WPF TabItem state lost when selected tab changes - wpf

Question

I have a TabControl with multiple tabs. Each tab has a multiline TextBox in it.
So in the visible textbox in Tab 1, the insertion point starts out at the beginning of line 1.
I move the insertion point to the end of line 2.
I change the selected tab to Tab 2, and then back to Tab 1.
The insertion point in the Tab 1 TextBox is always back at the start of line one.
Both tabs are being populated by viewmodels in a list:
<TabControl ItemsSource="{Binding TabItemViewModelList}">
<TabControl.Resources>
<DataTemplate DataType="TabItemViewModel">
<Grid>
<TextBox Text="{Binding Text}" />
</Grid>
</DataTemplate>
</TabControl.Resources>
<TabControl>
That's simplified, but not by much.
Is this because it's re-instantiating the template each time I change the selected item, or something like that? Is there any way around that other than agonizingly maintaining every tiny bit of UI state in the ViewModel?
Update
I was right, this is the virtualization bug, it's a known bug, and the designer probably considered it a feature. He's insane, of course: A tab control is not a list box. It becomes totally unusable long before you've got enough items in it for virtualization to make any sense at all.

Answer 1

The answer seems to be Ivan Krivyakov's TabContent.IsCached attached property, found in this question: Turning off Virtualization for Tabcontrol without itemsource - WPF. It's not the answer to his question, but it is the answer to mine.
Looks like this in the XAML:
<TabControl
xmlns:ikriv="clr-namespace:IKriv.Windows.Controls.Behaviors"
ikriv:TabContent.IsCached="True"
ItemsSource="{Binding DocumentList.Documents}"
SelectedItem="{Binding DocumentList.ActiveDocument}"
/>
And here's the complete source code to Krivyakov's solution, just in case CodeProject gets hit by an asteroid:
// TabContent.cs, version 1.2
// The code in this file is Copyright (c) Ivan Krivyakov
// See http://www.ikriv.com/legal.php for more information
//
using System;
using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Markup;
/// <summary>
/// http://www.codeproject.com/Articles/460989/WPF-TabControl-Turning-Off-Tab-Virtualization
/// </summary>
namespace IKriv.Windows.Controls.Behaviors
{
/// <summary>
/// Attached properties for persistent tab control
/// </summary>
/// <remarks>By default WPF TabControl bound to an ItemsSource destroys visual state of invisible tabs.
/// Set ikriv:TabContent.IsCached="True" to preserve visual state of each tab.
/// </remarks>
public static class TabContent
{
public static bool GetIsCached(DependencyObject obj)
{
return (bool)obj.GetValue(IsCachedProperty);
}
public static void SetIsCached(DependencyObject obj, bool value)
{
obj.SetValue(IsCachedProperty, value);
}
/// <summary>
/// Controls whether tab content is cached or not
/// </summary>
/// <remarks>When TabContent.IsCached is true, visual state of each tab is preserved (cached), even when the tab is hidden</remarks>
public static readonly DependencyProperty IsCachedProperty =
DependencyProperty.RegisterAttached("IsCached", typeof(bool), typeof(TabContent), new UIPropertyMetadata(false, OnIsCachedChanged));
public static DataTemplate GetTemplate(DependencyObject obj)
{
return (DataTemplate)obj.GetValue(TemplateProperty);
}
public static void SetTemplate(DependencyObject obj, DataTemplate value)
{
obj.SetValue(TemplateProperty, value);
}
/// <summary>
/// Used instead of TabControl.ContentTemplate for cached tabs
/// </summary>
public static readonly DependencyProperty TemplateProperty =
DependencyProperty.RegisterAttached("Template", typeof(DataTemplate), typeof(TabContent), new UIPropertyMetadata(null));
public static DataTemplateSelector GetTemplateSelector(DependencyObject obj)
{
return (DataTemplateSelector)obj.GetValue(TemplateSelectorProperty);
}
public static void SetTemplateSelector(DependencyObject obj, DataTemplateSelector value)
{
obj.SetValue(TemplateSelectorProperty, value);
}
/// <summary>
/// Used instead of TabControl.ContentTemplateSelector for cached tabs
/// </summary>
public static readonly DependencyProperty TemplateSelectorProperty =
DependencyProperty.RegisterAttached("TemplateSelector", typeof(DataTemplateSelector), typeof(TabContent), new UIPropertyMetadata(null));
[EditorBrowsable(EditorBrowsableState.Never)]
public static TabControl GetInternalTabControl(DependencyObject obj)
{
return (TabControl)obj.GetValue(InternalTabControlProperty);
}
[EditorBrowsable(EditorBrowsableState.Never)]
public static void SetInternalTabControl(DependencyObject obj, TabControl value)
{
obj.SetValue(InternalTabControlProperty, value);
}
// Using a DependencyProperty as the backing store for InternalTabControl. This enables animation, styling, binding, etc...
[EditorBrowsable(EditorBrowsableState.Never)]
public static readonly DependencyProperty InternalTabControlProperty =
DependencyProperty.RegisterAttached("InternalTabControl", typeof(TabControl), typeof(TabContent), new UIPropertyMetadata(null, OnInternalTabControlChanged));
[EditorBrowsable(EditorBrowsableState.Never)]
public static ContentControl GetInternalCachedContent(DependencyObject obj)
{
return (ContentControl)obj.GetValue(InternalCachedContentProperty);
}
[EditorBrowsable(EditorBrowsableState.Never)]
public static void SetInternalCachedContent(DependencyObject obj, ContentControl value)
{
obj.SetValue(InternalCachedContentProperty, value);
}
// Using a DependencyProperty as the backing store for InternalCachedContent. This enables animation, styling, binding, etc...
[EditorBrowsable(EditorBrowsableState.Never)]
public static readonly DependencyProperty InternalCachedContentProperty =
DependencyProperty.RegisterAttached("InternalCachedContent", typeof(ContentControl), typeof(TabContent), new UIPropertyMetadata(null));
[EditorBrowsable(EditorBrowsableState.Never)]
public static object GetInternalContentManager(DependencyObject obj)
{
return (object)obj.GetValue(InternalContentManagerProperty);
}
[EditorBrowsable(EditorBrowsableState.Never)]
public static void SetInternalContentManager(DependencyObject obj, object value)
{
obj.SetValue(InternalContentManagerProperty, value);
}
// Using a DependencyProperty as the backing store for InternalContentManager. This enables animation, styling, binding, etc...
public static readonly DependencyProperty InternalContentManagerProperty =
DependencyProperty.RegisterAttached("InternalContentManager", typeof(object), typeof(TabContent), new UIPropertyMetadata(null));
private static void OnIsCachedChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
if (obj == null) return;
var tabControl = obj as TabControl;
if (tabControl == null)
{
throw new InvalidOperationException("Cannot set TabContent.IsCached on object of type " + args.NewValue.GetType().Name +
". Only objects of type TabControl can have TabContent.IsCached property.");
}
bool newValue = (bool)args.NewValue;
if (!newValue)
{
if (args.OldValue != null && ((bool)args.OldValue))
{
throw new NotImplementedException("Cannot change TabContent.IsCached from True to False. Turning tab caching off is not implemented");
}
return;
}
EnsureContentTemplateIsNull(tabControl);
tabControl.ContentTemplate = CreateContentTemplate();
EnsureContentTemplateIsNotModified(tabControl);
}
private static DataTemplate CreateContentTemplate()
{
const string xaml =
"<DataTemplate><Border b:TabContent.InternalTabControl=\"{Binding RelativeSource={RelativeSource AncestorType=TabControl}}\" /></DataTemplate>";
var context = new ParserContext();
context.XamlTypeMapper = new XamlTypeMapper(new string[0]);
context.XamlTypeMapper.AddMappingProcessingInstruction("b", typeof(TabContent).Namespace, typeof(TabContent).Assembly.FullName);
context.XmlnsDictionary.Add("", "http://schemas.microsoft.com/winfx/2006/xaml/presentation");
context.XmlnsDictionary.Add("b", "b");
var template = (DataTemplate)XamlReader.Parse(xaml, context);
return template;
}
private static void OnInternalTabControlChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
if (obj == null) return;
var container = obj as Decorator;
if (container == null)
{
var message = "Cannot set TabContent.InternalTabControl on object of type " + obj.GetType().Name +
". Only controls that derive from Decorator, such as Border can have a TabContent.InternalTabControl.";
throw new InvalidOperationException(message);
}
if (args.NewValue == null) return;
if (!(args.NewValue is TabControl))
{
throw new InvalidOperationException("Value of TabContent.InternalTabControl cannot be of type " + args.NewValue.GetType().Name +", it must be of type TabControl");
}
var tabControl = (TabControl)args.NewValue;
var contentManager = GetContentManager(tabControl, container);
contentManager.UpdateSelectedTab();
}
private static ContentManager GetContentManager(TabControl tabControl, Decorator container)
{
var contentManager = (ContentManager)GetInternalContentManager(tabControl);
if (contentManager != null)
{
/*
* Content manager already exists for the tab control. This means that tab content template is applied
* again, and new instance of the Border control (container) has been created. The old container
* referenced by the content manager is no longer visible and needs to be replaced
*/
contentManager.ReplaceContainer(container);
}
else
{
// create content manager for the first time
contentManager = new ContentManager(tabControl, container);
SetInternalContentManager(tabControl, contentManager);
}
return contentManager;
}
private static void EnsureContentTemplateIsNull(TabControl tabControl)
{
if (tabControl.ContentTemplate != null)
{
throw new InvalidOperationException("TabControl.ContentTemplate value is not null. If TabContent.IsCached is True, use TabContent.Template instead of ContentTemplate");
}
}
private static void EnsureContentTemplateIsNotModified(TabControl tabControl)
{
var descriptor = DependencyPropertyDescriptor.FromProperty(TabControl.ContentTemplateProperty, typeof(TabControl));
descriptor.AddValueChanged(tabControl, (sender, args) =>
{
throw new InvalidOperationException("Cannot assign to TabControl.ContentTemplate when TabContent.IsCached is True. Use TabContent.Template instead");
});
}
public class ContentManager
{
TabControl _tabControl;
Decorator _border;
public ContentManager(TabControl tabControl, Decorator border)
{
_tabControl = tabControl;
_border = border;
_tabControl.SelectionChanged += (sender, args) => { UpdateSelectedTab(); };
}
public void ReplaceContainer(Decorator newBorder)
{
if (Object.ReferenceEquals(_border, newBorder)) return;
_border.Child = null; // detach any tab content that old border may hold
_border = newBorder;
}
public void UpdateSelectedTab()
{
_border.Child = GetCurrentContent();
}
private ContentControl GetCurrentContent()
{
var item = _tabControl.SelectedItem;
if (item == null) return null;
var tabItem = _tabControl.ItemContainerGenerator.ContainerFromItem(item);
if (tabItem == null) return null;
var cachedContent = TabContent.GetInternalCachedContent(tabItem);
if (cachedContent == null)
{
cachedContent = new ContentControl
{
DataContext = item,
ContentTemplate = TabContent.GetTemplate(_tabControl),
ContentTemplateSelector = TabContent.GetTemplateSelector(_tabControl)
};
cachedContent.SetBinding(ContentControl.ContentProperty, new Binding());
TabContent.SetInternalCachedContent(tabItem, cachedContent);
}
return cachedContent;
}
}
}
}

Answer 2

The carret position gets updated every time the text gets re-initialized in your case. You could put a behavior on the text box to set it to the end on focus... see the following: Set the caret/cursor position to the end of the string value WPF textbox