How to alter Event Dropdown values from Register Participant or New Event Registration Page?

Question

On Register Particpant Page, I see list of all Events under Event dropdown
As per the requirement, I am using Recurring Entity to have repeat
events,so I need to hide repeat events and display only parent events
How can we alter the results in the Event dropdown

The Event dropdown has this form structure (from hook_buildForm) When I add a condition and try to print variables inside it on New Participant Registration it does not show up.(probably since its entering )

 $apiRequest['entity'] == 'event'

/**
 * Implements hook_civicrm_apiWrappers
 */
function custom_changes_civicrm_apiWrappers(&$wrappers, $apiRequest) {

  if ($apiRequest['entity'] == 'event' ) {
    print_r('<pre>');
    print_r($apiRequest);
    print_r($wrappers);
    die('ed'); 
 }
}

On adding the code answered below

if ('CRM_Event_Form_Participant' == $formName) {
    $elementIndex = $form->_elementIndex['event_id'];
    $element = & $form->_elements[$elementIndex];
    $dataApiParams = json_decode($element->_attributes['data-api-params'], TRUE);
    $dataApiParams['params'] = ['parent_event_id' => ['IS NULL' => 1]];
    $element->_attributes['data-api-params'] = json_encode($dataApiParams);   }

I get an error (when I comment I don't get the error).

Error: Call to undefined method stdClass::getName() in HTML_QuickForm->accept() (line 1686 of csites\all\modules\civicrm\packages\HTML\QuickForm.php).

To fix that I actually went to QuickForm.php and added an if condition as seen below

if(method_exists($element,'getName')

Is that a correct method and if not , how can I debug that further based on the error log.

 function accept(&$renderer)
    {
        $renderer->startForm($this);
        foreach (array_keys($this->_elements) as $key) {
            $element =& $this->_elements[$key];
            if (method_exists($element,'getName')){
            $elementName = $element->getName();
            $required    = ($this->isElementRequired($elementName) && !$element->isFrozen());
            $error       = $this->getElementError($elementName);
            $element->accept($renderer, $required, $error);
            }
        }
        $renderer->finishForm($this);
    } // end func accept

Answer 1

The event field on participant form uses event api to retrieve events. You can easily alter the list by implementing hook_civicrm_apiWrappers hook by adding additional parameter to exclude recurring event in $params of api probably 'parent_event_id' => ['IS NULL' => 1].

Or using buildform hook try adding parent_event_id condition in data-api-params of the field(element).

if ('CRM_Event_Form_Participant' == $formName) {
    $elementIndex = CRM_Utils_Array::value('event_id', $form->_elementIndex);
    if (empty($elementIndex)) {
      return;
    }
    $element = & $form->_elements[$elementIndex];
    $dataApiParams = json_decode($element->_attributes['data-api-params'], TRUE);
    $dataApiParams['params'] = ['parent_event_id' => ['IS NULL' => 1]];
    $element->_attributes['data-api-params'] = json_encode($dataApiParams);
  }

Cheers

Pradeep