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<?php

# Basic Usage
new GW_List_Field_As_Choices( array(
    'form_id' => 1,
    'list_field_id' => 2,
    'choice_field_ids' => 3
) );

# Enable Sorting of Choices Alphanumerically
new GW_List_Field_As_Choices( array(
    'form_id' => 1,
    'list_field_id' => 2,
    'choice_field_ids' => 3,
    'sort' => true
) );

# Populating Multiple Choice Fields
new GW_List_Field_As_Choices( array(
    'form_id' => 384,
    'list_field_id' => 3,
    'choice_field_ids' => array( 6, 7 )
) );

# Customizing the Choice Label and Value
new GW_List_Field_As_Choices( array(
    'form_id' => 384,
    'list_field_id' => 2,
    'choice_field_ids' => array( 4, 5 ),
    'label_template' => '{Name} <span style="color:#999;font-style:italic;">({Age})</span>',
    'value_template' => '{Name}'
) );

<?php
/**
* Gravity Wiz // Use List Field as Choices for Gravity Forms
*
* Adds support for populating choice-based fields (i.e. checkboxes, selects, radio buttons) with values entered in a 
* List field. This functionality requires that the form has multiple pages and that the List field must be placed on 
* a page prior to the choice-based field that it will populate.
*
* @author    David Smith <david@gravitywiz.com>
* @license   GPL-2.0+
* @link      http://gravitywiz.com/use-list-field-choices-gravity-forms/
*/
class GW_List_Field_As_Choices {

    function __construct( $args ) {

        $this->_args = wp_parse_args( $args, array(
            'form_id'           => false,
            'list_field_id'     => false,
            'choice_field_ids'  => false,
            'label_template'    => '{0}',
            'sort'              => false
        ) );

        if( ! is_array( $this->_args['choice_field_ids'] ) )
            $this->_args['choice_field_ids'] = array( $this->_args['choice_field_ids'] );

        extract( $this->_args ); // gives us $form_id, $list_field_id, $choices_field_id

        add_filter( 'gform_pre_render', array( $this, 'populate_choice_fields' ), 9 );
        add_filter( 'gform_pre_validation', array( $this, 'populate_choice_fields' ), 9 );
        add_filter( 'gform_pre_submission_filter_' . $form_id, array( $this, 'populate_choice_fields' ) );

    }

    function populate_choice_fields( $form ) {
        
        if( $form['id'] != $this->_args['form_id'] )
            return $form;
            
        $list_field = GFFormsModel::get_field( $form, $this->_args['list_field_id'] );
        $values = GFFormsModel::get_field_value( $list_field );

        // if list field doesn't have any values, let's ditch this party
        if( ! is_array( $values ) )
            return $form;

        $choices = $this->get_list_choices( $values );

        foreach( $form['fields'] as &$field ) {

            if( ! $this->is_applicable_field( $field ) )
                continue;

            // set 'choices' for choice fields
            $field['choices'] = $choices;

            // only set inputs for 'checkbox' choice fields
            if( GFFormsModel::get_input_type( $field ) == 'checkbox' ) {
                $inputs = array();
                foreach( $choices as $index => $choice ) {
                    $inputs[] = array(
                        'label' => $choice['text'],
                        'id'    => $field['id'] . '.' . ( $index + 1 )
                    );
                }
                $field['inputs'] = $inputs;
            }
            
        }

        return $form;
    }
    
    function get_list_choices( $values ) {
        
        $choices = array();
        
        foreach( $values as $row ) {
            
            $label = $this->replace_template( $this->_args['label_template'], $row );
            $value = isset( $this->_args['value_template'] ) ? $this->replace_template( $this->_args['value_template'], $row ) : $label;
            
            $choices[] = array(
                'text' => $label,
                'value' => $value
            );
            
        }

        if( $this->_args['sort'] == true )
            usort( $choices, create_function( '$a, $b', 'return strnatcasecmp( $a["text"], $b["text"] );' ) );

        return $choices;
    }
    
    function replace_template( $template, $row ) {
        
        // combine our templates so we can find all matches at once
        preg_match_all( '/{(\w+)}/', $template, $matches, PREG_SET_ORDER );
        
        if( is_array( $row ) ) {

            $mega_row = array_merge( $row, array_values( $row ) );

            foreach( $matches as $match ) {
                $template = str_replace( $match[0], rgar( $mega_row, $match[1] ), $template );
            }
            
        } else {

            foreach( $matches as $match ) {
                $template = str_replace( $match[0], $row, $template );
            }
            
        }
        
        return $template;
    }
    
    function is_applicable_field( $field ) {
        
        $is_choice_field = is_array( rgar( $field, 'choices' ) );
        $is_registered_field = in_array( $field['id'], $this->_args['choice_field_ids'] );
        
        return $is_choice_field && $is_registered_field;
    }
    
}

/**
* Uncomment the code below by removing the pound symbols (#) in front of each line. See @link in the comment section 
* at the top for additional usage instructions.
*/

# Basic Usage
# new GW_List_Field_As_Choices( array(
#    'form_id' => 1,
#    'list_field_id' => 2,
#    'choice_field_ids' => 3
# ) );