显示正在上传的PFFile的进度

Question

用户注册然后他/她可以拍照将其用作Userimage（图像具有名称图像并保存在imageToUpload中）然后我将其上传到Test类。我已经放置了一个进度条，向用户显示上传图片的进度（进度条的名称为progressBar）

PFUser *currentUser = [PFUser currentUser];

NSData *imageToUpload = UIImageJPEGRepresentation(_image, 0.8f);

PFFile *photoFile = [PFFile fileWithData:imageToUpload];

PFObject *User = [PFObject objectWithClassName:@"Test"];
    User[@"username"] = currentUser.username;
    User[@"UserImage"] = photoFile;
    [User saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
    if (succeeded) {

        _useThisPicture.enabled = NO;

    } else {

        _useThisPicture.enabled = YES;

    } progressBlock:^(int percentDone) {

        [_progressBar setProgress: (float)percentDone/100.0f];
    };
}];

一切正常，我现在唯一困住的就是progressBlock。我的目标是更改进度的值，以便显示上传到解析图像的当前进度。