问题
I am trying to create a User Control that represents what I am calling a workspace (a reference from a blog by Josh Smith). The workspaces will be displayed in a tab control. I am aiming to use a tabbed interface to manage various documents that I have open much like in a browser of an excal work book.
Each time a user opens a new workspace, that workspace should be displayed in the tab control. Each Workspace takes the form of a user control, and each workspace has its own view model. I would like for the Tab Header to display a property from my view model which I think will likely have to be exposed as a property through my user control.
So far, the cleanest solution that I liked the best until I ran into numerous issues was by using datatemplates. Basically I Did the following:
<DataTemplate x:Key="WorkspaceItem">
<DockPanel Width="120">
<ContentPresenter
Content="{Binding Title}"
VerticalAlignment="Center"
/>
</DockPanel>
</DataTemplate>
<DataTemplate DataType="{x:Type CustomerViewModel}">
<workspace:CustomerWorkspace />
</DataTemplate>
<TabControl ItemsSource="{Binding Workspaces}"
ItemTemplate="{StaticResource WorkspaceItem}"/>
The TabControl.ItemsSource is bound to an observablecollection(of Object) which contains all of my workspaces.
This works great except for 2 things:
If I open multiple customers, then I have multiple workspaces open. Because of DataTemplate Recycling, I lose state when i swap from one tab to another. So everything that is not bound will lose state.
The performance of swapping between Different workspaces (that use different datatemplates) is terribly slow.
So... I found a suggestion from another user on SO to add the user controls to the ObservableCOllection and ditch the data templates. that now solves one of the problems of losing state. however, now I am faced with 2 remaining problems:
- How do i set the TabItem.Header property without using a DataTemplate
- The speed of swapping back and forth between tabs is still slow unless they are of the same DataTemplate.
I then proceeded to Actually add a TabItem to the ObservableCollection in my codebehind and Set the TabItem.Content Property to that of the user control. The speed issue was now eliminated as is the losing state issue since I have removed the use of the DataTemplates. However, I am now stuck with the issue of binding a TabItem.header to the Custome "Title" Property of my usercontrol that should be displayed in the Tab Header.
So after this terribly long post, my questions are:
Is there any way to use datatemplates and force them to create a new Instance for each item in the collection to prevent recycling and state loss.
1a. Is there a better alternative than what I mentioned in the post above?
is there a way to do all of this through the Xaml instead of through back end code construction of Tab Items?
回答1:
The default behavior of WPF is to unload items which are not visible, which includes unloading TabItems which are not visible. This means when you go back to the tab, the TabItem gets re-loaded, and anything not bound (such as a scroll position, control states, etc) will get reset.
There was a good site here which contains code to extend the TabControl and stop it from destroying its TabItems when switching tabs, however it no longer seems to exist now.
Here's a copy of the code, although I've made some changes to it. It preserves the ContentPresenter of TabItems when switching tabs, and uses it to redraw the TabItem when you go back to the page. It takes up a bit more memory, however I find it better on performance since the TabItem no longer has to re-create all the controls that were on it.
// Extended TabControl which saves the displayed item so you don't get the performance hit of
// unloading and reloading the VisualTree when switching tabs
// Obtained from http://eric.burke.name/dotnetmania/2009/04/26/22.09.28
// and made a some modifications so it reuses a TabItem's ContentPresenter when doing drag/drop operations
[TemplatePart(Name = "PART_ItemsHolder", Type = typeof(Panel))]
public class TabControlEx : System.Windows.Controls.TabControl
{
// Holds all items, but only marks the current tab's item as visible
private Panel _itemsHolder = null;
// Temporaily holds deleted item in case this was a drag/drop operation
private object _deletedObject = null;
public TabControlEx()
: base()
{
// this is necessary so that we get the initial databound selected item
this.ItemContainerGenerator.StatusChanged += ItemContainerGenerator_StatusChanged;
}
/// <summary>
/// if containers are done, generate the selected item
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
void ItemContainerGenerator_StatusChanged(object sender, EventArgs e)
{
if (this.ItemContainerGenerator.Status == GeneratorStatus.ContainersGenerated)
{
this.ItemContainerGenerator.StatusChanged -= ItemContainerGenerator_StatusChanged;
UpdateSelectedItem();
}
}
/// <summary>
/// get the ItemsHolder and generate any children
/// </summary>
public override void OnApplyTemplate()
{
base.OnApplyTemplate();
_itemsHolder = GetTemplateChild("PART_ItemsHolder") as Panel;
UpdateSelectedItem();
}
/// <summary>
/// when the items change we remove any generated panel children and add any new ones as necessary
/// </summary>
/// <param name="e"></param>
protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
{
base.OnItemsChanged(e);
if (_itemsHolder == null)
{
return;
}
switch (e.Action)
{
case NotifyCollectionChangedAction.Reset:
_itemsHolder.Children.Clear();
if (base.Items.Count > 0)
{
base.SelectedItem = base.Items[0];
UpdateSelectedItem();
}
break;
case NotifyCollectionChangedAction.Add:
case NotifyCollectionChangedAction.Remove:
// Search for recently deleted items caused by a Drag/Drop operation
if (e.NewItems != null && _deletedObject != null)
{
foreach (var item in e.NewItems)
{
if (_deletedObject == item)
{
// If the new item is the same as the recently deleted one (i.e. a drag/drop event)
// then cancel the deletion and reuse the ContentPresenter so it doesn't have to be
// redrawn. We do need to link the presenter to the new item though (using the Tag)
ContentPresenter cp = FindChildContentPresenter(_deletedObject);
if (cp != null)
{
int index = _itemsHolder.Children.IndexOf(cp);
(_itemsHolder.Children[index] as ContentPresenter).Tag =
(item is TabItem) ? item : (this.ItemContainerGenerator.ContainerFromItem(item));
}
_deletedObject = null;
}
}
}
if (e.OldItems != null)
{
foreach (var item in e.OldItems)
{
_deletedObject = item;
// We want to run this at a slightly later priority in case this
// is a drag/drop operation so that we can reuse the template
this.Dispatcher.BeginInvoke(DispatcherPriority.DataBind,
new Action(delegate()
{
if (_deletedObject != null)
{
ContentPresenter cp = FindChildContentPresenter(_deletedObject);
if (cp != null)
{
this._itemsHolder.Children.Remove(cp);
}
}
}
));
}
}
UpdateSelectedItem();
break;
case NotifyCollectionChangedAction.Replace:
throw new NotImplementedException("Replace not implemented yet");
}
}
/// <summary>
/// update the visible child in the ItemsHolder
/// </summary>
/// <param name="e"></param>
protected override void OnSelectionChanged(SelectionChangedEventArgs e)
{
base.OnSelectionChanged(e);
UpdateSelectedItem();
}
/// <summary>
/// generate a ContentPresenter for the selected item
/// </summary>
void UpdateSelectedItem()
{
if (_itemsHolder == null)
{
return;
}
// generate a ContentPresenter if necessary
TabItem item = GetSelectedTabItem();
if (item != null)
{
CreateChildContentPresenter(item);
}
// show the right child
foreach (ContentPresenter child in _itemsHolder.Children)
{
child.Visibility = ((child.Tag as TabItem).IsSelected) ? Visibility.Visible : Visibility.Collapsed;
}
}
/// <summary>
/// create the child ContentPresenter for the given item (could be data or a TabItem)
/// </summary>
/// <param name="item"></param>
/// <returns></returns>
ContentPresenter CreateChildContentPresenter(object item)
{
if (item == null)
{
return null;
}
ContentPresenter cp = FindChildContentPresenter(item);
if (cp != null)
{
return cp;
}
// the actual child to be added. cp.Tag is a reference to the TabItem
cp = new ContentPresenter();
cp.Content = (item is TabItem) ? (item as TabItem).Content : item;
cp.ContentTemplate = this.SelectedContentTemplate;
cp.ContentTemplateSelector = this.SelectedContentTemplateSelector;
cp.ContentStringFormat = this.SelectedContentStringFormat;
cp.Visibility = Visibility.Collapsed;
cp.Tag = (item is TabItem) ? item : (this.ItemContainerGenerator.ContainerFromItem(item));
_itemsHolder.Children.Add(cp);
return cp;
}
/// <summary>
/// Find the CP for the given object. data could be a TabItem or a piece of data
/// </summary>
/// <param name="data"></param>
/// <returns></returns>
ContentPresenter FindChildContentPresenter(object data)
{
if (data is TabItem)
{
data = (data as TabItem).Content;
}
if (data == null)
{
return null;
}
if (_itemsHolder == null)
{
return null;
}
foreach (ContentPresenter cp in _itemsHolder.Children)
{
if (cp.Content == data)
{
return cp;
}
}
return null;
}
/// <summary>
/// copied from TabControl; wish it were protected in that class instead of private
/// </summary>
/// <returns></returns>
protected TabItem GetSelectedTabItem()
{
object selectedItem = base.SelectedItem;
if (selectedItem == null)
{
return null;
}
if (_deletedObject == selectedItem)
{
}
TabItem item = selectedItem as TabItem;
if (item == null)
{
item = base.ItemContainerGenerator.ContainerFromIndex(base.SelectedIndex) as TabItem;
}
return item;
}
}
The TabControl template I usually use looks something like this:
<Style x:Key="TabControlEx_NoHeadersStyle" TargetType="{x:Type local:TabControlEx}">
<Setter Property="SnapsToDevicePixels" Value="true"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type localControls:TabControlEx}">
<DockPanel>
<!-- This is needed to draw TabControls with Bound items -->
<StackPanel IsItemsHost="True" Height="0" Width="0" />
<Grid x:Name="PART_ItemsHolder" />
</DockPanel>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
You can also simplify your XAML by using an implicit DataTemplate instead of an ItemTemplate since your ViewModel will be placed in your TabItem.Content. I'm also not too sure what you're asking about the header, but if I understand you correctly you can just set the header in another implicit style for the TabItem
<Window.Resources>
<DataTemplate DataType="{x:Type CustomerViewModel}">
<workspace:CustomerWorkspace />
</DataTemplate>
</Window.Resources>
<TabControl ItemsSource="{Binding Workspaces}">
<TabControl.Resources>
<Style TargetType="{x:Type TabItem}">
<Setter Property="Header" Value="{Binding HeaderProperty}" />
</Style>
</TabControl.Resources>
</TabControl>
来源：https://stackoverflow.com/questions/12432062/binding-to-itemssource-of-tabcontrol-in-wpf