如何在WPF TabControl中取消选项卡更改

Question

I have found multiple questions about this problem on SO, however I still can't quite get a realiable solution. Here is what I came up with after reading the answers.

Xaml:

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="MainWindow" Height="300" Width="300" x:Name="this">
    <TabControl IsSynchronizedWithCurrentItem="True" ItemsSource="{Binding Tabs, ElementName=this}" x:Name="TabControl"/>
</Window>

Code behind:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        var tabs = new ObservableCollection<string> {"Tab1", "Tab2", "Tab3"};
        Tabs = CollectionViewSource.GetDefaultView(tabs);
        Tabs.CurrentChanging += OnCurrentChanging;
        Tabs.CurrentChanged += OnCurrentChanged;
        Tabs.MoveCurrentToFirst();
        CurrentTab = tabs.First();
    }

    private void OnCurrentChanging(object sender, CurrentChangingEventArgs e)
    {
        //only show message box when tab is changed by user input
        if (!_cancelTabChange)
        {
            if (MessageBox.Show("Change tab?", "Message", MessageBoxButton.YesNo) == MessageBoxResult.No)
            {
                _cancelTabChange = true;
                return;
            }
        }
        _cancelTabChange = false;
    }

    private void OnCurrentChanged(object sender, EventArgs e)
    {
        if (!_cancelTabChange)
        {
            //Update current tab property, if user did not cancel transition
            CurrentTab = (string)Tabs.CurrentItem;
        }
        else
        {
            //navigate back to current tab otherwise
            Dispatcher.BeginInvoke(new Action(() => Tabs.MoveCurrentTo(CurrentTab)));
        }
    }

    public string CurrentTab { get; set; }

    public static readonly DependencyProperty TabsProperty = DependencyProperty.Register("Tabs", typeof(ICollectionView), typeof(MainWindow), new FrameworkPropertyMetadata(default(ICollectionView)));
    public ICollectionView Tabs
    {
        get { return (ICollectionView)GetValue(TabsProperty); }
        set { SetValue(TabsProperty, value); }
    }

    private bool _cancelTabChange;
}

Basically I want to display a confirmation message, when user navigates to different tab, and if he clicks "no" - abort the transition. This code does not work though. If you click multiple times on "Tab2", each time choosing "no" in message box, at some point it stops working: events stop triggering. Event will trigger again if you click on "Tab3", but if you choose "yes" it opens second tab and not third. I am having trouble figuring out wtf is going on. :)

Does anyone see a bug in my solution? Or is there an easier way to display a confirmation message, when user switches tabs? I am also willing to use any opensource tab control, which does have a proper SelectionChanging event. I could not find any though.

I am using .Net 4.0.

Edit: If I comment the message box out:

private void OnCurrentChanging(object sender, CurrentChangingEventArgs e)
{
    //only show message box when tab is changed by user input
    if (!_cancelTabChange)
    {
        //if (MessageBox.Show("Change tab?", "Message", MessageBoxButton.YesNo) == MessageBoxResult.No)
        //{
            Debug.WriteLine("Canceled");
            _cancelTabChange = true;
            return;
        //}
    }
    _cancelTabChange = false;
}

Everything works fine. Weird.

Answer 1

This solution http://coderelief.net/2011/11/07/fixing-issynchronizedwithcurrentitem-and-icollectionview-cancel-bug-with-an-attached-property/

seems to work quite well with

<TabControl ... yournamespace:SelectorAttachedProperties.IsSynchronizedWithCurrentItemFixEnabled="True" .../>

private void OnCurrentChanging(object sender, CurrentChangingEventArgs e)
{                   
    if (MessageBox.Show("Change tab?", "Message", MessageBoxButton.YesNo) == MessageBoxResult.No)
    {
        e.Cancel = true;                    
    }                     
}



public static class SelectorAttachedProperties
{
    private static Type _ownerType = typeof(SelectorAttachedProperties);

    #region IsSynchronizedWithCurrentItemFixEnabled

    public static readonly DependencyProperty IsSynchronizedWithCurrentItemFixEnabledProperty =
        DependencyProperty.RegisterAttached("IsSynchronizedWithCurrentItemFixEnabled", typeof(bool), _ownerType,
        new PropertyMetadata(false, OnIsSynchronizedWithCurrentItemFixEnabledChanged));

    public static bool GetIsSynchronizedWithCurrentItemFixEnabled(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsSynchronizedWithCurrentItemFixEnabledProperty);
    }

    public static void SetIsSynchronizedWithCurrentItemFixEnabled(DependencyObject obj, bool value)
    {
        obj.SetValue(IsSynchronizedWithCurrentItemFixEnabledProperty, value);
    }

    private static void OnIsSynchronizedWithCurrentItemFixEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Selector selector = d as Selector;
        if (selector == null || !(e.OldValue is bool && e.NewValue is bool) || e.OldValue == e.NewValue)
            return;

        bool enforceCurrentItemSync = (bool)e.NewValue;
        ICollectionView collectionView = null;

        EventHandler itemsSourceChangedHandler = null;
        itemsSourceChangedHandler = delegate
        {
            collectionView = selector.ItemsSource as ICollectionView;
            if (collectionView == null)
                collectionView = CollectionViewSource.GetDefaultView(selector);
        };

        SelectionChangedEventHandler selectionChangedHanlder = null;
        selectionChangedHanlder = delegate
        {
            if (collectionView == null)
                return;

            if (selector.IsSynchronizedWithCurrentItem == true && selector.SelectedItem != collectionView.CurrentItem)
            {
                selector.IsSynchronizedWithCurrentItem = false;
                selector.SelectedItem = collectionView.CurrentItem;
                selector.IsSynchronizedWithCurrentItem = true;
            }
        };

        if (enforceCurrentItemSync)
        {
            TypeDescriptor.GetProperties(selector)["ItemsSource"].AddValueChanged(selector, itemsSourceChangedHandler);
            selector.SelectionChanged += selectionChangedHanlder;
        }
        else
        {
            TypeDescriptor.GetProperties(selector)["ItemsSource"].RemoveValueChanged(selector, itemsSourceChangedHandler);
            selector.SelectionChanged -= selectionChangedHanlder;
        }
    }

    #endregion IsSynchronizedWithCurrentItemFixEnabled
}

Answer 2

For some reason adding TabControl.Focus() fixes things:

private void OnCurrentChanged(object sender, EventArgs e)
{
    if (!_cancelTabChange)
    {
        //Update current tab property, if user did not cancel transition
        CurrentTab = (string)Tabs.CurrentItem;
    }
    else
    {
        //navigate back to current tab otherwise
        Dispatcher.BeginInvoke(new Action(() => 
        {
            Tabs.MoveCurrentTo(CurrentTab);
            TabControl.Focus();
        }));
    }
}

I still have no clue what on Earth is going on here. So I will gladly accept the answer, which sheds some light on this issue.

Answer 3

He who must be obeyed requested that the application ask the user if they wish to leave the page so here is the slightly changed code:

    private Object _selectedTab;

    public Object SelectedTab
    {
        get
        {
            return _selectedTab;
        }
        set
        {
            if (
                  !(_selectedTab is ADR_Scanner.ViewModel.ConfigurationViewModel) || 
                  !_configurationViewModel.HasChanged ||
                  (System.Windows.Forms.MessageBox.Show("Are you sure you want to leave this page without saving the configuration changes", ADR_Scanner.App.Current.MainWindow.Title, System.Windows.Forms.MessageBoxButtons.YesNo, System.Windows.Forms.MessageBoxIcon.Error) == System.Windows.Forms.DialogResult.Yes)
                )
            {
                _selectedTab = value;
            }
            OnPropertyChanged("SelectedTab");
        }
    }

I think this small change does pretty much what you wanted.

Answer 4

private void MainTabControl_OnSelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (ReasonBecauseLeaveTabItemIsForbidden)
        {
            if (MainTabControl.SelectedIndex == IndexOfTabItem)
            {
                MessageBox.Show(SomeMessageWhyLeaveTabItemIsForbidden);
            }
            MainTabControl.SelectedIndex = IndexOfTabItem;
        }
    }

IndexOfTabItem - index of TabItem that disabled for leaving.

Answer 5

inside the tabControl_SelectionChanged event handler:

if (e.OriginalSource == tabControl) //if this event fired from your tabControl
            {
                e.Handled = true;

                if (!forbiddenPage.IsSelected)  //User leaving the tab
                {
                    if (forbiddenTest())
                    {
                        forbiddenPage.IsSelected = true;
                        MessageBox.Show("you must not leave this page");
                    }
             }

Note that setting forbiddenPage.IsSelected = true causes a loop and you reenter this event handler. This time, however, we exit because the page selected IS the forbidden page.

Answer 6

There is a much easier solution. Add a binding to the selected item in the XAML:

    <TabControl SelectedItem="{Binding SelectedTab}" ... 

Then in the view model:

    private Object _selectedTab;

    public Object SelectedTab
    {
        get
        {
            return _selectedTab;
        }
        set
        {
            if (_selectedTab is ADR_Scanner.ViewModel.ConfigurationViewModel && _configurationViewModel.HasChanged)
            {
                System.Windows.Forms.MessageBox.Show("Please save the configuration changes", ADR_Scanner.App.ResourceAssembly.GetName().Name, System.Windows.Forms.MessageBoxButtons.OK, System.Windows.Forms.MessageBoxIcon.Error);
            }
            else
            {
                _selectedTab = value;
            }
            OnPropertyChanged("SelectedTab");
        }
    }

Obviously you replace ADR_Scanner.ViewModel.ConfigurationViewModel with your own view model class. Lastly make sure you initialise _selectedTab in your constructor otherwise the TabControl will have no initial selection.