namespace backend\assets;
use yii\web\AssetBundle;
/**
 * @author Qiang Xue 
 * @since 2.0
 */
class AppAsset extends AssetBundle
{
  public $basePath = "@webroot";
  public $baseUrl = "@web";
  //默认自动加载样式
  public $css = [
    "css/site.css",
  ];
  //默认自动加载js
  public $js = [
  ];
  //依赖关系管理
  public $depends = [
    "yii\web\YiiAsset",
    "yii\bootstrap\BootstrapAsset",
  ];  
  //定义按需加载JS方法，注意加载顺序在最后 
  public static function addJs($view, $jsfile) { 
    $view->registerJsFile(
      $jsfile, 
      [
        AppAsset::className(), 
        "depends" => "backend\assets\AppAsset"
      ]
    ); 
  } 
  //定义按需加载css方法，注意加载顺序在最后 
  public static function addCss($view, $cssfile) { 
    $view->registerCssFile(
      $cssfile, 
      [
       AppAsset::className(), 
       "depends" => "backend\assets\AppAsset"
      ]
    ); 
  } 
}

<?php

use backend\assets\AppAsset;
AppAsset::register($this);
AppAsset::addJs($this,Yii::$app->request->baseUrl."/js/a.js");
AppAsset::addCss($this,Yii::$app->request->baseUrl."/css/b.css");
?>

<?php
$this->registerJs("
  $(function () {
  //为所欲为的写你想要写的js代码吧
  
  });
", \yii\web\View::POS_END);

<?php $this->beginBlock('js') ?> 

  //js代码
  
<?php $this->endBlock() ?> 
<?php $this->registerJs($this->blocks['js'], \yii\web\View::POS_END); ?> 

<script type="text/javascript">
<?php $this->beginBlock('js') ?> 

  //js代码
  
<?php $this->endBlock() ?> 
<?php $this->registerJs($this->blocks['js'], \yii\web\View::POS_END); ?> 
</script>