WPF, create Custom DataGridTextColumn to Prevent unwanted Character

i newbie to WPF, i want to prevent user to input character, eg. character "-", so i created custom DataGridTextColumn with following code :

public class DataGridNumericColumn : DataGridTextColumn
{
    protected override object PrepareCellForEdit(FrameworkElement editingElement, RoutedEventArgs editingEventArgs)
    {
        var textBox = (TextBox) editingElement;
        textBox.PreviewTextInput += OnPreviewTextInput;
        return base.PrepareCellForEdit(editingElement, editingEventArgs);
    }


    private void OnPreviewTextInput(object sender, TextCompositionEventArgs e)
    {
        var textBox = (TextBox)sender;
        if (e.Text == "-")
            return;
        if (!this.IsNumeric(e.Text))
            e.Handled = true;
    }
}

and XAML :

<ZF:ZFDataGrid
        Grid.Row="4" Grid.Column="0" 
        HorizontalAlignment="Stretch" VerticalAlignment="Top"
        HorizontalContentAlignment="Stretch"
        VerticalContentAlignment="Stretch"
        CanUserAddRows="True"
        CanUserDeleteRows="False"
        CanUserResizeRows="False"
        CanUserReorderColumns="False"
        CanUserSortColumns="False"
        IsSynchronizedWithCurrentItem="True"
        SelectionUnit="Cell"
        SelectionMode="Single"
        Margin="3,3,3,0" 
        AutoGenerateColumns="False"
        AlternatingRowBackground="WhiteSmoke"
        RowHeaderWidth="30"
        FontSize="18"
        ItemsSource="{Binding POSModel}">
    <ZF:DataGridNumericColumn Header="Qty" Width="80" />
</ZF:ZFDataGrid>

the Custom DataGridNumericColumn work well, except when i press the character for the first time. if i press F2 to edit or double click the column and then press the key, everything works well.

but if i press the key without editing the cell first, the custom DataGridNumericColumn not work.

i put breakpoint on PrepareCellForEdit, and the coding works. but method OnPreviewTextInput works the second time when i press the key. not the first one.

can anyone give me another solution ?

EDITED:

protected override object PrepareCellForEdit(FrameworkElement editingElement, RoutedEventArgs editingEventArgs)
    {
        var textBox = (TextBox) editingElement;
        textBox.PreviewTextInput += OnPreviewTextInput;
        textBox.TextChanged += OnTextChanged; //change here
        return base.PrepareCellForEdit(editingElement, editingEventArgs);
    }

this code only run ONCE, the rest will be handled by OnPreviewTextInput

  private void OnTextChanged(object sender, TextChangedEventArgs e)
    {
        var textBox = (TextBox)sender;

        if (textBox.Text.Contains("-"))
        {
            textBox.TextChanged -= OnTextChanged;
            textBox.Text = "";
        }
    }