使用具有预调整大小的ajax进行多个文件上载

Question

我有一个预调整大小和ajax文件上传的功能。我根据文件输入长度大小在FOR bucle中调用此函数。当我打电话一次。工作正常，但有些时候，（我不知道为什么只有时候）当我多次调用时，函数上传重复的图像。也许对于下一个是什么时候前一个还没有完成并使用相同的画布？我怎么能解决这个问题？非常感谢！

这是我的代码：

...

for (var i = 0; i < filesToUpload.length; i++) {
   j=j+1;
   preandupload(filesToUpload[i],filesToUpload.length,j);
}
...

function preandupload(file,cuantos,actual){

var dataurl = null;
var img=null;
var canvas=null;
var ctx=null;
// Create an image
img = new Image();


// Create a file reader
var reader = new FileReader();
// Set the image once loaded into file reader
reader.onloadend = function(e)
{
    img.src = e.target.result;

    img.onload = function () {
        var canvas = document.createElement("canvas");
        canvas.id = "mycanvas"+actual;
        var ctx = canvas.getContext("2d");

        ctx.drawImage(img, 0, 0);


        var MAX_WIDTH = 1280;
        var MAX_HEIGHT = 1280;
        var width = img.width;
        var height = img.height;

        if (width > height) {
          if (width > MAX_WIDTH) {
            height *= MAX_WIDTH / width;
            width = MAX_WIDTH;
          }
        } else {
          if (height > MAX_HEIGHT) {
            width *= MAX_HEIGHT / height;
            height = MAX_HEIGHT;
          }
        }
        canvas.width = width;
        canvas.height = height;
        var fd = new FormData();
        getOrientation(file, function(orientation) {

        ctx.drawImage(img, 0, 0, width, height);

        dataurl = canvas.toDataURL("image/jpeg",0.6);

        // Post the data

        fd.append("ori", orientation);
        fd.append("image", dataurl);

        $.ajax({
            url: 'savetofile.php',
            data: fd,
            cache: false,
            contentType: false,
            processData: false,
            enctype: 'multipart/form-data',
            type: 'POST',
            success: function(data){

               uploadComplete2(cuantos,actual)


            }
        });
      });



    } // img.onload
 }
// Load files into file reader
reader.readAsDataURL(file);
}