jQuery's “uploadProgress” not firing in “$.ajax”

问题

I am new to jQuery and now, I am currently working on file uploads. And I want to add some progress bar each time I upload image. I used the uploadProgress in jQuery but it seems doesn't work. Here's my code:

$('#_form_').on('submit', function(e){

   var file_and_desc = new FormData($(this)[0]),
       form_url = "_pages/_form_";

       var ext = choose.val(),
           allowed = ['jpg','png'];

       if(ext){
          var get_ext = ext.split('.');
              get_ext.reverse();

              if($.inArray(get_ext[0].toLowerCase(), allowed) > -1){
                   //upload image
                   $.ajax({
                         url : form_url,
                         type: 'POST',
                         data: file_and_desc,
                         contentType: false,
                         processData: false,
                         uploadProgress: function(event, positio, total, percentComplete){
                          $('h1').html(percentComplete);
                         },
                         success: function(data){
                              // some code here...
                         }
                   });
              }
       }
});

That's it! What should I do?

回答1:

According to the $.ajax() reference, uploadProgress is not a valid option.

Instead, the xhr option is used instead, which lets you set progress listeners on the XMLHttpRequest that is used by the ajax request.

this answer shows how to do that:

$.ajax({
    xhr: function() {
        var xhr = new window.XMLHttpRequest();
        xhr.upload.addEventListener("progress", function(evt) {
            if (evt.lengthComputable) {
                var percentComplete = (evt.loaded / evt.total) * 100;
                //Do something with upload progress here
            }
       }, false);
       return xhr;
    },
    type: 'POST',
    url: "/",
    data: {},
    success: function(data){
        //Do something on success
    }
});

来源：https://stackoverflow.com/questions/42216409/jquerys-uploadprogress-not-firing-in-ajax