WPF Cancel tab selection with message box - wpf

Question

I have a need to detect a dirty page when the user tries to move to a new tab and give the user the option to cancel the move off the current tab. I can get this to work when I don't need to ask the user, but showing the messagebox is breaking the functionality. I have provided a simple app that exhibits the problem. Sorry it's in VB, but that's what we use here.
<Window x:Class="MainWindow"
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"
xmlns:local="clr-namespace:ChangeTab"
mc:Ignorable="d"
Title="MainWindow" Height="350" Width="525"
DataContext="{Binding RelativeSource={RelativeSource Self}}">
<TabControl SelectedIndex="{Binding SelectedIndex}" Initialized="TabControl_Initialized" IsSynchronizedWithCurrentItem="True">
<TabItem Header="Tab 1">
<TextBlock Text="Content 1"/>
</TabItem>
<TabItem Header="Tab 2">
<StackPanel>
<TextBlock Text="Content 2"/>
<CheckBox Content="Locked" IsChecked="{Binding IsChecked}"/>
</StackPanel>
</TabItem>
</TabControl>
</Window>
Imports System.ComponentModel
Imports System.Windows.Threading
Class MainWindow
Implements INotifyPropertyChanged
Dim tc As TabControl
Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
Public Function SetProperty(Of T)(ByRef storage As T, value As T, PropertyName As String) As Boolean
If Object.Equals(storage, value) Then Return False
storage = value
NotifyPropertyChanged(PropertyName)
Return True
End Function
Public Sub NotifyPropertyChanged(ByVal propertyName As String)
RaiseEvent PropertyChanged(Me, New System.ComponentModel.PropertyChangedEventArgs(propertyName))
End Sub
Private _IsChecked As Boolean
Public Property IsChecked As Boolean
Get
Return _IsChecked
End Get
Set(value As Boolean)
SetProperty(_IsChecked, value, "IsChecked")
End Set
End Property
Private _SelectedIndex As Integer
Public Property SelectedIndex As Integer
Get
Return _SelectedIndex
End Get
Set(value As Integer)
SetProperty(_SelectedIndex, value, "SelectedIndex")
End Set
End Property
Private Sub TabControl_Initialized(sender As Object, e As EventArgs)
tc = DirectCast(sender, TabControl)
AddHandler tc.Items.CurrentChanging, AddressOf Items_CurrentChanging
End Sub
Private Sub Items_CurrentChanging(sender As Object, e As CurrentChangingEventArgs)
Dim Result As MessageBoxResult
If SelectedIndex = 0 And IsChecked Then
Result = MessageBox.Show("Do you want to leave this tab?", "Leave", MessageBoxButton.YesNo, MessageBoxImage.Question, MessageBoxResult.No, MessageBoxOptions.ServiceNotification)
If Result = MessageBoxResult.No Then
e.Cancel = True
tc.SelectedItem = DirectCast(sender, ICollectionView).CurrentItem
End If
End If
End Sub
End Class
If you run this app and follow these steps you will find the tab control stops responding to tab changes.
1. Click TAB 2 (we move to TAB 2)
2. Check the Locked checkbox
3. Click TAB 1
4. Respond NO to the popup (we stay on TAB 2)
5. Click TAB 1 again
6. Response YES to the popup (we return to TAB 1)
7. Click TAB 2 again (we move to TAB 2)
8. Click on TAB 1
9. Response NO to the popup (we stay on TAB 2)
10. Click on TAB 1 again -- nothing happens!
NB: If you clear and check the Locked checkbox then functionality returns.
If you put a breakpoint in Items_CurrentChanging everything works correctly. Surely this is a threading issue - can anyone tell me what is wrong and, more importantly, how to fix it?
Thanks

Answer 1

It looks like the Items_CurrentChanging event is not being raised because the tab I'm clicking on still has focus (even though it is not the current item). I don't know why we have to go through two iterations to see the problem. The solution is to add a line of code after we set SelectedIndex. This sets focus back to the tab we didn't move away from.
Dispatcher.BeginInvoke(Sub() DirectCast(tc.SelectedItem, UIElement).Focus())
If anyone can shine a light on what's going on here, I'd be very grateful.