Custom Ajax forms with managed_file form element

Question

We have a form with three fields - where depending on the value of the first select field, we will show the other two form elements - one select field (the options depends on the value of the first select field) and one file upload field.

The issue is that when I upload a file the second select list value is resetting. Because managed_file form element is again calling an ajax function which overrides the value of my first ajax call

Code is below:

public function buildForm(array $form, FormStateInterface $form_state) {
    $texts = array();
    $text_values = db_query("SELECT text_id, title FROM `heritage_text_structure`")->fetchAll();
    foreach($text_values AS $key => $value){
        $texts[$value->text_id] = $value->title;
    }
    $form['text'] = array(
        '#type' => 'select',
        '#title' => $this->t('Select the heritage text'),
        '#required' => TRUE,
        '#options' => $texts,
        '#default_value' => isset($form['text']['#default_value'])?$form['text']['#default_value']:null,
        '#ajax' => array(
            'event' => 'change',
            'wrapper' => 'text-info',
            'callback' => '::_ajax_text_callback',
        ),
    );
    if(!empty($form_state->getTriggeringElement())) {
        $description = '';
        $text_id = $form_state->getTriggeringElement()['#value'];
        $labels = db_query("SELECT level_labels FROM `heritage_text_structure` WHERE text_id = :text_id", array(':text_id' => $text_id))->fetchField();
        $labels_array = explode(',', $labels);
        $label_count = count($labels_array);
        for($i=0; $i<count($labels_array); $i++){
            if($i < $label_count-1)
                $description = $description.$labels_array[$i].', ';
            else $description = $description.$labels_array[$i];
        }
        $description = $description.', Content, Language';
        $csvCount = $label_count + 2;
    }
    $form['text_info'] = array(
        '#type' => 'container',
        '#prefix' => '<div id="text-info">',
        '#suffix' => '</div>'
    );
    if(isset($text_id) && $text_id > 0){
        $validators = array(
            'file_validate_extensions' => array('csv '),
        );
        $form['text_info']['fieldset'] = array(
            '#type' => 'fieldset',
            '#title' => $this->t('Select the Target Source'),
            '#description' => $this->t('Choose the source into which you are importing content'), 
        );
        $sources = array();
        $source_values = db_query("SELECT id, title FROM `heritage_source_nodes` WHERE text_id = :text_id", array('text_id' => $text_id))->fetchAll();
        foreach($source_values AS $key => $value){
            $sources[$value->id] = $value->title;
        }
        $form['text_info']['fieldset']['sources'] = array(
            '#type' => 'select',
            '#title' => $this->t('Select the Source to which content is imported'),
            '#required' => TRUE,
            '#options' => $sources,
            '#default_value' => isset($form['text_info']['fieldset']['sources']['widget']['#default_value'])?$form['text_info']['fieldset']['sources']['widget']['#default_value']:null,
        );
        $form['text_info']['fieldset']['file'] = array(
            '#type' => 'managed_file',
            '#title' => t('Upload the Content in CSV format'),
            '#size' => 20,
            '#description' => t('CSV file (eg: '.$description.'). Please maintain the order of the fields.'),
            '#upload_location' => 'public://file_uploads/',
            '#upload_validators' => $validators,
        );
        $form['text_info']['fieldset']['csv_count'] = array(
            '#type' => 'hidden',
            '#value' => $csvCount,
        );
    }
    $form['actions']['submit'] = array(
        '#type' => 'submit',
        '#value' => $this->t('Import Content'),
    );
    return $form;
}
/**
* {@inheritdoc} Map content to the corresponding fields.
*/
public function submitForm(array &$form, FormStateInterface $form_state) {
    $text = $form_state->getValue('text');
    print("<pre>");print_r($form_state->getValue());exit;
}

/**
* Ajax callback function
*/
public function _ajax_text_callback(array $form, FormStateInterface $form_state) {
    return $form['text_info'];
}

Answer 1

This is how we solved it:

During ajax callback, instead of getting the triggered element value, we got the value from $form_state->getUserInput()

ie, in the above code replaced

$text_id = $form_state->getTriggeringElement()['#value'];

with

$text_id = $form_state->getUserInput()['text'];

Since the managed_field will also do an ajax request the text_id was getting replaced in the above code.