Standardize Element API requests

Custom_ElementApiHelper.php

<?php
namespace Craft;

class Custom_ElementApiHelper
{
	public static function getParams($params)
	{
		$requestParams = array_filter([
			'elementType' => craft()->request->getParam('elementType'),
			'criteria' => Custom_ElementApiHelper::prepCriteria(craft()->request->getParam('criteria')),
			'transformer' => Custom_ElementApiHelper::prepTransformer(craft()->request->getParam('transformer', 'element')),
			'first' => (bool) craft()->request->getParam('first', false),
			'paginate' => (bool) craft()->request->getParam('paginate', true),
			'elementsPerPage' => craft()->request->getParam('elementsPerPage'),
			'pageParam' => craft()->request->getParam('pageParam'),
		], function ($i) {
			return $i !== null;
		});

		$params = array_merge($params, $requestParams);

		if (!isset($params['criteria']))
		{
			throw new Exception('Element API configs must specify a criteria.');
		}

		return $params;
	}

	public static function prepTransformer($transformer)
	{
	    if (!$transformer) {
	        return;
	    }

	    $transformerClass = 'Custom_'.ucfirst($transformer).'Transformer';

	    return class_exists('Craft\\'.$transformerClass) ? Craft::createComponent('Craft\\'.$transformerClass) : null;
	}

	public static function prepCriteria($criteria)
	{
	    if (!$criteria) {
	        return;
	    }

	    $criteria['relatedTo'] = self::prepRelatedCriteria(isset($criteria['relatedTo']) && $criteria['relatedTo'] ? $criteria['relatedTo'] : []);

	    return $criteria;
	}

	private static function prepRelatedCriteria($relatedTo)
	{
		$relatedTo = is_array($relatedTo) ? $relatedTo : [$relatedTo];
		$operator = isset($relatedTo['operator']) && $relatedTo['operator'] ? $relatedTo['operator'] : 'and';
		unset($relatedTo['operator']);

		$relatedTo = array_filter(array_map(function ($field, $element) {
			$criteria['element'] = array_filter(is_array($element) ? $element : [$element]);
			if (is_string($field)) {
				$criteria['field'] = $field;
			}

			return empty($criteria['element']) ? null : $criteria;
		}, array_keys($relatedTo), $relatedTo));

		if (count($relatedTo)) {
			array_unshift($relatedTo, $operator);
		}

		return $relatedTo;
	}
}

Custom_ElementTransformer.php

<?php
namespace Craft;
require realpath(implode('/', [
	craft()->path->getPluginsPath(),
	'/elementapi/vendor/autoload.php'
]));

use League\Fractal\TransformerAbstract;

class Custom_ElementTransformer extends TransformerAbstract
{
    protected $fields = ['title', 'slug', 'id'];

    public function transform(BaseElementModel $element)
    {
        $values = array();

        foreach ($this->fields as $fieldName) {
            $values[$fieldName] = $this->transformAttribute($element->$fieldName);
        }

        return $values;
    }

    protected function transformAttribute($value)
    {
        if ($value instanceof \DateTime) {
            return $value->format(\DateTime::ISO8601);
        } elseif ($value instanceOf ElementCriteriaModel) {
            if ($value->elementType instanceof AssetElementType) {
                return array_map(function($item) {
                    return [
                        'url' => $item->url,
                        'imgAltText' => $item->imgAltText,
                    ];
                }, $value->find());
            }

            return $value->ids();
        } elseif ($value instanceOf RichTextData) {
            return $value->getParsedContent();
        }

        return ModelHelper::packageAttributeValue($value);
    }
}

elementapi.php

<?php

namespace Craft;

Craft::import('plugins.custom.integrations.elementapi.*');

return [
    'endpoints' => [
        'api/elements.json' => function () {
		    return Custom_ElementApiHelper::getParams([
		        'transformer' => 'elements',
		    ]);
        },
	    'api/products.json' => function () {
	        return Custom_ElementApiHelper::getParams([
	            'transformer' => 'product',
	            'elementType' => ElementType::Entry,
	        ]);
	    },
    ],
];