How to preserve control state within tab items in a TabControl

Question

I am a newcomer to WPF, attempting to build a project that follows the recommendations of Josh Smith's excellent article describing The Model-View-ViewModel Design Pattern.

Using Josh's sample code as a base, I have created a simple application that contains a number of "workspaces", each represented by a tab in a TabControl. In my application, a workspace is a document editor that allows a hierarchical document to be manipulated via a TreeView control.

Although I have succeeded in opening multiple workspaces and viewing their document content in the bound TreeView control, I find that the TreeView "forgets" its state when switching between tabs. For example, if the TreeView in Tab1 is partially expanded, it will be shown as fully collapsed after switching to Tab2 and returning to Tab1. This behaviour appears to apply to all aspects of control state for all controls.

After some experimentation, I have realized that I can preserve state within a TabItem by explicitly binding each control state property to a dedicated property on the underlying ViewModel. However, this seems like a lot of additional work, when I simply want all my controls to remember their state when switching between workspaces.

I assume I am missing something simple, but I am not sure where to look for the answer. Any guidance would be much appreciated.

Thanks, Tim

Update:

As requested, I will attempt to post some code that demonstrates this problem. However, since the data that underlies the TreeView is complex, I will post a simplified example that exhibits the same symtoms. Here is the XAML from the main window:

<TabControl IsSynchronizedWithCurrentItem="True" ItemsSource="{Binding Path=Docs}">
    <TabControl.ItemTemplate>
        <DataTemplate>
            <ContentPresenter Content="{Binding Path=Name}" />
        </DataTemplate>
    </TabControl.ItemTemplate>

    <TabControl.ContentTemplate>
        <DataTemplate>
            <view:DocumentView />
        </DataTemplate>
    </TabControl.ContentTemplate>
</TabControl>

The above XAML correctly binds to an ObservableCollection of DocumentViewModel, whereby each member is presented via a DocumentView.

For the simplicity of this example, I have removed the TreeView (mentioned above) from the DocumentView and replaced it with a TabControl containing 3 fixed tabs:

<TabControl>
    <TabItem Header="A" />
    <TabItem Header="B" />
    <TabItem Header="C" />
</TabControl>

In this scenario, there is no binding between the DocumentView and the DocumentViewModel. When the code is run, the inner TabControl is unable to remember its selection when the outer TabControl is switched.

However, if I explicitly bind the inner TabControl's SelectedIndex property ...

<TabControl SelectedIndex="{Binding Path=SelectedDocumentIndex}">
    <TabItem Header="A" />
    <TabItem Header="B" />
    <TabItem Header="C" />
</TabControl>

... to a corresponding dummy property on the DocumentViewModel ...

public int SelecteDocumentIndex { get; set; }

... the inner tab is able to remember its selection.

I understand that I can effectively solve my problem by applying this technique to every visual property of every control, but I am hoping there is a more elegant solution.

Answer 1

The Writer sample application of the WPF Application Framework (WAF) shows how to solve your issue. It creates a new UserControl for every TabItem. So the state is preserved when the user changes the active Tab.

Answer 2

I got resolve it with this tip WPF TabControl: Turning Off Tab Virtualization at http://www.codeproject.com/Articles/460989/WPF-TabControl-Turning-Off-Tab-Virtualization this a class of TabContent with property IsCached.

Answer 3

i had the same problem and found a nice solution you can use it like a normal TabControl as far as i tested it. In case it it important for you here the Current License

Here the Code on case the Link goes down:

using System;
using System.Collections.Specialized;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;

namespace CefSharp.Wpf.Example.Controls
{
    /// <summary>
    /// Extended TabControl which saves the displayed item so you don't get the performance hit of
    /// unloading and reloading the VisualTree when switching tabs
    /// </summary>
    /// <remarks>
    /// Based on example from http://stackoverflow.com/a/9802346, which in turn is based on
    /// http://www.pluralsight-training.net/community/blogs/eburke/archive/2009/04/30/keeping-the-wpf-tab-control-from-destroying-its-children.aspx
    /// with some modifications so it reuses a TabItem's ContentPresenter when doing drag/drop operations
    /// </remarks>
    [TemplatePart(Name = "PART_ItemsHolder", Type = typeof(Panel))]
    public class NonReloadingTabControl : TabControl
    {
        private Panel itemsHolderPanel;

        public NonReloadingTabControl()
        {
            // This is necessary so that we get the initial databound selected item
            ItemContainerGenerator.StatusChanged += ItemContainerGeneratorStatusChanged;
        }

        /// <summary>
        /// If containers are done, generate the selected item
        /// </summary>
        /// <param name="sender">The sender.</param>
        /// <param name="e">The <see cref="EventArgs"/> instance containing the event data.</param>
        private void ItemContainerGeneratorStatusChanged(object sender, EventArgs e)
        {
            if (ItemContainerGenerator.Status == GeneratorStatus.ContainersGenerated)
            {
                ItemContainerGenerator.StatusChanged -= ItemContainerGeneratorStatusChanged;
                UpdateSelectedItem();
            }
        }

        /// <summary>
        /// Get the ItemsHolder and generate any children
        /// </summary>
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            itemsHolderPanel = 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">The <see cref="NotifyCollectionChangedEventArgs"/> instance containing the event data.</param>
        protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
        {
            base.OnItemsChanged(e);

            if (itemsHolderPanel == null)
                return;

            switch (e.Action)
            {
                case NotifyCollectionChangedAction.Reset:
                itemsHolderPanel.Children.Clear();
                break;

                case NotifyCollectionChangedAction.Add:
                case NotifyCollectionChangedAction.Remove:
                if (e.OldItems != null)
                {
                    foreach (var item in e.OldItems)
                    {
                        var cp = FindChildContentPresenter(item);
                        if (cp != null)
                            itemsHolderPanel.Children.Remove(cp);
                    }
                }

                // Don't do anything with new items because we don't want to
                // create visuals that aren't being shown

                UpdateSelectedItem();
                break;

                case NotifyCollectionChangedAction.Replace:
                throw new NotImplementedException("Replace not implemented yet");
            }
        }

        protected override void OnSelectionChanged(SelectionChangedEventArgs e)
        {
            base.OnSelectionChanged(e);
            UpdateSelectedItem();
        }

        private void UpdateSelectedItem()
        {
            if (itemsHolderPanel == null)
                return;

            // Generate a ContentPresenter if necessary
            var item = GetSelectedTabItem();
            if (item != null)
                CreateChildContentPresenter(item);

            // show the right child
            foreach (ContentPresenter child in itemsHolderPanel.Children)
                child.Visibility = ((child.Tag as TabItem).IsSelected) ? Visibility.Visible : Visibility.Collapsed;
        }

        private ContentPresenter CreateChildContentPresenter(object item)
        {
            if (item == null)
                return null;

            var cp = FindChildContentPresenter(item);

            if (cp != null)
                return cp;

            var tabItem = item as TabItem;
            cp = new ContentPresenter
            {
                Content = (tabItem != null) ? tabItem.Content : item,
                ContentTemplate = this.SelectedContentTemplate,
                ContentTemplateSelector = this.SelectedContentTemplateSelector,
                ContentStringFormat = this.SelectedContentStringFormat,
                Visibility = Visibility.Collapsed,
                Tag = tabItem ?? (this.ItemContainerGenerator.ContainerFromItem(item))
            };
            itemsHolderPanel.Children.Add(cp);
            return cp;
        }

        private ContentPresenter FindChildContentPresenter(object data)
        {
            if (data is TabItem)
                data = (data as TabItem).Content;

            if (data == null)
                return null;

            if (itemsHolderPanel == null)
                return null;

            foreach (ContentPresenter cp in itemsHolderPanel.Children)
            {
                if (cp.Content == data)
                    return cp;
            }

            return null;
        }

        protected TabItem GetSelectedTabItem()
        {
            var selectedItem = SelectedItem;
            if (selectedItem == null)
                return null;

            var item = selectedItem as TabItem ?? ItemContainerGenerator.ContainerFromIndex(SelectedIndex) as TabItem;

            return item;
        }
    }
}

License at Copietime

// Copyright © 2010-2016 The CefSharp Authors
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//    * Redistributions of source code must retain the above copyright
//      notice, this list of conditions and the following disclaimer.
//
//    * Redistributions in binary form must reproduce the above
//      copyright notice, this list of conditions and the following disclaimer
//      in the documentation and/or other materials provided with the
//      distribution.
//
//    * Neither the name of Google Inc. nor the name Chromium Embedded
//      Framework nor the name CefSharp nor the names of its contributors
//      may be used to endorse or promote products derived from this software
//      without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Answer 4

Based on @Arsen's answer above, here is another behavior, that:

1. Doesn't need any additional references. (unless you put the code in an external library)
2. It doesn't use a base class.
3. It handles both Reset and Add collection changes.

To use it

Declare the namespace in the xaml:

<ResourceDictionary
    ...
    xmlns:behaviors="clr-namespace:My.Behaviors;assembly=My.Wpf.Assembly"
    ...
    >

Update the style:

<Style TargetType="TabControl" x:Key="TabControl">
    ...
    <Setter Property="behaviors:TabControlBehavior.DoNotCacheControls" Value="True" />
    ...
</Style>

Or update the TabControl directly:

<TabControl behaviors:TabControlBehavior.DoNotCacheControls="True" ItemsSource="{Binding Tabs}" SelectedItem="{Binding SelectedTab}">

And here is the code for the behavior:

using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Windows;
using System.Windows.Controls;

namespace My.Behaviors
{
    /// <summary>
    /// Wraps tab item contents in UserControl to prevent TabControl from re-using its content
    /// </summary>
    public class TabControlBehavior
    {
        private static readonly HashSet<TabControl> _tabControls = new HashSet<TabControl>();
        private static readonly Dictionary<ItemCollection, TabControl> _tabControlItemCollections = new Dictionary<ItemCollection, TabControl>();

        public static bool GetDoNotCacheControls(TabControl tabControl)
        {
            return (bool)tabControl.GetValue(DoNotCacheControlsProperty);
        }

        public static void SetDoNotCacheControls(TabControl tabControl, bool value)
        {
            tabControl.SetValue(DoNotCacheControlsProperty, value);
        }

        public static readonly DependencyProperty DoNotCacheControlsProperty = DependencyProperty.RegisterAttached(
            "DoNotCacheControls",
            typeof(bool),
            typeof(TabControlBehavior),
            new UIPropertyMetadata(false, OnDoNotCacheControlsChanged));

        private static void OnDoNotCacheControlsChanged(
            DependencyObject depObj,
            DependencyPropertyChangedEventArgs e)
        {
            var tabControl = depObj as TabControl;
            if (null == tabControl)
                return;
            if (e.NewValue is bool == false)
                return;

            if ((bool)e.NewValue)
                Attach(tabControl);
            else
                Detach(tabControl);
        }

        private static void Attach(TabControl tabControl)
        {
            if (!_tabControls.Add(tabControl))
                return;
            _tabControlItemCollections.Add(tabControl.Items, tabControl);
            ((INotifyCollectionChanged)tabControl.Items).CollectionChanged += TabControlUcWrapperBehavior_CollectionChanged;
        }

        private static void Detach(TabControl tabControl)
        {
            if (!_tabControls.Remove(tabControl))
                return;
            _tabControlItemCollections.Remove(tabControl.Items);
            ((INotifyCollectionChanged)tabControl.Items).CollectionChanged -= TabControlUcWrapperBehavior_CollectionChanged;
        }

        private static void TabControlUcWrapperBehavior_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
        {
            var itemCollection = (ItemCollection)sender;
            var tabControl = _tabControlItemCollections[itemCollection];
            IList items;
            if (e.Action == NotifyCollectionChangedAction.Reset)
            {   /* our ObservableArray<T> swops out the whole collection */
                items = (ItemCollection)sender;
            }
            else
            {
                if (e.Action != NotifyCollectionChangedAction.Add)
                    return;

                items = e.NewItems;
            }

            foreach (var newItem in items)
            {
                var ti = tabControl.ItemContainerGenerator.ContainerFromItem(newItem) as TabItem;
                if (ti != null)
                {
                    var userControl = ti.Content as UserControl;
                    if (null == userControl)
                        ti.Content = new UserControl { Content = ti.Content };
                }
            }
        }
    }
}

Answer 5

Using the idea of WAF I come to this simple solution which seems to solve the issue.

I use Interactivity Behavior, but the same can be done with attached property if Interactivity library is not referenced

/// <summary>
/// Wraps tab item contents in UserControl to prevent TabControl from re-using its content
/// </summary>
public class TabControlUcWrapperBehavior 
    : Behavior<UIElement>
{
    private TabControl AssociatedTabControl { get { return (TabControl) AssociatedObject; } }

    protected override void OnAttached()
    {
        ((INotifyCollectionChanged)AssociatedTabControl.Items).CollectionChanged += TabControlUcWrapperBehavior_CollectionChanged;
        base.OnAttached();
    }

    protected override void OnDetaching()
    {
        ((INotifyCollectionChanged)AssociatedTabControl.Items).CollectionChanged -= TabControlUcWrapperBehavior_CollectionChanged;
        base.OnDetaching();
    }

    void TabControlUcWrapperBehavior_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if (e.Action != NotifyCollectionChangedAction.Add) 
            return;

        foreach (var newItem in e.NewItems)
        {
            var ti = AssociatedTabControl.ItemContainerGenerator.ContainerFromItem(newItem) as TabItem;

            if (ti != null && !(ti.Content is UserControl)) 
                ti.Content = new UserControl { Content = ti.Content };
        }
    }
}

And usage

<TabControl ItemsSource="...">
    <i:Interaction.Behaviors>
        <controls:TabControlUcWrapperBehavior/>
    </i:Interaction.Behaviors>
</TabControl>

Answer 6

I have posted an answer for similar question. In my case manually creating the TabItems have solved the problem of creating the View again and again. Check here