使用XMLHttpRequest的open（）函数循环访问某些URL

Question

我正在从for循环中进行XMLHttpRequest open（）调用。循环遍历一组链接。我想等到readyState变为4（响应到来），然后再进行下一次迭代。我该怎么做？请帮忙

Answer 1

Rather than using a for loop to control the iterations, set up an object that :

1) On creation is passed a 'nextIter' callback object

2) Makes it's own call

3) Then on readystate4 ...

3a) Does it's own processing

3b) Calls the member 'nextIter' function. This object would be another instance of the same object.

This way your for loop would just create an object structure where,

objA <whenreadystate4 refs> objB <whenreadystate4 refs> objC ...

... and the calls just chain up as each ajax call finishes, and delegates on to the next in the chain ... that is until the bottom element references null.

Hopefully that makes sense.

Answer 2

Btw my existing program looks like this and i want to wait till the readyState becomes 4 before going to the next iteration

onMenuItemCommand: function() {
  var i;//iteration value of the loop

  var xhr=new XMLHttpRequest();//create the XMLHttp request
  var e=content.document.getElementsByTagName('a');// get all the links in the content page

  for( i=0;i<e.length;i++){ // loop goes through the selected links
    webassist.setLink(e[i]);      //set the link element

    if(e[i]!=""){  // check the link's href value is null

      xhr.open('HEAD',e[i] ,true);
      xhr.send(null);

    }
    else{ //if the href value is null set it to color red
      webassist.setColor("#FF0000");//red
    }

    xhr.onreadystatechange=function(evnt){
      if(xhr.readyState==4){
        if(xhr.status==200){
          webassist.setColor("#99FF66");//green
        }
        else{
          webassist.setColor("#FF0000");//red
        }
      }
      else{
        webassist.setColor("#FF00FF"); //pink
      }
    }
  }  
}