问在冗长的服务器端文件处理之前，如何知道ajax文件上传何时完全完成

EN

var formData = new FormData($('#form-file-upload')[0]);
$.ajax({
  url: 'ajax/file-upload.php',
  type: 'POST',
  dataType : "json",
  xhr: function() {
    var myXhr = $.ajaxSettings.xhr();
    if(myXhr.upload){
      myXhr.upload.addEventListener('progress', function(e){
        if(e.lengthComputable){
          var percent = (100*e.loaded/e.total).toFixed(0);
          $('#form-file-upload-progress-percent').html(percent + ' %');
          $('#form-file-upload-progress-bar').attr('aria-valuenow', percent).css({width: percent + '%'});
        }
      }, false);
    }
    return myXhr;
  },
  beforeSend: // code executed before submission
  success: // code executed after successful server-side processing
  complete: // code executed after server-side processing
  data: formData,
  cache: false,
  contentType: false,
  processData: false
});