如果在拥有不同大小项的TreeView
中启用了虚拟化,则会出现多个问题:
垂直滚动条会随机更改其大小,并且在查看整个树后不会记住元素的大小,使用鼠标滚动很困难。
上下滚动一段时间后,框架代码会抛出
ArgumentNullException
异常。
复现很简单:创建一个新的WPF应用程序,然后将此代码放入 MainWindow.xaml 中。
<Window x:Class="VirtualTreeView.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="800" Width="400" Left="0" Top="0"
DataContext="{Binding RelativeSource={RelativeSource Self}}">
<Grid>
<TreeView x:Name="tvwItems" ItemsSource="{Binding Items}"
VirtualizingPanel.IsVirtualizing="True" VirtualizingPanel.VirtualizationMode="Recycling">
<TreeView.ItemTemplate>
<DataTemplate>
<Border Height="{Binding Height}" Width="{Binding Height}"
BorderThickness="1" Background="DarkGray" BorderBrush="DarkBlue"/>
</DataTemplate>
</TreeView.ItemTemplate>
</TreeView>
</Grid>
</Window>
将此代码复制粘贴到MainWindow.xaml.cs中。
using System.Collections.ObjectModel;
using System.Linq;
namespace VirtualTreeView
{
public partial class MainWindow
{
public ObservableCollection<Item> Items { get; set; }
public MainWindow ()
{
Items = new ObservableCollection<Item>(Enumerable.Range(0, 20).Select(i => new Item {
Height = i*20,
}));
InitializeComponent();
}
}
public class Item
{
public double Height { get; set; }
}
}
当应用程序运行时,将鼠标指针移动到树形视图中,使用鼠标滚轮向下滚动到底部,然后向上滚动,再开始向下滚动。在中间的某个位置,会抛出以下异常:
System.ArgumentNullException was unhandled
HResult=-2147467261
Message=Value cannot be null.
Parameter name: element
Source=PresentationCore
ParamName=element
StackTrace:
at MS.Internal.Media.VisualTreeUtils.AsNonNullVisual(DependencyObject element, Visual& visual, Visual3D& visual3D)
at System.Windows.Media.VisualTreeHelper.GetParent(DependencyObject reference)
at System.Windows.Controls.VirtualizingStackPanel.FindScrollOffset(Visual v)
at System.Windows.Controls.VirtualizingStackPanel.OnAnchorOperation(Boolean isAnchorOperationPending)
at System.Windows.Controls.VirtualizingStackPanel.OnAnchorOperation()
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.DispatcherOperation.InvokeImpl()
at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Windows.Threading.DispatcherOperation.Invoke()
at System.Windows.Threading.Dispatcher.ProcessQueue()
at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
at System.Windows.Threading.Dispatcher.Run()
at System.Windows.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at System.Windows.Application.Run(Window window)
at System.Windows.Application.Run()
at VirtualTreeView.App.Main() in d:\Docs\Projects\_Try\VirtualTreeView\obj\Debug\App.g.cs:line 0
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
您还可以看到,异常不是唯一的问题。在向上和向下滚动时,滚动条的大小不断改变。(在ListBox
中不会出现相同的问题,因为它无法预测大小,但在查看整个列表后会记住总高度。)
问题:如何使滚动条正常工作并消除异常?(我不介意链接到替代 TreeView
控件或支持此场景的虚拟面板。)