHorizontalOffset(HorizontalOffset - _viewport.Width * 0.8);
}
public void PageRight()
{
SetHorizontalOffset(HorizontalOffset + _viewport.Width * 0.8);
}
public void PageUp()
{
SetVerticalOffset(VerticalOffset - _viewport.Height * 0.8);
}
private ScrollViewer _owner;
public ScrollViewer ScrollOwner
{
get { return _owner; }
set { _owner = value; }
}
public void SetHorizontalOffset(double offset)
{
if (offset < 0 || _viewport.Width >= _extent.Width)
{
offset = 0;
}
else
{
if (offset + _viewport.Width >= _extent.Width)
{
offset = _extent.Width - _viewport.Width;
}
}
_offset.X = offset;
if (_owner != null)
_owner.InvalidateScrollInfo();
InvalidateMeasure();
firstIndex = GetFirstVisibleIndex();
}
public void SetVerticalOffset(double offset)
{
if (offset < 0 || _viewport.Height >= _extent.Height)
{
offset = 0;
}
else
{
if (offset + _viewport.Height >= _extent.Height)
{
offset = _extent.Height - _viewport.Height;
}
}
_offset.Y = offset;
if (_owner != null)
_owner.InvalidateScrollInfo();
//_trans.Y = -offset;
InvalidateMeasure();
firstIndex = GetFirstVisibleIndex();
}
public double ViewportHeight
{
get { return _viewport.Height; }
}
public double ViewportWidth
{
get { return _viewport.Width; }
}
#endregion
#region helper data structures
class ItemAbstraction
{
public ItemAbstraction(WrapPanelAbstraction panel, int index)
{
_panel = panel;
_index = index;
}
WrapPanelAbstraction _panel;
public readonly int _index;
int _sectionIndex = -1;
public int SectionIndex
{
get
{
if (_sectionIndex == -1)
{
return _index % _panel._averageItemsPerSection - 1;
}
return _sectionIndex;
}
set
{
if (_sectionIndex == -1)
_sectionIndex = value;
}
}
int _section = -1;
public int Section
{
get
{
if (_section == -1)
{
return _index / _panel._averageItemsPerSection;
}
return _section;
}
set
{
if (_section == -1)
_section = value;
}
}
}
class WrapPanelAbstraction : IEnumerable<ItemAbstraction>
{
public WrapPanelAbstraction(int itemCount)
{
List<ItemAbstraction> items = new List<ItemAbstraction>(itemCount);
for (int i = 0; i < itemCount; i++)
{
ItemAbstraction item = new ItemAbstraction(this, i);
items.Add(item);
}
Items = new ReadOnlyCollection<ItemAbstraction>(items);
_averageItemsPerSection = itemCount;
_itemCount = itemCount;
}
public readonly int _itemCount;
public int _averageItemsPerSection;
private int _currentSetSection = -1;
private int _currentSetItemIndex = -1;
private int _itemsInCurrentSecction = 0;
private object _syncRoot = new object();
public int SectionCount
{
get
{
int ret = _currentSetSection + 1;
if (_currentSetItemIndex + 1 < Items.Count)
{
int itemsLeft = Items.Count - _currentSetItemIndex;
ret += itemsLeft / _averageItemsPerSection + 1;
}
return ret;
}
}
private ReadOnlyCollection<ItemAbstraction> Items { get; set; }
public void SetItemSection(int index, int section)
{
lock (_syncRoot)
{
if (section <= _currentSetSection + 1 && index == _currentSetItemIndex + 1)
{
_currentSetItemIndex++;
Items[index].Section = section;
if (section == _currentSetSection + 1)
{
_currentSetSection = section;
if (section > 0)
{
_averageItemsPerSection = (index) / (section);
}
_itemsInCurrentSecction = 1;
}
else
_itemsInCurrentSecction++;
Items[index].SectionIndex = _itemsInCurrentSecction - 1;
}
}
}
public ItemAbstraction this[int index]
{
get { return Items[index]; }
}
#region IEnumerable<ItemAbstraction> Members
public IEnumerator<ItemAbstraction> GetEnumerator()
{
return Items.GetEnumerator();
}
#endregion
#region IEnumerable Members
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
#endregion
}
#endregion
}
来源：http://www.codeproject.com/Articles/75847/Virtualizing-WrapPanel
// class from: https://github.com/samueldjack/VirtualCollection/blob/master/VirtualCollection/VirtualCollection/VirtualizingWrapPanel.cs
// MakeVisible() method from: http://www.switchonthecode.com/tutorials/wpf-tutorial-implementing-iscrollinfo
public class VirtualLizingTilePanel : VirtualizingPanel, IScrollInfo
{
private const double ScrollLineAmount = 16.0;
private Size _extentSize;
private Size _viewportSize;
private Point _offset;
private ItemsControl _itemsControl;
private readonly Dictionary<UIElement, Rect> _childLayouts = new Dictionary<UIElement, Rect>();
public static readonly DependencyProperty ItemWidthProperty =
DependencyProperty.Register("ItemWidth", typeof(double), typeof(VirtualLizingTilePanel), new PropertyMetadata(1.0, HandleItemDimensionChanged));
public static readonly DependencyProperty ItemHeightProperty =
DependencyProperty.Register("ItemHeight", typeof(double), typeof(VirtualLizingTilePanel), new PropertyMetadata(1.0, HandleItemDimensionChanged));
private static readonly DependencyProperty VirtualItemIndexProperty =
DependencyProperty.RegisterAttached("VirtualItemIndex", typeof(int), typeof(VirtualLizingTilePanel), new PropertyMetadata(-1));
private IRecyclingItemContainerGenerator _itemsGenerator;
private bool _isInMeasure;
private static int GetVirtualItemIndex(DependencyObject obj)
{
return (int)obj.GetValue(VirtualItemIndexProperty);
}
private static void SetVirtualItemIndex(DependencyObject obj, int value)
{
obj.SetValue(VirtualItemIndexProperty, value);
}
public double ItemHeight
{
get { return (double)GetValue(ItemHeightProperty); }
set { SetValue(ItemHeightProperty, value); }
}
public double ItemWidth
{
get { return (double)GetValue(ItemWidthProperty); }
set { SetValue(ItemWidthProperty, value); }
}
public VirtualLizingTilePanel()
{
if (!DesignerProperties.GetIsInDesignMode(this))
{
Dispatcher.BeginInvoke((Action)Initialize);
}
}
private void Initialize()
{
_itemsControl = ItemsControl.GetItemsOwner(this);
_itemsGenerator = (IRecyclingItemContainerGenerator)ItemContainerGenerator;
InvalidateMeasure();
}
protected override void OnItemsChanged(object sender, ItemsChangedEventArgs args)
{
base.OnItemsChanged(sender, args);
InvalidateMeasure();
}
protected override Size MeasureOverride(Size availableSize)
{
if (_itemsControl == null)
{
return availableSize;
}
_isInMeasure = true;
_childLayouts.Clear();
var extentInfo = GetExtentInfo(availableSize, ItemHeight);
EnsureScrollOffsetIsWithinConstrains(extentInfo);
var layoutInfo = GetLayoutInfo(availableSize, ItemHeight, extentInfo);
RecycleItems(layoutInfo);
// Determine where the first item is in relation to previously realized items
var generatorStartPosition = _itemsGenerator.GeneratorPositionFromIndex(layoutInfo.FirstRealizedItemIndex);
var visualIndex = 0;
var currentX = layoutInfo.FirstRealizedItemLeft;
var currentY = layoutInfo.FirstRealizedLineTop;
using (_itemsGenerator.StartAt(generatorStartPosition, GeneratorDirection.Forward, true))
{
for (var itemIndex = layoutInfo.FirstRealizedItemIndex; itemIndex <= layoutInfo.LastRealizedItemIndex; itemIndex++, visualIndex++)
{
bool newlyRealized;
var child = (UIElement)_itemsGenerator.GenerateNext(out newlyRealized);
SetVirtualItemIndex(child, itemIndex);
if (newlyRealized)
{
InsertInternalChild(visualIndex, child);
}
else
{
// check if item needs to be moved into a new position in the Children collection
if (visualIndex < Children.Count)
{
if (Children[visualIndex] != child)
{
var childCurrentIndex = Children.IndexOf(child);
if (childCurrentIndex >= 0)
{
RemoveInternalChildRange(childCurrentIndex, 1);
}
InsertInternalChild(visualIndex, child);
}
}
else
{
// we know that the child can't already be in the children collection
// because we've been inserting children in correct visualIndex order,
// and this child has a visualIndex greater than the Children.Count
AddInternalChild(child);
}
}
// only prepare the item once it has been added to the visual tree
_itemsGenerator.PrepareItemContainer(child);
child.Measure(new Size(ItemWidth, ItemHeight));
_childLayouts.Add(child, new Rect(currentX, currentY, ItemWidth, ItemHeight));
if (currentX + ItemWidth * 2 >= availableSize.Width)
{
// wrap to a new line
currentY += ItemHeight;
currentX = 0;
}
else
{
currentX += ItemWidth;
}
}
}
RemoveRedundantChildren();
UpdateScrollInfo(availableSize, extentInfo);
var desiredSize = new Size(double.IsInfinity(availableSize.Width) ? 0 : availableSize.Width,
double.IsInfinity(availableSize.Height) ? 0 : availableSize.Height);
_isInMeasure = false;
return desiredSize;
}
private void EnsureScrollOffsetIsWithinConstrains(ExtentInfo extentInfo)
{
_offset.Y = Clamp(_offset.Y, 0, extentInfo.MaxVerticalOffset);
}
private void RecycleItems(ItemLayoutInfo layoutInfo)
{
foreach (UIElement child in Children)
{
var virtualItemIndex = GetVirtualItemIndex(child);
if (virtualItemIndex < layoutInfo.FirstRealizedItemIndex || virtualItemIndex > layoutInfo.LastRealizedItemIndex)
{
var generatorPosition = _itemsGenerator.GeneratorPositionFromIndex(virtualItemIndex);
if (generatorPosition.Index >= 0)
{
_itemsGenerator.Recycle(generatorPosition, 1);
}
}
SetVirtualItemIndex(child, -1);
}
}
protected override Size ArrangeOverride(Size finalSize)
{
foreach (UIElement child in Children)
{
child.Arrange(_childLayouts[child]);
}
return finalSize;
}
private void UpdateScrollInfo(Size availableSize, ExtentInfo extentInfo)
{
_viewportSize = availableSize;
_extentSize = new Size(availableSize.Width, extentInfo.ExtentHeight);
InvalidateScrollInfo();
}
private void RemoveRedundantChildren()
{
// iterate backwards through the child collection because we're going to be
// removing items from it
for (var i = Children.Count - 1; i >= 0; i--)
{
var child = Children[i];
// if the virtual item index is -1, this indicates
// it is a recycled item that hasn't been reused this time round
if (GetVirtualItemIndex(child) == -1)
{
RemoveInternalChildRange(i, 1);
}
}
}
private ItemLayoutInfo GetLayoutInfo(Size availableSize, double itemHeight, ExtentInfo extentInfo)
{
if (_itemsControl == null)
{
return new ItemLayoutInfo();
}
// we need to ensure that there is one realized item prior to the first visible item, and one after the last visible item,
// so that keyboard navigation works properly. For example, when focus is on the first visible item, and the user
// navigates up, the ListBox selects the previous item, and the scrolls that into view - and this triggers the loading of the rest of the items
// in that row
var firstVisibleLine = (int)Math.Floor(VerticalOffset / itemHeight);
var firstRealizedIndex = Math.Max(extentInfo.ItemsPerLine * firstVisibleLine - 1, 0);
var firstRealizedItemLeft = firstRealizedIndex % extentInfo.ItemsPerLine * ItemWidth - HorizontalOffset;
var firstRealizedItemTop = (firstRealizedIndex / extentInfo.ItemsPerLine) * itemHeight - VerticalOffset;
var firstCompleteLineTop = (firstVisibleLine == 0 ? firstRealizedItemTop : firstRealizedItemTop + ItemHeight);
var completeRealizedLines = (int)Math.Ceiling((availableSize.Height - firstCompleteLineTop) / itemHeight);
var lastRealizedIndex = Math.Min(firstRealizedIndex + completeRealizedLines * extentInfo.ItemsPerLine + 2, _itemsControl.Items.Count - 1);
return new ItemLayoutInfo
{
FirstRealizedItemIndex = firstRealizedIndex,
FirstRealizedItemLeft = firstRealizedItemLeft,
FirstRealizedLineTop = firstRealizedItemTop,
LastRealizedItemIndex = lastRealizedIndex,
};
}
private ExtentInfo GetExtentInfo(Size viewPortSize, double itemHeight)
{
if (_itemsControl == null)
{
return new ExtentInfo();
}
var itemsPerLine = Math.Max((int)Math.Floor(viewPortSize.Width / ItemWidth), 1);
var totalLines = (int)Math.Ceiling((double)_itemsControl.Items.Count / itemsPerLine);
var extentHeight = Math.Max(totalLines * ItemHeight, viewPortSize.Height);
return new ExtentInfo
{
ItemsPerLine = itemsPerLine,
TotalLines = totalLines,
ExtentHeight = extentHeight,
MaxVerticalOffset = extentHeight - viewPortSize.Height,
};
}
public void LineUp()
{
SetVerticalOffset(VerticalOffset - ScrollLineAmount);
}
public void LineDown()
{
SetVerticalOffset(VerticalOffset + ScrollLineAmount);
}
public void LineLeft()
{
SetHorizontalOffset(HorizontalOffset + ScrollLineAmount);
}
public void LineRight()
{
SetHorizontalOffset(HorizontalOffset - ScrollLineAmount);
}
public void PageUp()
{
SetVerticalOffset(VerticalOffset - ViewportHeight);
}
public void PageDown()
{
SetVerticalOffset(VerticalOffset + ViewportHeight);
}
public void PageLeft()
{
SetHorizontalOffset(HorizontalOffset + ItemWidth);
}
public void PageRight()
{
SetHorizontalOffset(HorizontalOffset - ItemWidth);
}
public void MouseWheelUp()
{
SetVerticalOffset(VerticalOffset - ScrollLineAmount * SystemParameters.WheelScrollLines);
}
public void MouseWheelDown()
{
SetVerticalOffset(VerticalOffset + ScrollLineAmount * SystemParameters.WheelScrollLines);
}
public void MouseWheelLeft()
{
SetHorizontalOffset(HorizontalOffset - ScrollLineAmount * SystemParameters.WheelScrollLines);
}
public void MouseWheelRight()
{
SetHorizontalOffset(HorizontalOffset + ScrollLineAmount * SystemParameters.WheelScrollLines);
}
public void SetHorizontalOffset(double offset)
{
if (_isInMeasure)
{
return;
}
offset = Clamp(offset, 0, ExtentWidth - ViewportWidth);
_offset = new Point(offset, _offset.Y);
InvalidateScrollInfo();
InvalidateMeasure();
}
public void SetVerticalOffset(double offset)
{
if (_isInMeasure)
{
return;
}
offset = Clamp(offset, 0, ExtentHeight - ViewportHeight);
_offset = new Point(_offset.X, offset);
InvalidateScrollInfo();
InvalidateMeasure();
}
public Rect MakeVisible(Visual visual, Rect rectangle)
{
if (rectangle.IsEmpty ||
visual == null ||
visual == this ||
!IsAncestorOf(visual))
{
return Rect.Empty;
}
rectangle = visual.TransformToAncestor(this).TransformBounds(rectangle);
var viewRect = new Rect(HorizontalOffset, VerticalOffset, ViewportWidth, ViewportHeight);
rectangle.X += viewRect.X;
rectangle.Y += viewRect.Y;
viewRect.X = CalculateNewScrollOffset(viewRect.Left, viewRect.Right, rectangle.Left, rectangle.Right);
viewRect.Y = CalculateNewScrollOffset(viewRect.Top, viewRect.Bottom, rectangle.Top, rectangle.Bottom);
SetHorizontalOffset(viewRect.X);
SetVerticalOffset(viewRect.Y);
rectangle.Intersect(viewRect);
rectangle.X -= viewRect.X;
rectangle.Y -= viewRect.Y;
return rectangle;
}
private static double CalculateNewScrollOffset(double topView, double bottomView, double topChild, double bottomChild)
{
var offBottom = topChild < topView && bottomChild < bottomView;
var offTop = bottomChild > bottomView && topChild > topView;
var tooLarge = (bottomChild - topChild) > (bottomView - topView);
if (!offBottom && !offTop)
return topView;
if ((offBottom && !tooLarge) || (offTop && tooLarge))
return topChild;
return bottomChild - (bottomView - topView);
}
public ItemLayoutInfo GetVisibleItemsRange()
{
return GetLayoutInfo(_viewportSize, ItemHeight, GetExtentInfo(_viewportSize, ItemHeight));
}
public bool CanVerticallyScroll
{
get;
set;
}
public bool CanHorizontallyScroll
{
get;
set;
}
public double ExtentWidth
{
get { return _extentSize.Width; }
}
public double ExtentHeight
{
get { return _extentSize.Height; }
}
public double ViewportWidth
{
get { return _viewportSize.Width; }
}
public double ViewportHeight
{
get { return _viewportSize.Height; }
}
public double HorizontalOffset
{
get { return _offset.X; }
}
public double VerticalOffset
{
get { return _offset.Y; }
}
public ScrollViewer ScrollOwner
{
get;
set;
}
private void InvalidateScrollInfo()
{
if (ScrollOwner != null)
{
ScrollOwner.InvalidateScrollInfo();
}
}
private static void HandleItemDimensionChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var wrapPanel = (d as VirtualLizingTilePanel);
if (wrapPanel != null)
wrapPanel.InvalidateMeasure();
}
private double Clamp(double value, double min, double max)
{
return Math.Min(Math.Max(value, min), max);
}
internal class ExtentInfo
{
public int ItemsPerLine;
public int TotalLines;
public double ExtentHeight;
public double MaxVerticalOffset;
}
public class ItemLayoutInfo
{
public int FirstRealizedItemIndex;
public double FirstRealizedLineTop;
public double FirstRealizedItemLeft;
public int LastRealizedItemIndex;
}
}
来源：
// class from: https://github.com/samueldjack/VirtualCollection/blob/master/VirtualCollection/VirtualCollection/VirtualizingWrapPanel.cs
// MakeVisible() method from: http://www.switchonthecode.com/tutorials/wpf-tutorial-implementing-iscrollinfo
二、数据方面的一个处理
/// <summary>
/// 为ListBox支持数据虚拟化技术
/// </summary>
public class VirtualDataForListBox<T> : IDisposable, INotifyPropertyChanged where T : class
{
public event PropertyChangedEventHandler PropertyChanged;
private DelayHelper delay;
private ListBox listBox;
/// <summary>
/// 垂直滚动条
/// </summary>
private ScrollBar bar;
/// <summary>
/// 滚动视图
/// </summary>
private ScrollViewer viewer;
/// <summary>
/// 数据源
/// </summary>
private ObservableCollection<T> sources;
/// <summary>
/// 是否已初始化完毕
/// </summary>
protected bool Inited { get; set; }
/// <summary>
/// 偏移量
/// </summary>
protected double Offset { get; set; }
/// <summary>
/// 偏移数量
/// </summary>
protected int OffsetCount { get; set; }
/// <summary>
/// 偏移方向
/// <para>True：向上</para>
/// <para>False：向下</para>
/// </summary>
protected bool OffsetDirection { get; set; }
public Func<bool> CheckCanScrollToBottom;
#region 数据绑定
private ObservableCollection<T> virtualData;
/// <summary>
/// 虚拟数据
/// </summary>
public ObservableCollection<T> VirtualData
{
get { return virtualData; }
protected set
{
virtualData = value;
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(nameof(VirtualData)));
}
}
}
#endregion
#region 配置参数
/// <summary>
/// 初始化时最多加载的数据量
/// <para>需要保证:如果数据未完全加载,ListBox一定可以出现滚动条</para>
/// </summary>
[DefaultValue(20)]
public int InitLoadCount { get; set; }
/// <summary>
/// 递增的数量值
/// <para>滚动条滚动到两端时，每次自动加载的数据量</para>
/// <para>子项数量超过容器的最大数量<paramref name="MaxCount"/>时，自动减少的数量</para>
/// </summary>
[DefaultValue(20)]
public int IncreasingCount { get; set; }
/// <summary>
/// 子项的最大数量
/// </summary>
[DefaultValue(60)]
public int MaxCount { get; set; }
#endregion
/// <summary>
/// 当前显示的虚拟数据起始索引
/// </summary>
protected int StartVirtualIndex { get; set; }
/// <summary>
/// 当前显示的虚拟数据的终止索引
/// </summary>
protected int EndVirtualIndex { get; set; }
/// <summary>
/// 忽略滚动条滚动事件
/// </summary>
protected bool IgnoreBarChanged { get; set; }
public VirtualDataForListBox(ListBox listBox, ObservableCollection<T> sources)
{
if (listBox == null || sources == null)
throw new ArgumentException(" listBox or sources is null ");
this.delay = new DelayHelper(25, DelayLayout);
this.Inited = false;
this.Offset = 0;
this.listBox = listBox;
this.sources = sources;
this.InitLoadCount = 20;
this.IncreasingCount = 20;
this.MaxCount = 60;
this.EndVirtualIndex = -1;
this.StartVirtualIndex = -1;
this.VirtualData = new ObservableCollection<T>();
}
/// <summary>
/// 初始化
/// </summary>
public void Init()
{
if (this.Inited)
return;
if (this.listBox == null)
{
LogHelper.Warning("数据虚拟化-初始化失败");
return;
}
// 监控滚动条
this.bar = this.listBox.GetFirstChildT<ScrollBar, ListBoxItem>(t => t.Orientation == Orientation.Vertical);
this.viewer = this.listBox.GetFirstChildT<ScrollViewer, ListBoxItem>(null);
if (this.bar == null || this.viewer == null)
{
LogHelper.Warning("数据虚拟化-初始化失败");
return;
}
// 绑定数据源
this.listBox.SetBinding(ListBox.ItemsSourceProperty, new Binding(nameof(this.VirtualData)) { Source = this, });
this.ReloadEndData();
// 监控滚动条
this.bar.ValueChanged += Bar_ValueChanged;
// 监控滚动视图
this.viewer.LayoutUpdated += Viewer_LayoutUpdated;
// 监控数据源
this.sources.CollectionChanged += Sources_CollectionChanged;
Inited = true;
}
private void Viewer_LayoutUpdated(object sender, EventArgs e)
{
if (!this.Inited)
return;
Console.WriteLine(" Viewer_LayoutUpdated ");
if (this.Offset == 0 || this.IgnoreBarChanged)
return;
this.delay.DelayAction();
}
private void DelayLayout()
{
if (!this.Inited)
return;
var view = new ViewDecorate(this.viewer);
view.DispatcherAction(() =>
{
if (this.Offset == 0)
return;
try
{
this.IgnoreBarChanged = true;
double temp = 0;
// 向上
if (this.OffsetDirection)
{
for (int i = 0; i < this.OffsetCount && i < this.VirtualData.Count; i++)
{
temp += (this.listBox.ItemContainerGenerator.ContainerFromIndex(i) as ListBoxItem).ActualHeight;
}
}
this.viewer.ScrollToVerticalOffset(this.Offset + temp);
Console.WriteLine(" Viewer_LayoutUpdated ----------------------- Over ");
}
finally
{
this.Offset = 0;
this.IgnoreBarChanged = false;
}
});
}
/// <summary>
/// 滚动条滚动
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void Bar_ValueChanged(object sender, System.Windows.RoutedPropertyChangedEventArgs<double> e)
{
if (!this.Inited)
return;
if (this.IgnoreBarChanged || this.Offset != 0)
{
e.Handled = true;
return;
}
try
{
this.IgnoreBarChanged = true;
const int count = 100;
// 向下滚动到端部
if (e.NewValue > e.OldValue && e.NewValue + count >= this.bar.Maximum)
{
TryScrollDown(e.NewValue - e.OldValue);
}
// 向上滚动到端部
else if (e.NewValue < e.OldValue && e.NewValue - count <= 0)
{
TryScrollUp(e.OldValue - e.NewValue);
}
}
finally
{
e.Handled = true;
this.IgnoreBarChanged = false;
}
}
/// <summary>
/// 数据源发生变化
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void Sources_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
if (!this.Inited)
return;
if (e.Action == NotifyCollectionChangedAction.Add)
{
// 新消息到达、尝试将滚动条滚动到底部
this.MoveToBottom();
}
else if (e.Action == NotifyCollectionChangedAction.Remove)
{
this.IgnoreBarChanged = true;
// 移除旧数据
foreach (var item in e.OldItems)
{
if (item is T)
this.VirtualData.Remove(item as T);
}
this.ReCalIndex();
if (this.StartVirtualIndex == -1 || this.EndVirtualIndex == -1)
{
this.ReloadEndData();
}
else
{
if (this.VirtualData.Count < this.InitLoadCount)
{
// 数量过少、尝试填充数据
this.LoadMoreData();
}
}
this.IgnoreBarChanged = false;
}
// 撤回消息
else if (e.Action == NotifyCollectionChangedAction.Replace)
{
if (e.OldItems != null && e.OldItems.Count == 1 && e.NewItems != null && e.NewItems.Count == 1)
{
var oldT = e.OldItems[0] as T;
var newT = e.NewItems[0] as T;
int index = this.VirtualData.IndexOf(oldT);
if (index > -1)
{
this.VirtualData[index] = newT;
}
}
}
else if (e.Action == NotifyCollectionChangedAction.Reset)
{
this.IgnoreBarChanged = true;
this.ReloadEndData();
this.IgnoreBarChanged = false;
}
}
/// <summary>
/// 将视图移动到某个索引的位置
/// </summary>
/// <param name="index"></param>
public void MoveToIndex(int index)
{
if (!this.Inited)
return;
if (index < 0 || index >= this.sources.Count)
return;
var t = this.sources[index];
if (this.VirtualData.IndexOf(t) > -1)
{
listBox.ScrollIntoView(t);
return;
}
int start = index - this.InitLoadCount;
if (start < 0)
start = 0;
int end = index + this.InitLoadCount;
if (end >= this.sources.Count)
end = this.sources.Count - 1;
int count = end - start + 1;
if (count == 0)
return;
try
{
this.IgnoreBarChanged = true;
var list = this.sources.Skip(start).Take(count);
this.VirtualData.Clear();
foreach (var item in list)
{
this.VirtualData.Add(item);
}
this.ReCalIndex();
listBox.ScrollIntoView(t);
}
finally
{
this.IgnoreBarChanged = false;
}
}
/// <summary>
/// 将视图移动到底部
/// </summary>
public void MoveToBottom()
{
if (!this.Inited)
return;
try
{
this.IgnoreBarChanged = true;
// 询问是否可以将滚动条滚动到底部
if (this.CheckCanScrollToBottom != null && !this.CheckCanScrollToBottom())
return;
// 超过最大显示容量、则重新加载末端数据
if (this.StartVirtualIndex == -1 || this.sources.Count == 0 || this.sources.Count - this.StartVirtualIndex > this.MaxCount)
{
this.ReloadEndData();
return;
}
// 没有需要加载的数据
if (this.EndVirtualIndex == this.sources.Count - 1)
{
this.listBox.ScrollViewToBottom();
return;
}
// 平滑加载
var count = this.EndVirtualIndex + 1;
if (this.sources.Count > count)
{
var list = this.sources.Skip(count).ToList();
foreach (var item in list)
{
this.VirtualData.Add(item);
}
this.ReCalIndex();
this.listBox.ScrollViewToBottom();
}
}
catch (Exception ex)
{
LogHelper.Execption(ex, "数据虚拟化");
}
finally
{
this.IgnoreBarChanged = false;
}
}
/// <summary>
/// 重新计算索引值
/// </summary>
private void ReCalIndex()
{
if (this.VirtualData.Count > 0)
{
this.StartVirtualIndex = this.sources.IndexOf(this.VirtualData[0]);
this.EndVirtualIndex = this.sources.IndexOf(this.VirtualData[this.VirtualData.Count - 1]);
if (this.StartVirtualIndex == -1 || this.EndVirtualIndex == -1 || this.EndVirtualIndex < this.StartVirtualIndex)
{
this.StartVirtualIndex = -1;
this.EndVirtualIndex = -1;
LogHelper.Warning("数据虚拟化-逻辑错误");
}
}
else
{
this.StartVirtualIndex = -1;
this.EndVirtualIndex = -1;
}
}
/// <summary>
/// 重新初始化数据
/// </summary>
private void ReloadEndData()
{
if (this.VirtualData.Count > 0)
{
this.VirtualData.Clear();
this.EndVirtualIndex = -1;
this.StartVirtualIndex = -1;
}
if (this.sources != null && this.sources.Count > 0)
{
var list = this.sources.ListLastMaxCount(this.InitLoadCount);
if (list.Count > 0)
{
foreach (var item in list)
{
this.VirtualData.Add(item);
}
this.ReCalIndex();
// 滚动条滚动到最底部
this.listBox.ScrollViewToBottom();
}
}
}
/// <summary>
/// 删除数据时加载更多数据
/// </summary>
private void LoadMoreData()
{
List<T> data = this.sources.ListFindRangeWithMaxCount(this.StartVirtualIndex, this.InitLoadCount);
if (data.Count <= this.VirtualData.Count)
{
// 没有加载到更多数据
return;
}
int start = data.IndexOf(this.VirtualData[0]);
int end = data.LastIndexOf(this.VirtualData[this.VirtualData.Count - 1]);
if (start == -1 || end == -1 || end < start)
{
LogHelper.Warning("数据虚拟化-逻辑错误");
return;
}
for (int i = 0; i < data.Count; i++)
{
if (i < start)
{
this.VirtualData.Insert(i, data[i]);
}
else if (i > end)
{
this.VirtualData.Add(data[i]);
}
}
this.ReCalIndex();
}
/// <summary>
/// 向上滚动
/// </summary>
private void TryScrollUp(double offset)
{
// 没有数据了
if (this.StartVirtualIndex == -1 || this.StartVirtualIndex == 0)
return;
double tempOffset = this.viewer.ContentVerticalOffset;
// 释放捕获的鼠标
this.bar.Track.Thumb.ReleaseMouseCapture();
this.bar.Track.DecreaseRepeatButton.ReleaseMouseCapture();
int tempCount = 0;
var list = this.sources.ListLastMaxCount(this.StartVirtualIndex, this.IncreasingCount, false);
// list 为反序结果
foreach (var item in list)
{
this.VirtualData.Insert(0, item);
tempCount++;
}
if (this.VirtualData.Count > this.MaxCount)
{
for (int i = 0; i < this.IncreasingCount; i++)
{
this.VirtualData.RemoveAt(this.VirtualData.Count - 1);
}
}
this.ReCalIndex();
this.OffsetDirection = true;
this.OffsetCount = tempCount;
this.Offset = tempOffset - offset;
if (this.Offset == 0)
this.Offset = 1;
}
/// <summary>
/// 向下滚动
/// </summary>
private void TryScrollDown(double offest)
{
// 没有数据了
if (this.EndVirtualIndex == -1 || this.EndVirtualIndex == this.sources.Count - 1)
return;
// 释放捕获的鼠标
this.bar.Track.Thumb.ReleaseMouseCapture();
this.bar.Track.IncreaseRepeatButton.ReleaseMouseCapture();
double tempOffset = this.viewer.ContentVerticalOffset;
var list = this.sources.Skip(this.EndVirtualIndex + 1).Take(this.IncreasingCount);
foreach (var item in list)
{
this.VirtualData.Add(item);
}
if (this.VirtualData.Count > this.MaxCount)
{
for (int i = 0; i < this.IncreasingCount; i++)
{
tempOffset -= (this.listBox.ItemContainerGenerator.ContainerFromIndex(0) as ListBoxItem).ActualHeight;
this.VirtualData.RemoveAt(0);
}
}
this.ReCalIndex();
this.OffsetDirection = false;
this.OffsetCount = 0;
this.Offset = tempOffset + offest;
if (this.Offset == 0)
this.Offset = 1;
}
public void Dispose()
{
if (!this.Inited)
return;
this.Inited = false;
this.VirtualData.Clear();
// 监控滚动条
this.bar.ValueChanged -= Bar_ValueChanged;
// 监控滚动视图
this.viewer.LayoutUpdated -= Viewer_LayoutUpdated;
// 监控数据源
this.sources.CollectionChanged -= Sources_CollectionChanged;
this.CheckCanScrollToBottom = null;
this.delay.Dispose();
}
}
该处理方式相当于根据滚动条的滚动适时增减items，当然该类的应用有一定的局限性，不过操作滚动条的方式还是具有借鉴意义的。
源码出处不详。
三、补充
启用UI虚拟化的两个附加属性：
1、ScrollViewer.CanContentScroll="True"
2、VirtualizingStackPanel.IsVirtualizing="True"