HTTP加载失败（错误代码：12 [1:12]）

Question

在我的应用程序中，我尝试从服务器下载一批图像。

我遇到许多错误，其中一个我找不到解释。我检查了Apple的URL加载系统错误代码，但无法对错误代码12进行罚款。我也得到了HTTP load failed (error code: -999)（是NSURLErrorCancelled）和Task finished with error - code: -1001（是NSURLErrorTimedOut）。

我最近从使用过时的NSURLConnection切换为NSURLSession。对于前者，我也遇到了999和1001错误，但是我试图找出错误代码12的含义。

- (void)loadImage:(LeafletURL*)leafletURLInput isThumbnail:(BOOL)isThumbnailInput isBatchDownload:(BOOL)isBatchDownload isRetina:(BOOL)isRetina
{

    isRetina_ = isRetina;

    if (session)
    {
        /*is this the right call here? */
        [session invalidateAndCancel];
        [session release];
        session = nil;
    }
    if (mImageData)
    {
        [mImageData release];
        mImageData = nil;
    }

    self.leafletURL = leafletURLInput;
    self.isThumbnail = isThumbnailInput;

    NSString* location = (self.isThumbnail) ?leafletURL.thumbnailLocation :leafletURL.hiResImageLocation;

    //// Check if the image needs to be downloaded from server. If it is a batch download, then override the local resources////
    if ( ([location isEqualToString:kLocationServer] || (isBatchDownload && [location isEqualToString:kLocationResource])) && self.leafletURL.rawURL != nil )
    {
        //NSLog(@"final loadimage called server");
        //// tell the delegate to get ride of the old image while waiting. ////
        if([delegate respondsToSelector:@selector(leafletImageLoaderWillBeginLoadingImage:)])
        {
            [delegate leafletImageLoaderWillBeginLoadingImage:self];
        }

        mImageData = [[NSMutableData alloc] init];

        /*download tasks have their data written to a local temp file. It’s the responsibility of the completion handler to move the file from its temporary location to a permanent location.*/

        NSURL* url = [NSURL URLWithString:[leafletURL pathForImageOnServerUsingThumbnail:self.isThumbnail isRetina:isRetina]];
        NSURLRequest* request = [NSURLRequest requestWithURL:url];
        session = [NSURLSession sharedSession];
        dataTask = [session dataTaskWithRequest:request
                              completionHandler:^(NSData *data, NSURLResponse *response, NSError *error)
                    {
                        // do something with the data
                    }];
        [dataTask resume];
    }

    //// if not, tell the delegate that the image is already cached. ////
    else
    {
        if([delegate respondsToSelector:@selector(leafletImageLoaderDidFinishLoadingImage:)])
        {

            [delegate leafletImageLoaderDidFinishLoadingImage:self];

        }
    }
}