Yii2框架加载css和js文件的方法分析

<?php
/**
 * @link http://www.yiiframework.com/
 * @copyright Copyright (c) 2008 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */
namespace app\assets;
use yii\web\AssetBundle;
/**
 * @author Qiang Xue <qiang.xue@gmail.com 
 * @since 2.0
 */
class AppAsset extends AssetBundle
{
  public $basePath = '@webroot';
  public $baseUrl = '@web';
  // public $css = [
  //   'css/site.css',
  //   'bootstrap/css/bootstrap.min.css',
  // ];
  // public $js = [
  //   'jquery/jquery.min.js',
  //   'bootstrap/js/bootstrap.min.js'
  // ];
  public function registerAssetFiles( $view )
  {
    //加一个版本号
    //目的：使浏览器获取最新的文件
    $release = "20170226";
    $this- css = [
      "bootstrap/css/bootstrap.min.css?v={$release}"
    ];
    $this- js = [
       'jquery/jquery.min.js',
       'bootstrap/js/bootstrap.min.js'
    ];
    parent::registerAssetFiles( $view );
  }
}