xmlhttprequest在函数中无法正常工作

Question

有人知道为什么upload.onprogress如果在单独的函数上不能正常工作？

代码正常工作（进度条缓慢移动）：

        xhr.upload.onprogress = function(e) {
            if (e.lengthComputable) {
                progress.value = (e.loaded / e.total) * 100;
            }
        };  

但如果我把它投入使用，它就不再起作用了：

xhr.upload.onprogress = uploadProgress(event);

function uploadProgress(e) {
    if (e.lengthComputable) {
        progress.value = (e.loaded / e.total) * 100;
    }
}   

在第二个代码上，进度条在文件完成上传后直接跳转到100％而不是，在上传期间很好地移动到100％

---

所以，我已经尝试了所提供的解决方案，它实际上是有效的，如果我把功能放在里面。 有没有办法把它放在功能之外？

        function uploadFile(blobFile, fileName) {
            ...
            ...

            // Listen to the upload progress for each upload.
            xhr.upload.onprogress = uploadProgress;

            // Progress Bar Calculation why it has to be in uploadFile function..
            function uploadProgress(e) {
                if (e.lengthComputable) {
                    progress.value = (e.loaded / e.total) * 100;
                }
            }                            

            uploaders.push(xhr);
            xhr.send(fd);
        }  

        //it does not work if I put it outside the function. is there anyway to do this?  
        function uploadProgress(e) {
             if (e.lengthComputable) {
                 progress.value = (e.loaded / e.total) * 100;
             }
        }   

Answer 1

使用uploadProgress(event); 您调用函数本身并将返回值xhr.upload.onprogress而不是将其指定为回调函数：

xhr.upload.onprogress = uploadProgress;

Answer 2

在第二个例子中你应该使用

xhr.upload.onprogress = uploadProgress;

不

xhr.upload.onprogress = uploadProgress(event);

您已经分配了调用函数的结果，而不是对函数的引用。

Answer 3

如何在分配回调之前定义函数？ JavasCript有时在以后定义函数时会遇到问题。

我的意思是你可以替换：

// Listen to the upload progress for each upload.
xhr.upload.onprogress = uploadProgress;

// Progress Bar Calculation
function uploadProgress(e) {
    if (e.lengthComputable) {
         progress.value = (e.loaded / e.total) * 100;
    }
}

同

// Progress Bar Calculation
function uploadProgress(e) {
    if (e.lengthComputable) {
         progress.value = (e.loaded / e.total) * 100;
    }
}
// Listen to the upload progress for each upload.
xhr.upload.onprogress = uploadProgress;