R – How to properly handle a PreviewMouseDown event with a MessageBox confirmation

savechangestreeviewwpf

Earlier I asked how to cancel a WPF TreeViewItem.Selected event.

The answerers suggested I instead handle the PreviewMouseDown event before the selection even takes place. That makes sense.

I've tried to do that…

XAML…

<TreeView Name="TreeViewThings"
    ...
    PreviewMouseDown="TreeViewThings_PreviewMouseDown"
    TreeViewItem.Expanded="TreeViewThings_Expanded"
    TreeViewItem.Selected="TreeViewThings_Selected" >

Visual Basic…

Sub TreeViewThings_PreviewMouseDown(...)
    If UnsavedChangesExist() Then
        e.Handled = UserCancelled()
    Else
        e.Handled = False
    End If
End Sub

Function UnsavedChangesExist() As Boolean
    ...
End Function

Function UserCancelled() As Boolean
    Return MessageBox.Show("Discard your unsaved changes?", _
                           "Unsaved Changes", _
                           MessageBoxButton.OKCancel, _
                           MessageBoxImage.Question) = MessageBoxResult.Cancel
End Function

This is only sort of working…

If there are no unsaved changes, then it proceeds just fine and executes TreeViewThings_Selected().

If there are unsaved changes, then I see the MessageBox…

MessageBox: Continue and discard your unsaved changes? OK/Cancel http://img25.imageshack.us/img25/141/discard2yk0.gif

If I then choose Cancel, it cancels.
However, If I instead choose OK to discard my unsaved changes, then it just cancels anyway–even though e.Handled = False. It does not continue on and execute TreeViewThings_Selected().

I think the fact that there's a MessageBox screws it up somehow.

What am I doing wrong?

Best Answer

The problem is that the messagebox causes your tree to lose focus. Have you tried setting the focus back to the tree after the messagebox is dismissed?

Related Solutions

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If you want to bind to another property on the object:

{Binding Path=PathToProperty, RelativeSource={RelativeSource Self}}

If you want to get a property on an ancestor:

{Binding Path=PathToProperty,
    RelativeSource={RelativeSource AncestorType={x:Type typeOfAncestor}}}

If you want to get a property on the templated parent (so you can do 2 way bindings in a ControlTemplate)

{Binding Path=PathToProperty, RelativeSource={RelativeSource TemplatedParent}}

or, shorter (this only works for OneWay bindings):

{TemplateBinding Path=PathToProperty}

Wpf – How to cancel a user’s WPF TreeView click

UPDATE

Realized I could put the logic in SelectedItemChanged instead. A little cleaner solution.

Xaml

<TreeView Name="c_treeView"
          SelectedItemChanged="c_treeView_SelectedItemChanged">
    <TreeView.ItemContainerStyle>
        <Style TargetType="{x:Type TreeViewItem}">
            <Setter Property="IsSelected" Value="{Binding Path=IsSelected, Mode=TwoWay}" />
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

Code behind. I have some classes that is my ItemsSource of the TreeView so I made an interface (MyInterface) that exposes the IsSelected property for all of them.

private MyInterface m_selectedTreeViewItem = null;
private void c_treeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
    if (m_selectedTreeViewItem != null)
    {
        if (e.NewValue == m_selectedTreeViewItem)
        {
            // Will only end up here when reversing item
            // Without this line childs can't be selected
            // twice if "No" was pressed in the question..   
            c_treeView.Focus();   
        }
        else
        {
            if (MessageBox.Show("Change TreeViewItem?",
                                "Really change",
                                MessageBoxButton.YesNo,
                                MessageBoxImage.Question) != MessageBoxResult.Yes)
            {
                EventHandler eventHandler = null;
                eventHandler = new EventHandler(delegate
                {
                    c_treeView.LayoutUpdated -= eventHandler;
                    m_selectedTreeViewItem.IsSelected = true;
                });
                // Will be fired after SelectedItemChanged, to early to change back here
                c_treeView.LayoutUpdated += eventHandler;
            }   
            else
            {
                m_selectedTreeViewItem = e.NewValue as MyInterface;
            }        
        }
    }
    else
    {
        m_selectedTreeViewItem = e.NewValue as MyInterface;
    }
}

I haven't found any situation where it doesn't revert back to the previous item upon pressing "No".

Related Topic

.net – WPF: Does MessageBox Break PreviewMouseDown