Custom plugin to limit how many checkboxes can be checked for some fields on content submission forms.

gf-limit-checkboxes.php

<?php
	/*
	Plugin Name: Gravity Forms Limit Selected Checkboxes
	Plugin URI: http://gravitywiz.com/2012/06/11/limiting-how-many-checkboxes-can-be-checked
	Description: Custom plugin to limit how many checkboxes can be checked for some fields on content submission forms. No interface provided. Manually edit plugin code to add or remove fields to limit. Created from this code: http://gravitywiz.com/2012/06/11/limiting-how-many-checkboxes-can-be-checked
	Version: 1.0
	Author: Christi Nickerson
	Author URI: http://cnickerson.com
	License: GPL2
	*/

class GFLimitCheckboxes {

    private $form_id;
    private $field_limits;
    private $output_script;

    function __construct($form_id, $field_limits) {

        $this->form_id = $form_id;
        $this->field_limits = $this->set_field_limits($field_limits);

        add_filter("gform_pre_render_$form_id", array(&$this, 'pre_render'));
        add_filter("gform_validation_$form_id", array(&$this, 'validate'));

    }

    function pre_render($form) {

        $script = '';
        $output_script = false;

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

            $field_id = $field['id'];
            $field_limits = $this->get_field_limits($field['id']);

            if( !$field_limits   // if field limits not provided for this field
                || RGFormsModel::get_input_type($field) != 'checkbox'   // or if this field is not a checkbox
                || !isset($field_limits['max'])   // or if 'max' is not set for this field
                )
                continue;

            $output_script = true;
            $max = $field_limits['max'];
            $selectors = array();

            foreach($field_limits['field'] as $checkbox_field) {
                $selectors[] = "#field_{$form['id']}_{$checkbox_field} .gfield_checkbox input:checkbox";
            }

            $script .= "jQuery(\"" . implode(', ', $selectors) . "\").checkboxLimit({$max});";

        }

        GFFormDisplay::add_init_script($form['id'], 'limit_checkboxes', GFFormDisplay::ON_PAGE_RENDER, $script);

        if($output_script):
            ?>

            <script type="text/javascript">
            jQuery(document).ready(function($) {
                $.fn.checkboxLimit = function(n) {

                    var checkboxes = this;

                    this.toggleDisable = function() {

                        // if we have reached or exceeded the limit, disable all other checkboxes
                        if(this.filter(':checked').length >= n) {
                            var unchecked = this.not(':checked');
                            unchecked.prop('disabled', true);
                        }
                        // if we are below the limit, make sure all checkboxes are available
                        else {
                            this.prop('disabled', false);
                        }

                    }

                    // when form is rendered, toggle disable
                    checkboxes.bind('gform_post_render', checkboxes.toggleDisable());

                    // when checkbox is clicked, toggle disable
                    checkboxes.click(function(event) {

                        checkboxes.toggleDisable();

                        // if we are equal to or below the limit, the field should be checked
                        return checkboxes.filter(':checked').length <= n;
                    });

                }
            });
            </script>

            <?php
        endif;

        return $form;
    }

    function validate($validation_result) {

        $form = $validation_result['form'];
        $checkbox_counts = array();

        // loop through and get counts on all checkbox fields (just to keep things simple)
        foreach($form['fields'] as $field) {

            if( RGFormsModel::get_input_type($field) != 'checkbox' )
                continue;

            $field_id = $field['id'];
            $count = 0;

            foreach($_POST as $key => $value) {
                if(strpos($key, "input_{$field['id']}_") !== false)
                    $count++;
            }

            $checkbox_counts[$field_id] = $count;

        }

        // loop through again and actually validate
        foreach($form['fields'] as &$field) {

            if(!$this->should_field_be_validated($form, $field))
                continue;

            $field_id = $field['id'];
            $field_limits = $this->get_field_limits($field_id);

            $min = isset($field_limits['min']) ? $field_limits['min'] : false;
            $max = isset($field_limits['max']) ? $field_limits['max'] : false;

            $count = 0;
            foreach($field_limits['field'] as $checkbox_field) {
                $count += rgar($checkbox_counts, $checkbox_field);
            }

            if($count < $min) {
                $field['failed_validation'] = true;
                $field['validation_message'] = sprintf( _n('You must select at least %s item.', 'You must select at least %s items.', $min), $min );
                $validation_result['is_valid'] = false;
            }
            else if($count > $max) {
                $field['failed_validation'] = true;
                $field['validation_message'] = sprintf( _n('You may only select %s item.', 'You may only select %s items.', $max), $max );
                $validation_result['is_valid'] = false;
            }

        }

        $validation_result['form'] = $form;

        return $validation_result;
    }

    function should_field_be_validated($form, $field) {

        if( $field['pageNumber'] != GFFormDisplay::get_source_page( $form['id'] ) )
    		return false;

        // if no limits provided for this field
        if( !$this->get_field_limits($field['id']) )
            return false;

        // or if this field is not a checkbox
        if( RGFormsModel::get_input_type($field) != 'checkbox' )
            return false;

        // or if this field is hidden
        if( RGFormsModel::is_field_hidden($form, $field, array()) )
            return false;

        return true;
    }

    function get_field_limits($field_id) {

        foreach($this->field_limits as $key => $options) {
            if(in_array($field_id, $options['field']))
                return $options;
        }

        return false;
    }

    function set_field_limits($field_limits) {

        foreach($field_limits as $key => &$options) {

            if(isset($options['field'])) {
                $ids = is_array($options['field']) ? $options['field'] : array($options['field']);
            } else {
                $ids = array($key);
            }

            $options['field'] = $ids;

        }

        return $field_limits;
    }

}

/*
Template:
new GFLimitCheckboxes(FORM_ID, array(
    FIELD_ID => array(
        'min' => MIN_NUMBER,
        'max' => MAX_NUMBER
        )
    ));
Add new field IDs as needed.
Add a new array for each field as needed.
Add a new class for each form as needed.
See https://gravitywiz.com/limiting-how-many-checkboxes-can-be-checked/
for examples.
*/

new GFLimitCheckboxes(1, array(
	48 => array(
				'max' => 3
			),
	52 => array(
				'max' => 3
			),
	60 => array(
				'max' => 5
			)
));