WPF中ItemsControl應用虛擬化時找到子元素的方法 · WPF編程之路

wpf的虛擬化技術會使UI的控件只初始化看的到的子元素，而不是所有子元素都被初始化，這樣會提高UI性能。但是我們經常會遇到一個問題：應用虛擬化后看不見的子元素因為沒有實際產生導致ItemContainerGenerator的查找元素方法（ContainerFromIndex /?ContainerFromItem）失效。解決辦法1：（1）監聽ItemsControl的ItemContainerGenerator的StatusChanged事件，當GeneratorStatus為ContainerGenerated時再進行查找，（2）遍歷ItemsControl的Items，獲取當前需要查找元素的Index，（3）利用反射調用VirtualizingPanel的BringIndexIntoView，或者直接調用BringIntoView，然后強制滾動以產生Item（具體可以參考TreeViewItem的ExpandRecursicve的內部實現）。需要注意的是：（1）ItemContainerGenerator的StatuChanged事件會多次被觸發，原因是每次初始化的Item數量就是當前空間所能看到的數量，StatusChanged觸發的次數就是總共Items除以每次初始的Item數量。（2）調用BringIndexInToView不正確會導致InvalidOperationException，具體為“Cannot call StartAt when content generation is in progress.” ?或者 ”無法在正在進行內容生成時調用StartAt。”。可以用Dispatcher.BeginInvoke來解決，如下面代碼。（3）當然ItemsControl中的虛擬化模式設置為Recycling，即?VirtualizingStackPanel.VirtualizationMode?="Recycling"時，后端存儲的子元素選中項會在ItermContainerGenerator重新產生子項時變為DisconnectedItem。可以把模式設置為Standard解決。具體代碼如下： - 1. 查找入口? ???????private?ItemsControl?_currentSelectedItem?=?null; ???????private?void?BtnFind_Click(?object?sender?,?System.?Windows.RoutedEventArgs?e) ?????? { ???????????if?(string?.IsNullOrEmpty(?txtContent.Text?)) ?????????? { ???????????????return; ?????????? } ???????????if?(_currentSelectedItem?==?null) ?????????? { ???????????????_currentSelectedItem?=?_treeView?; ?????????? } ???????????else ?????????? { ???????????????if?(_currentSelectedItem?is?TreeViewItem) ?????????????? { ?????????????????? (?_currentSelectedItem?as?TreeViewItem).?IsExpanded?=?true?; ?????????????? } ?????????? } ???????????if?(_currentSelectedItem?.ItemContainerGenerator.?Status?!=?GeneratorStatus?.ContainersGenerated) ?????????? { ???????????????_currentSelectedItem.ItemContainerGenerator?.StatusChanged?-=?new?EventHandler(ItemContainerGenerator_StatusChanged?); ???????????????_currentSelectedItem.ItemContainerGenerator?.StatusChanged?+=?new?EventHandler(ItemContainerGenerator_StatusChanged?); ?????????? } ???????????else ?????????? { ???????????????treeViewItem_BringIntoView(txtContent?.Text); ?????????? } ?????? } - 2.StatusChanged事件的處理 ????void?ItemContainerGenerator_StatusChanged?(object?sender,?EventArgs?e) ?????? { ???????????var?generator?=?sender?as?ItemContainerGenerator?; ???????????if?(null?==?generator) ?????????? { ???????????????return; ?????????? } ???????????//once the children have been generated, expand those children's children then remove the event handler ???????????if?(generator?.Status?==?GeneratorStatus.ContainersGenerated?&&?_currentSelectedItem?.ItemContainerGenerator.?Status?==?GeneratorStatus?.ContainersGenerated) ?????????? { ???????????????treeViewItem_BringIntoView(txtContent?.Text); ?????????? } ?????? } - 3.具體虛擬化時的強制產生子元素及查找處理 private?void?treeViewItem_BringIntoView(string?findItem) ?????? { ?????????? System.Diagnostics.?Debug.WriteLine("enter treeViewItem_BringIntoview"?); ???????????try ?????????? { ?????????????? _currentSelectedItem.ApplyTemplate(); ???????????????ItemsPresenter?itemsPresenter = (ItemsPresenter)_currentSelectedItem.Template.FindName("ItemsHost", (FrameworkElement)_currentSelectedItem); ???????????????if?(itemsPresenter !=?null?) ?????????????????? itemsPresenter.ApplyTemplate(); ???????????????else ?????????????????? _currentSelectedItem.UpdateLayout(); ???????????????VirtualizingPanel?virtualizingPanel = _currentSelectedItem.GetItemsHost()?as?VirtualizingPanel; ?????????????? virtualizingPanel.CallEnsureGenerator(); ???????????????int?selectedIndex = -1; ???????????????int?count1 = _currentSelectedItem.Items.Count; ???????????????for?(int?i = 0; i < count1; i++) ?????????????? { ???????????????????ItemsItem1?tviItem = _currentSelectedItem.Items.GetItemAt(i)?as?ItemsItem1; ???????????????????if?(null?!= tviItem && tviItem.Label.Equals(findItem)) ?????????????????? { ?????????????????????? selectedIndex = i; ???????????????????????break; ?????????????????? } ?????????????? } ???????????????if?(selectedIndex < 0) ?????????????? { ???????????????????return; ?????????????? } ???????????????Action?action = () => ?????????????? { ???????????????????TreeViewItem?itemSelected =?null?; ???????????????????//Force to generate every treeView item by using scroll item ???????????????????if?(virtualizingPanel !=?null?) ?????????????????? { ???????????????????????try ?????????????????????? { ?????????????????????????? virtualizingPanel.CallBringIndexIntoView(selectedIndex); ?????????????????????? } ???????????????????????catch?(System.Exception?ex) ?????????????????????? { ?????????????????????????? System.Diagnostics.?Debug.WriteLine("CallBringIndexIntoView exception : "?+ ex.Message); ?????????????????????? } ?????????????????????? itemSelected = (TreeViewItem)_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(selectedIndex); ?????????????????? } ???????????????????else ?????????????????? { ?????????????????????? itemSelected = (TreeViewItem)_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(selectedIndex); ?????????????????????? itemSelected.BringIntoView(); ?????????????????? } ???????????????????if?(null?!= itemSelected) ?????????????????? { ?????????????????????? _currentSelectedItem = itemSelected; ?????????????????????? (_currentSelectedItem?as?TreeViewItem?).IsSelected =?true; ?????????????????????? _currentSelectedItem.BringIntoView(); ?????????????????? } ?????????????? }; ?????????????? Dispatcher.BeginInvoke(?DispatcherPriority.Background, action); ?????????? } ???????????catch?(System.Exception?ex) ?????????? { ???????????????// ?????????? } ?????? } - 4.xaml代碼 <Window ???xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" ???xmlns:x?="http://schemas.microsoft.com/winfx/2006/xaml" ???xmlns:d?="http://schemas.microsoft.com/expression/blend/2008" ???xmlns:mc?="http://schemas.openxmlformats.org/markup-compatibility/2006" ???mc:Ignorable?="d" ???x:Class?="WpfApplication1.MainWindow" ???x:Name?="Window" ???Title="MainWindow" ???Width="640" ???Height="480" > ???<Window.Resources> ???????<HierarchicalDataTemplate ???????????x:Key?="ItemsItem1Template" ???????????ItemsSource="{Binding?Items}" > ???????????<StackPanel> ???????????????<TextBlock ???????????????????Text="{Binding?Label}" /> ???????????</StackPanel> ???????</HierarchicalDataTemplate> ???</Window.Resources> ???<Grid ???????x:Name?="LayoutRoot" > ???????<TreeView ???????????x:Name?="_treeView" ???????????HorizontalAlignment="Left" ???????????Width="287" ???????????d:DataContext?="{Binding}" ???????????ItemsSource="{Binding?Items,?Source?={StaticResource?SampleDataSource?}}" ???????????ItemTemplate="{DynamicResource?ItemsItem1Template}" ???????????VirtualizingStackPanel.IsVirtualizing?="True" ? ? ? ? ? ?VirtualizingStackPanel.VirtualizationMode?="Standard" ? ? ? ? ? ? /> ???????<Button ???????????x:Name?="btnFind" ???????????Content="Find" ???????????HorizontalAlignment="Right" ???????????Margin="0,8,8,0" ???????????VerticalAlignment="Top" ???????????Width="75" ???????????Click="BtnFind_Click" /> ???????<TextBox ???????????x:Name?="txtContent" ???????????Margin="291,8,87,0" ???????????TextWrapping="Wrap" ???????????VerticalAlignment="Top" ???????????Height="21.837" /> ???</Grid> </Window> - 5.反射系統控件私有方法的類 public?static?class?WPFUIElementExtension ?? { ?????? #region?Functions to get internal members using reflection ???????// Some functionality we need is hidden in internal members, so we use reflection to get them ?????? #region?ItemsControl.ItemsHost ???????static?readonly?PropertyInfo?ItemsHostPropertyInfo?=?typeof?(ItemsControl).?GetProperty("ItemsHost"?,?BindingFlags.Instance?|?BindingFlags.?NonPublic); ???????public?static?Panel?GetItemsHost(this?ItemsControl?itemsControl) ?????? { ???????????Debug.Assert?(itemsControl?!=?null); ???????????return?ItemsHostPropertyInfo?.GetValue(?itemsControl,?null?)?as?Panel; ?????? } ?????? #endregion?ItemsControl.ItemsHost ?????? #region?Panel.EnsureGenerator ???????private?static?readonly?MethodInfo?EnsureGeneratorMethodInfo?=?typeof(Panel?).GetMethod(?"EnsureGenerator",?BindingFlags?.Instance?|?BindingFlags.NonPublic?); ???????public?static?void?CallEnsureGenerator(this?Panel?panel) ?????? { ???????????Debug.Assert?(panel?!=?null); ???????????EnsureGeneratorMethodInfo.Invoke?(panel,?null); ?????? } ?????? #endregion?Panel.EnsureGenerator ?????? #region?VirtualizingPanel.?BringIndexIntoView ???????private?static?readonly?MethodInfo?BringIndexIntoViewMethodInfo?=?typeof(VirtualizingPanel?).GetMethod(?"BringIndexIntoView",?BindingFlags?.Instance?|?BindingFlags.NonPublic?); ???????public?static?void?CallBringIndexIntoView(this?VirtualizingPanel?virtualizingPanel,?int?index) ?????? { ???????????Debug.Assert?(virtualizingPanel?!=?null); ???????????BringIndexIntoViewMethodInfo.Invoke?(virtualizingPanel,?new?object?[] {?index?}); ?????? } ?????? #endregion?VirtualizingPanel.?BringIndexIntoView ?????? #endregion?Functions to get internal members using reflection ?? } 解決方法2：（1）參考方法1的第一步解決方法（2）遍歷ItemsControl的Items，根據ContainerFromIndex去找到當前可見的元素的index。（3）利用BringInoView去滾動現有的Item以便UI產生后續的子元素，然后循環直到找見要查找的子元素。（遍歷分為2部分，向前遍歷和向后遍歷）注意事項：（1）參考方法1的第一注意事項（2）因為比方法1多了一次循環遍歷，當items很多時有點卡頓，不過還在可以忍受的范圍。具體代碼： 1.參考方法1的代碼，具體只有強制生成子元素的方法有區別，即與方法1中的步驟3有區別。 2.如下： ?private?void?treeViewItem_BringIntoView2(string?findItem) ?????? { ?????????? System.Diagnostics.?Debug?.WriteLine("enter treeViewItem_BringIntoview"?); ???????????try ?????????? { ?????????????? _currentSelectedItem.ApplyTemplate(); ???????????????ItemsPresenter?itemsPresenter = (ItemsPresenter?)_currentSelectedItem.Template.FindName(?"ItemsHost", (FrameworkElement?)_currentSelectedItem); ???????????????if?(itemsPresenter !=?null?) ?????????????????? itemsPresenter.ApplyTemplate(); ???????????????else ?????????????????? _currentSelectedItem.UpdateLayout(); ???????????????VirtualizingPanel?virtualizingPanel = _currentSelectedItem.GetItemsHost()?as?VirtualizingPanel?; ?????????????? virtualizingPanel.CallEnsureGenerator(); ???????????????TreeViewItem?itemTemp =?null?; ???????????????ItemsItem1?objTemp =?null?; ???????????????int?visiableIndex = -1; ???????????????int?findIndex = -1; ???????????????int?count1 = _currentSelectedItem.Items.Count; ???????????????for?(int?i = 0; i < count1; i++) ?????????????? { ?????????????????? itemTemp = (?TreeViewItem?)_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(i); ???????????????????if?(null?!= itemTemp) ?????????????????? { ?????????????????????? visiableIndex = i; ?????????????????? } ?????????????????? objTemp = _currentSelectedItem.Items.GetItemAt(i)?as?ItemsItem1?; ???????????????????if?(null?!= objTemp && objTemp.Label.Equals(findItem)) ?????????????????? { ?????????????????????? findIndex = i; ?????????????????? } ?????????????? } ???????????????if?(findIndex == -1 || visiableIndex == -1) ?????????????? { ???????????????????return?; ?????????????? } ???????????????if?(findIndex < visiableIndex) ?????????????? { ???????????????????for?(int?j = visiableIndex; j >= findIndex; j--) ?????????????????? { ?????????????????????? itemTemp = (TreeViewItem?)_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(j); ???????????????????????if?(null?!= itemTemp) ?????????????????????? { ?????????????????????????? itemTemp.BringIntoView(); ?????????????????????? } ?????????????????? } ?????????????? } ???????????????else?if?(findIndex > visiableIndex) ?????????????? { ???????????????????for?(int?j = visiableIndex; j <= findIndex; j++) ?????????????????? { ?????????????????????? itemTemp = (TreeViewItem?)_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(j); ???????????????????????if?(null?!= itemTemp) ?????????????????????? { ?????????????????????????? itemTemp.BringIntoView(); ?????????????????????? } ?????????????????? } ?????????????? } ???????????????else ?????????????? { ?????????????????? itemTemp = (TreeViewItem?)_currentSelectedItem.ItemContainerGenerator.ContainerFromIndex(visiableIndex); ???????????????????if?(null?!= itemTemp) ?????????????????? { ?????????????????????? itemTemp.BringIntoView(); ?????????????????? } ?????????????? } ???????????????if?(null?!= itemTemp) ?????????????? { ?????????????????? _currentSelectedItem = itemTemp; ?????????????????? (_currentSelectedItem?as?TreeViewItem?).IsSelected =?true; ?????????????????? _currentSelectedItem.BringIntoView(); ?????????????? } ?????????? } ???????????catch?(System.Exception?ex) ?????????? { ???????????????// ?????????? } ?????? }