source: contact-form-7/tags/5.9.6/modules/textarea.php

<?php
/**
** A base module for [textarea] and [textarea*]
**/

/* form_tag handler */

add_action( 'wpcf7_init', 'wpcf7_add_form_tag_textarea', 10, 0 );

function wpcf7_add_form_tag_textarea() {
        wpcf7_add_form_tag( array( 'textarea', 'textarea*' ),
                'wpcf7_textarea_form_tag_handler', array( 'name-attr' => true )
        );
}

function wpcf7_textarea_form_tag_handler( $tag ) {
        if ( empty( $tag->name ) ) {
                return '';
        }

        $validation_error = wpcf7_get_validation_error( $tag->name );

        $class = wpcf7_form_controls_class( $tag->type );

        if ( $validation_error ) {
                $class .= ' wpcf7-not-valid';
        }

        $atts = array();

        $atts['cols'] = $tag->get_cols_option( '40' );
        $atts['rows'] = $tag->get_rows_option( '10' );
        $atts['maxlength'] = $tag->get_maxlength_option( '400' );
        $atts['minlength'] = $tag->get_minlength_option();

        if ( $atts['maxlength'] and $atts['minlength']
        and $atts['maxlength'] < $atts['minlength'] ) {
                unset( $atts['maxlength'], $atts['minlength'] );
        }

        $atts['class'] = $tag->get_class_option( $class );
        $atts['id'] = $tag->get_id_option();
        $atts['tabindex'] = $tag->get_option( 'tabindex', 'signed_int', true );
        $atts['readonly'] = $tag->has_option( 'readonly' );

        $atts['autocomplete'] = $tag->get_option(
                'autocomplete', '[-0-9a-zA-Z]+', true
        );

        if ( $tag->is_required() ) {
                $atts['aria-required'] = 'true';
        }

        if ( $validation_error ) {
                $atts['aria-invalid'] = 'true';
                $atts['aria-describedby'] = wpcf7_get_validation_error_reference(
                        $tag->name
                );
        } else {
                $atts['aria-invalid'] = 'false';
        }

        $value = empty( $tag->content )
                ? (string) reset( $tag->values )
                : $tag->content;

        if ( $tag->has_option( 'placeholder' )
        or $tag->has_option( 'watermark' ) ) {
                $atts['placeholder'] = $value;
                $value = '';
        }

        $value = $tag->get_default_option( $value );

        $value = wpcf7_get_hangover( $tag->name, $value );

        $atts['name'] = $tag->name;

        $html = sprintf(
                '<span class="wpcf7-form-control-wrap" data-name="%1$s"><textarea %2$s>%3$s</textarea>%4$s</span>',
                esc_attr( $tag->name ),
                wpcf7_format_atts( $atts ),
                esc_textarea( $value ),
                $validation_error
        );

        return $html;
}


add_action(
        'wpcf7_swv_create_schema',
        'wpcf7_swv_add_textarea_rules',
        10, 2
);

function wpcf7_swv_add_textarea_rules( $schema, $contact_form ) {
        $tags = $contact_form->scan_form_tags( array(
                'basetype' => array( 'textarea' ),
        ) );

        foreach ( $tags as $tag ) {
                if ( $tag->is_required() ) {
                        $schema->add_rule(
                                wpcf7_swv_create_rule( 'required', array(
                                        'field' => $tag->name,
                                        'error' => wpcf7_get_message( 'invalid_required' ),
                                ) )
                        );
                }

                if ( $minlength = $tag->get_minlength_option() ) {
                        $schema->add_rule(
                                wpcf7_swv_create_rule( 'minlength', array(
                                        'field' => $tag->name,
                                        'threshold' => absint( $minlength ),
                                        'error' => wpcf7_get_message( 'invalid_too_short' ),
                                ) )
                        );
                }

                if ( $maxlength = $tag->get_maxlength_option( '400' ) ) {
                        $schema->add_rule(
                                wpcf7_swv_create_rule( 'maxlength', array(
                                        'field' => $tag->name,
                                        'threshold' => absint( $maxlength ),
                                        'error' => wpcf7_get_message( 'invalid_too_long' ),
                                ) )
                        );
                }
        }
}


/* Tag generator */

add_action( 'wpcf7_admin_init', 'wpcf7_add_tag_generator_textarea', 20, 0 );

function wpcf7_add_tag_generator_textarea() {
        $tag_generator = WPCF7_TagGenerator::get_instance();
        $tag_generator->add( 'textarea', __( 'text area', 'contact-form-7' ),
                'wpcf7_tag_generator_textarea' );
}

function wpcf7_tag_generator_textarea( $contact_form, $args = '' ) {
        $args = wp_parse_args( $args, array() );
        $type = 'textarea';

        $description = __( "Generate a form-tag for a multi-line text input field. For more details, see %s.", 'contact-form-7' );

        $desc_link = wpcf7_link( __( 'https://contactform7.com/text-fields/', 'contact-form-7' ), __( 'Text fields', 'contact-form-7' ) );

?>
<div class="control-box">
<fieldset>
<legend><?php echo sprintf( esc_html( $description ), $desc_link ); ?></legend>

<table class="form-table">
<tbody>
        <tr>
        <th scope="row"><?php echo esc_html( __( 'Field type', 'contact-form-7' ) ); ?></th>
        <td>
                <fieldset>
                <legend class="screen-reader-text"><?php echo esc_html( __( 'Field type', 'contact-form-7' ) ); ?></legend>
                <label><input type="checkbox" name="required" /> <?php echo esc_html( __( 'Required field', 'contact-form-7' ) ); ?></label>
                </fieldset>
        </td>
        </tr>

        <tr>
        <th scope="row"><label for="<?php echo esc_attr( $args['content'] . '-name' ); ?>"><?php echo esc_html( __( 'Name', 'contact-form-7' ) ); ?></label></th>
        <td><input type="text" name="name" class="tg-name oneline" id="<?php echo esc_attr( $args['content'] . '-name' ); ?>" /></td>
        </tr>

        <tr>
        <th scope="row"><label for="<?php echo esc_attr( $args['content'] . '-values' ); ?>"><?php echo esc_html( __( 'Default value', 'contact-form-7' ) ); ?></label></th>
        <td><input type="text" name="values" class="oneline" id="<?php echo esc_attr( $args['content'] . '-values' ); ?>" /><br />
        <label><input type="checkbox" name="placeholder" class="option" /> <?php echo esc_html( __( 'Use this text as the placeholder of the field', 'contact-form-7' ) ); ?></label></td>
        </tr>

        <tr>
        <th scope="row"><label for="<?php echo esc_attr( $args['content'] . '-id' ); ?>"><?php echo esc_html( __( 'Id attribute', 'contact-form-7' ) ); ?></label></th>
        <td><input type="text" name="id" class="idvalue oneline option" id="<?php echo esc_attr( $args['content'] . '-id' ); ?>" /></td>
        </tr>

        <tr>
        <th scope="row"><label for="<?php echo esc_attr( $args['content'] . '-class' ); ?>"><?php echo esc_html( __( 'Class attribute', 'contact-form-7' ) ); ?></label></th>
        <td><input type="text" name="class" class="classvalue oneline option" id="<?php echo esc_attr( $args['content'] . '-class' ); ?>" /></td>
        </tr>

</tbody>
</table>
</fieldset>
</div>

<div class="insert-box">
        <input type="text" name="<?php echo $type; ?>" class="tag code" readonly="readonly" onfocus="this.select()" />

        <div class="submitbox">
        <input type="button" class="button button-primary insert-tag" value="<?php echo esc_attr( __( 'Insert Tag', 'contact-form-7' ) ); ?>" />
        </div>

        <br class="clear" />

        <p class="description mail-tag"><label for="<?php echo esc_attr( $args['content'] . '-mailtag' ); ?>"><?php echo sprintf( esc_html( __( "To use the value input through this field in a mail field, you need to insert the corresponding mail-tag (%s) into the field on the Mail tab.", 'contact-form-7' ) ), '<strong><span class="mail-tag"></span></strong>' ); ?><input type="text" class="mail-tag code hidden" readonly="readonly" id="<?php echo esc_attr( $args['content'] . '-mailtag' ); ?>" /></label></p>
</div>
<?php
}