Re: Adding custom action in view page of modeladmin

class GridFieldDeleteAllButton implements GridField_HTMLProvider, GridField_ActionProvider {
   protected $targetFragment;

   public function __construct($targetFragment = "before") {
      $this->targetFragment = $targetFragment;
   }

   public function getHTMLFragments($gridField) {
      $button = new GridField_FormAction(
         $gridField,
         'deleteall',
         _t('TableListField.DELETEALL', 'Delete All'),
         'deleteall',
         null
      );
      $button->setAttribute('data-icon', 'cross-circle');
      $button->addExtraClass('no-ajax');
      return array(
         $this->targetFragment => '<p class="grid-csv-button">' . $button->Field() . '</p>',
      );
   }

   public function getActions($gridField) {
      return array('deleteall');
   }

   public function handleAction(GridField $gridField, $actionName, $arguments, $data) {
      if($actionName == 'deleteall') {
         return $this->handleDeleteAll($gridField);
      }
   }

   public function handleDeleteAll($gridField, $request = null) {
      //Add your own DELETE logic here !
      $items=$gridField->getList();
      foreach ($items as $item) {
         $item->delete();
      }
      Controller::curr()->redirectBack();
   }
}

public function updateEditForm(&$form) {
$gridField = $form->Fields()->fieldByName($this->sanitiseClassName($this->owner->modelClass));
$gridField->getConfig()->addComponent(new GridFieldDeleteAllButton('before'));
}