API.php

<?php

class API {
    private static $endpoints = array();

    public static function add_endpoint($endpoint) {
        self::$endpoints[] = $endpoint;
    }

    public static function construct_endpoints() {
        add_filter('query_vars', function ($query_vars) {
            $query_vars[] = 'api';
            return $query_vars;
        });

        foreach (self::$endpoints as $key => $endpoint) {
            add_action('init', function () use ($endpoint) {
                add_rewrite_rule('api' . $endpoint->get_url_regex(), 'index.php?api=' . $endpoint->get_name(), 'top');
            });

            add_filter('template_include', function ($template) use ($endpoint) {
                global $wp_query;

                if (isset( $wp_query->query_vars['api'] ) && !is_page() && !is_single()) {
                    $wp_query->is_404 = false;
                    $wp_query->is_archive = true;
                    $wp_query->is_category = true;

                    $base_name = str_replace('$matches[1]', '', $endpoint->get_name());
                    if (strpos($wp_query->query_vars['api'], $base_name) !== false) {
                        $match = str_replace($base_name, '', $wp_query->query_vars['api']);
                        header('Content-Type: application/json');
                        header('Access-Control-Request-Headers: X-Requested-With, accept, content-type');
                        header('Access-Control-Allow-Methods: GET, POST');

                        // Disable caching in debug mode
                        if (WP_DEBUG) {
                            die($output = json_encode($endpoint->get_output($match)));
                        }

                        // If it's a POST request, don't cache anything
                        if ($_SERVER['REQUEST_METHOD'] == 'POST') {
                            die(json_encode($endpoint->get_output($match)));
                        }

                        $filename = dirname(__FILE__) . '/cache/' . $wp_query->query_vars['api'] . '.json';

                        // How long the cache for endpoints lasts for in seconds
                        $cache_ttl = 15;

                        if (file_exists($filename) && time() - filemtime($filename) < $cache_ttl) {
                            $output = file_get_contents($filename);
                        }
                        else {
                            $output = json_encode($endpoint->get_output($match));
                            file_put_contents($filename, $output);
                        }

                        die($output);
                    }
                } else {
                    return $template;
                }
            });
        }
    }
}