angularjs是如何监听XMLHttpRequest事件的？

Question

在线DEMO：https://plnkr.co/edit/KAwhab7...(function(angular) {  'use strict';angular.module('scopeExample', [])  .controller('MyController', ['$scope','$timeout', function($scope,$timeout) {   $scope.greeting='hhh'   var httpRequest = new XMLHttpRequest();    httpRequest.onreadystatechange = function(){       console.log(httpRequest)      if(httpRequest.readyState==2){ //不更新        $scope.greeting='000'      }      // if(httpRequest.readyState==1){//更新       //   $scope.greeting='000'      // }    };    httpRequest.onload = function(){      // 不更新       // $scope.greeting='333'    }     httpRequest.open('GET', 'https://www.json-generator.com/api/json/get/bHEfYDDHEy?indent=2');    httpRequest.send();  }]);})(window.angular);为什么angularjs只有在readyState==1会更新数据？最好可以结合源代码解释