yii2使用ajax返回json的实现方法

public function actionAjax()
{
  if(isset(Yii::$app->request->post('test'))){
    $test = "Ajax Worked!";
    // do your query stuff here
  }else{
    $test = "Ajax failed";
    // do your query stuff here
  }
  // return Json
  return \yii\helpers\Json::encode($test);
}