private void Attach(FrameworkElement element)
 {
     element.MouseLeftButtonDown += this.OnHeaderMouseLeftButtonDown;
     this.dataSource = element.ParentOfType<RadGanttView>().DataContext as TasksDataSource;
 }
 private void Detach(FrameworkElement element)
 {
     element.MouseLeftButtonDown -= this.OnHeaderMouseLeftButtonDown;
     this.dataSource = null;
     this.proxy = null;
 }