public function boot()
 {
     UserController::extend(function ($controller) {
         $controller->implement[] = 'VojtaSvoboda.UserExportPdf.Behaviors.PdfExportBehavior';
     });
     // extend user listing
     Event::listen('backend.list.extendColumns', function ($widget) {
         // Only for the User controller
         if (!$widget->getController() instanceof UserController) {
             return;
         }
         // Only for the User model
         if (!$widget->model instanceof UserModel) {
             return;
         }
         // Add an extra birthday column
         $widget->addColumns(['export' => ['label' => 'vojtasvoboda.userexportpdf::lang.list.export.column', 'sortable' => false, 'clickable' => false]]);
     });
     // extend export column value to add export button
     Event::listen('backend.list.overrideColumnValue', function ($widget, $model, $column, $value) {
         // Only for the User model
         if (!$model instanceof UserModel) {
             return;
         }
         if ($column->columnName == 'export') {
             $button = '<a';
             $button .= ' href="' . url('/backend/rainlab/user/users/pdf/' . $model->id) . '"';
             $button .= ' class="btn btn-xs btn-warning"';
             $button .= '>' . Lang::get('vojtasvoboda.userexportpdf::lang.list.export.button') . '</a>';
             return $button;
         }
     });
 }