当同步选项不可用时，如何等待ajax请求在javascript中完成？

Question

I'm trying to wait for the AJAX request to complete. It would be easy if the method xmlhttp.open would support async = false but Ant Galio does not support this option and only asynchronous requests are permitted. The question is how can I wait for the callback to be called.

    var ajaxFinished = false;
    var xmlhttp = new XMLHttpRequest();
    this.debug("-- onreadystatechange is being defined");
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4) {
            ajaxFinished = true;
                var data = xmlhttp.responseText;
            if (xmlhttp.status == 200) {
                that.debug('downloadSettings: SUCCESS');
                [...]
            } else {
                that.debug('downloadSettings:');
                that.debug('-- Error: ');
                that.debug('-- ResponseText: "'+data+'"')
            }
        }
    }

    while (ajaxFinished == false) {

    }

    this.debug("-- open connection");
    xmlhttp.open("GET", requestUrl, true); /* Ant Galio does not support synchronous  */
    this.debug("-- send");
    xmlhttp.send();                 

I'm looking for some kind of active waiting. I know about another solution but I'm interested in a solution that would not require changing more of the code than is my example above.

Thanks!

Answer 1

yes, you can

function getFile(url) {
  if (window.XMLHttpRequest) {              
    AJAX=new XMLHttpRequest();              
  } else {                                  
    AJAX=new ActiveXObject("Microsoft.XMLHTTP");
  }
  if (AJAX) {
     AJAX.open("GET", url, false);                             
     AJAX.send(null);
     return AJAX.responseText;                                         
  } else {
     return false;
  }                                             
}

var fileFromServer = getFile('http://somedomain.com/somefile.txt');

w3c definition http://www.w3.org/TR/XMLHttpRequest/#the-open()-method

client . open(method, url [, async = true [, user = null [, password = null]]])

Answer 2

You can't. There is no "active waiting" in JavaScript, there can be only one active execution a time ("single-threaded").

Answer 3

There is a workaround. Instead of using the the blocking while loop for poll use the nonblocking setInterval ().. so your code might look something like this.

    var ajaxFinished = false; 
    var xmlhttp = new XMLHttpRequest(); 
    this.debug("-- onreadystatechange is being defined"); 
    xmlhttp.onreadystatechange = function() { 
     if (xmlhttp.readyState == 4) { 
      ajaxFinished = true; 
      var data = xmlhttp.responseText; 
      if (xmlhttp.status == 200) { 
        that.debug('downloadSettings: SUCCESS'); 
         [...] 
      } else { 
         that.debug('downloadSettings:'); 
         that.debug('-- Error: "); 
         that.debug('-- ResponseText: "'+data+'"') 
      } 
     } 
    } 

    //Polling function 

    function checkEvent(){
    if(ajaxFinished == true){
    //your code i.e xmlhttp.open("GET", requestUrl, true);
    }
    clearInterval(chkeventid);//Clear Interval via ID for single time execution
    }

    var chkeventid=self.setInterval("checkEvent()",100);//The poll call

    The setInterval method is treated a bit differently in JS as you know so you may use it as against the while loop.