【ワードプレス】スラッグが重複していないかチェック

functions.php

new WP_Check_Slug;

class WP_Check_Slug {

	public function __construct()
	{
		add_action('admin_footer-post-new.php', array($this, 'admin_footer'));
		add_action('admin_footer-post.php', array($this, 'admin_footer'));
		add_action('wp_ajax_check-slug', array($this, 'check_slug'));
		
	}
	
	/**
	 * ポスト投稿時にスラッグチェックする
	 * @global type $hook_suffix
	 */
	public function admin_footer()
	{
		global $hook_suffix;
		if ('post-new.php' === $hook_suffix || 'post.php' === $hook_suffix) {
			?>
			<script type="text/javascript">
				document.getElementById('post_name').onchange = function () {
					var post_name = document.getElementById('post_name').value;
					var xhr = new XMLHttpRequest();
					xhr.onreadystatechange = function () {
						switch (xhr.readyState) {
							case 4:
								if ((200 <= xhr.status && xhr.status < 300) || (xhr.status == 304)) {
									var response = JSON.parse(xhr.response);
									if (response.data.post_id > 0)
										alert("登録済みのスラッグです。");
								}
								break;
						}
					};
					xhr.open("POST", '<?php echo admin_url('admin-ajax.php'); ?>');
					xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
					xhr.send('action=check-slug&post_name=' + post_name);
					return false;
				};
			</script>
			<?php
		}
	}

	public function check_slug()
	{
		$post = get_page_by_path($_POST['post_name'], OBJECT, 'post');
		$post_ID = ($post) ? $post->ID : 0;

		wp_send_json_success( array(
			'post_id' => $post_ID
		));				
	}
}