面试被问到了，应该是和Android的RecycleView一样的东西，对长列表，只渲染需要显的部分。作用于ListItem和TreeItem。
怎么实现没看明白，留份样板备用。
源码在这里：WPFControls
public class VirtualizingWrapPanel : VirtualizingPanel, IScrollInfo
{
private TranslateTransform trans = new TranslateTransform();
public VirtualizingWrapPanel()
{
this.RenderTransform = trans;
}
#region DependencyProperties
public static readonly DependencyProperty ChildWidthProperty = DependencyProperty.RegisterAttached("ChildWidth", typeof(double), typeof(VirtualizingWrapPanel), new FrameworkPropertyMetadata(200.0, FrameworkPropertyMetadataOptions.AffectsMeasure | FrameworkPropertyMetadataOptions.AffectsArrange));
public static readonly DependencyProperty ChildHeightProperty = DependencyProperty.RegisterAttached("ChildHeight", typeof(double), typeof(VirtualizingWrapPanel), new FrameworkPropertyMetadata(200.0, FrameworkPropertyMetadataOptions.AffectsMeasure | FrameworkPropertyMetadataOptions.AffectsArrange));
//鼠标每一次滚动 UI上的偏移
public static readonly DependencyProperty ScrollOffsetProperty = DependencyProperty.RegisterAttached("ScrollOffset", typeof(int), typeof(VirtualizingWrapPanel), new PropertyMetadata(10));
public int ScrollOffset
{
get { return Convert.ToInt32(GetValue(ScrollOffsetProperty)); }
set { SetValue(ScrollOffsetProperty, value); }
}
public double ChildWidth
{
get => Convert.ToDouble(GetValue(ChildWidthProperty));
set => SetValue(ChildWidthProperty, value);
}
public double ChildHeight
{
get => Convert.ToDouble(GetValue(ChildHeightProperty));
set => SetValue(ChildHeightProperty, value);
}
#endregion
int GetItemCount(DependencyObject element)
{
var itemsControl = ItemsControl.GetItemsOwner(element);
return itemsControl.HasItems ? itemsControl.Items.Count : 0;
}
int CalculateChildrenPerRow(Size availableSize)
{
int childPerRow = 0;
if (availableSize.Width == double.PositiveInfinity)
childPerRow = this.Children.Count;
else
childPerRow = Math.Max(1, Convert.ToInt32(Math.Floor(availableSize.Width / this.ChildWidth)));
return childPerRow;
}
/// <summary>
/// width不超过availableSize的情况下，自身实际需要的Size(高度可能会超出availableSize)
/// </summary>
/// <param name="availableSize"></param>
/// <param name="itemsCount"></param>
/// <returns></returns>
Size CalculateExtent(Size availableSize, int itemsCount)
{
int childPerRow = CalculateChildrenPerRow(availableSize);//现有宽度下 一行可以最多容纳多少个
return new Size(childPerRow * this.ChildWidth, this.ChildHeight * Math.Ceiling(Convert.ToDouble(itemsCount) / childPerRow));
}
/// <summary>
/// 更新滚动条
/// </summary>
/// <param name="availableSize"></param>
void UpdateScrollInfo(Size availableSize)
{
var extent = CalculateExtent(availableSize, GetItemCount(this));//extent 自己实际需要
if (extent != this.extent)
{
this.extent = extent;
this.ScrollOwner.InvalidateScrollInfo();
}
if (availableSize != this.viewPort)
{
this.viewPort = availableSize;
this.ScrollOwner.InvalidateScrollInfo();
}
}
/// <summary>
/// 获取所有item，在可视区域内第一个item和最后一个item的索引
/// </summary>
/// <param name="firstIndex"></param>
/// <param name="lastIndex"></param>
void GetVisiableRange(ref int firstIndex, ref int lastIndex)
{
int childPerRow = CalculateChildrenPerRow(this.extent);
firstIndex = Convert.ToInt32(Math.Floor(this.offset.Y / this.ChildHeight)) * childPerRow;
lastIndex = Convert.ToInt32(Math.Ceiling((this.offset.Y + this.viewPort.Height) / this.ChildHeight)) * childPerRow - 1;
int itemsCount = GetItemCount(this);
if (lastIndex >= itemsCount)
lastIndex = itemsCount - 1;
}
/// <summary>
/// 将不在可视区域内的item 移除
/// </summary>
/// <param name="startIndex">可视区域开始索引</param>
/// <param name="endIndex">可视区域结束索引</param>
void CleanUpItems(int startIndex, int endIndex)
{
var children = this.InternalChildren;
var generator = this.ItemContainerGenerator;
for (int i = children.Count - 1; i >= 0; i--)
{
var childGeneratorPosi = new GeneratorPosition(i, 0);
int itemIndex = generator.IndexFromGeneratorPosition(childGeneratorPosi);
if (itemIndex < startIndex || itemIndex > endIndex)
{
generator.Remove(childGeneratorPosi, 1);
RemoveInternalChildRange(i, 1);
}
}
}
/// <summary>
/// scroll/availableSize/添加删除元素 改变都会触发 edit元素不会改变
/// </summary>
/// <param name="availableSize"></param>
/// <returns></returns>
protected override Size MeasureOverride(Size availableSize)
{
this.UpdateScrollInfo(availableSize);//availableSize更新后，更新滚动条
int firstVisiableIndex = 0, lastVisiableIndex = 0;
GetVisiableRange(ref firstVisiableIndex, ref lastVisiableIndex);//availableSize更新后，获取当前viewport内可放置的item的开始和结束索引 firstIdnex-lastIndex之间的item可能部分在viewport中也可能都不在viewport中。
UIElementCollection children = this.InternalChildren;//因为配置了虚拟化，所以children的个数一直是viewport区域内的个数,如果没有虚拟化则是ItemSource的整个的个数
IItemContainerGenerator generator = this.ItemContainerGenerator;
//获得第一个可被显示的item的位置
GeneratorPosition startPosi = generator.GeneratorPositionFromIndex(firstVisiableIndex);
int childIndex = (startPosi.Offset == 0) ? startPosi.Index : startPosi.Index + 1;//startPosi在chilren中的索引
using (generator.StartAt(startPosi, GeneratorDirection.Forward, true))
{
int itemIndex = firstVisiableIndex;
while (itemIndex <= lastVisiableIndex)//生成lastVisiableIndex-firstVisiableIndex个item
{
bool newlyRealized = false;
var child = generator.GenerateNext(out newlyRealized) as UIElement;
if (newlyRealized)
{
if (childIndex >= children.Count)
base.AddInternalChild(child);
else
{
base.InsertInternalChild(childIndex, child);
}
generator.PrepareItemContainer(child);
}
else
{
//处理 正在显示的child被移除了这种情况
if (!child.Equals(children[childIndex]))
{
base.RemoveInternalChildRange(childIndex, 1);
}
}
child.Measure(new Size(this.ChildWidth, this.ChildHeight));
//child.DesiredSize//child想要的size
itemIndex++;
childIndex++;
}
}
CleanUpItems(firstVisiableIndex, lastVisiableIndex);
return new Size(double.IsInfinity(availableSize.Width) ? 0 : availableSize.Width, double.IsInfinity(availableSize.Height) ? 0 : availableSize.Height);//自身想要的size
}
protected override Size ArrangeOverride(Size finalSize)
{
Debug.WriteLine("----ArrangeOverride");
var generator = this.ItemContainerGenerator;
UpdateScrollInfo(finalSize);
int childPerRow = CalculateChildrenPerRow(finalSize);
double availableItemWidth = finalSize.Width / childPerRow;
for (int i = 0; i <= this.Children.Count - 1; i++)
{
var child = this.Children[i];
int itemIndex = generator.IndexFromGeneratorPosition(new GeneratorPosition(i, 0));
int row = itemIndex / childPerRow;//current row
int column = itemIndex % childPerRow;
double xCorrdForItem = 0;
xCorrdForItem = column * availableItemWidth + (availableItemWidth - this.ChildWidth) / 2;
Rect rec = new Rect(xCorrdForItem, row * this.ChildHeight, this.ChildWidth, this.ChildHeight);
child.Arrange(rec);
}
return finalSize;
}
protected override void OnRenderSizeChanged(SizeChangedInfo sizeInfo)
{
base.OnRenderSizeChanged(sizeInfo);
this.SetVerticalOffset(this.VerticalOffset);
}
protected override void OnClearChildren()
{
base.OnClearChildren();
this.SetVerticalOffset(0);
}
protected override void BringIndexIntoView(int index)
{
if (index < 0 || index >= Children.Count)
throw new ArgumentOutOfRangeException();
int row = index / CalculateChildrenPerRow(RenderSize);
SetVerticalOffset(row * this.ChildHeight);
}
#region IScrollInfo Interface
public bool CanVerticallyScroll { get; set; }
public bool CanHorizontallyScroll { get; set; }
private Size extent = new Size(0, 0);
public double ExtentWidth => this.extent.Width;
public double ExtentHeight => this.extent.Height;
private Size viewPort = new Size(0, 0);
public double ViewportWidth => this.viewPort.Width;
public double ViewportHeight => this.viewPort.Height;
private Point offset;
public double HorizontalOffset => this.offset.X;
public double VerticalOffset => this.offset.Y;
public ScrollViewer ScrollOwner { get; set; }
public void LineDown()
{
this.SetVerticalOffset(this.VerticalOffset + this.ScrollOffset);
}
public void LineLeft()
{
throw new NotImplementedException();
}
public void LineRight()
{
throw new NotImplementedException();
}
public void LineUp()
{
this.SetVerticalOffset(this.VerticalOffset - this.ScrollOffset);
}
public Rect MakeVisible(Visual visual, Rect rectangle)
{
return new Rect();
}
public void MouseWheelDown()
{
this.SetVerticalOffset(this.VerticalOffset + this.ScrollOffset);
}
public void MouseWheelLeft()
{
throw new NotImplementedException();
}
public void MouseWheelRight()
{
throw new NotImplementedException();
}
public void MouseWheelUp()
{
this.SetVerticalOffset(this.VerticalOffset - this.ScrollOffset);
}
public void PageDown()
{
this.SetVerticalOffset(this.VerticalOffset + this.viewPort.Height);
}
public void PageLeft()
{
throw new NotImplementedException();
}
public void PageRight()
{
throw new NotImplementedException();
}
public void PageUp()
{
this.SetVerticalOffset(this.VerticalOffset - this.viewPort.Height);
}
public void SetHorizontalOffset(double offset)
{
throw new NotImplementedException();
}
public void SetVerticalOffset(double offset)
{
if (offset < 0 || this.viewPort.Height >= this.extent.Height)
offset = 0;
else
if (offset + this.viewPort.Height >= this.extent.Height)
offset = this.extent.Height - this.viewPort.Height;
this.offset.Y = offset;
this.ScrollOwner?.InvalidateScrollInfo();
this.trans.Y = -offset;
this.InvalidateMeasure();
//接下来会触发MeasureOverride()
}
#endregion
}