问题
I've found a couple of alternatives, this CodePlex project and this commercial one, but the former is extremely inefficient in my testing, and the latter, well, costs $$. I've also found this implementation, but it's not so much a WrapPanel as a "TilePanel", and doesn't support automatically-sized items (though it does appear to be fast).
I've also found some "hints" on how to write your own here, but as the developer developed it on company time, he couldn't post his full source.
Has anyone taken the time to implement a decent VirtualizingWrapPanel and posted it publically, or am I going to have to take a day and write my own?
回答1:
I've been seeking for the answer lately, and this is what I end up with, hope it helps:
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Media;
namespace Wpf.Controls
{
// 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 VirtualizingWrapPanel : 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(VirtualizingWrapPanel), new PropertyMetadata(1.0, HandleItemDimensionChanged));
public static readonly DependencyProperty ItemHeightProperty =
DependencyProperty.Register("ItemHeight", typeof(double), typeof(VirtualizingWrapPanel), new PropertyMetadata(1.0, HandleItemDimensionChanged));
private static readonly DependencyProperty VirtualItemIndexProperty =
DependencyProperty.RegisterAttached("VirtualItemIndex", typeof(int), typeof(VirtualizingWrapPanel), 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 VirtualizingWrapPanel()
{
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 VirtualizingWrapPanel);
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;
}
}
}
回答2:
We have used this one with some success: http://virtualwrappanel.codeplex.com
There is an issue with removing items from the collection, but you can resolved it with a small code tweak: http://virtualwrappanel.codeplex.com/workitem/1
来源：https://stackoverflow.com/questions/3485821/is-there-a-good-free-virtualizingwrappanel-available-for-wpf