NSURLSessionDownloadTask 中的 totalBytesExpectedToWrite 为 -1答案

【问题标题】：totalBytesExpectedToWrite is -1 in NSURLSessionDownloadTaskNSURLSessionDownloadTask 中的 totalBytesExpectedToWrite 为 -1
【发布时间】：2014-06-28 10:08:31
【问题描述】：

我遇到了一个奇怪的问题。我使用 NSURLSession 和 NSURLSessionDownloadTask 从 Internet 加载文件。这是代码

NSURLSessionConfiguration *sessionConfiguration =
[NSURLSessionConfiguration backgroundSessionConfiguration:kSessionId];
self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration
                                             delegate:self
                                        delegateQueue:[NSOperationQueue new]];
NSURL *url = [NSURL URLWithString:urlString];
NSURLRequest *request = [NSURLRequest requestWithURL:url];
NSURLSessionDownloadTask *downloadTask = [self.session downloadTaskWithRequest:request];
[downloadTask resume];

我的班级被声明为NSURLSessionDownloadDelegate，我得到了很好的回调。但是当系统调用委托方法时

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    NSLog(@"totalBytesExpectedToWrite: %lld", totalBytesExpectedToWrite);
    NSLog(@"%lld", totalBytesWritten);
}

totalBytesExpectedToWrite 总是等于-1，我无法向用户显示进度，因为我不知道下载文件的大小。

你能提示我哪里出错了吗？

【问题讨论】：

这可能是一个服务器问题，它没有正确发送Content-Length 标头。如果您在浏览器中使用相同的 URL，您的浏览器是否显示正确的进度？

标签： ios nsurlsession

【解决方案1】：

-1是NSURLSessionTransferSizeUnknown，表示http服务器没有提供一个“Content-Length”标头（数据使用“Transfer-Encoding: chunked”发送）。

您可能无能为力。如果https://stackoverflow.com/a/12599242/1187415 的解决方法也适用于您的情况，您可以尝试：

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:anURL];
[request addValue:@"" forHTTPHeaderField:@"Accept-Encoding"];

【讨论】：

【解决方案2】：

Web 服务可能未在标头字段 Content-Length 中提供总大小。

如果未提供总大小，则您的应用无法知道长度，这会提供一个进度条。

使用 Charles Proxy 等分析器检查来自 Web 服务器的内容。

【讨论】：

【解决方案3】：

Content-Length 可以是非 0 和 totalBytesExpectedToWrite:-1

//TRACK PROGRESS - MOVED DOWN as also used in BACKGROUND REFRESH > DOWNLOAD FILE > CALL DELEGATE
-(void)URLSession:(NSURLSession *)session
     downloadTask:(NSURLSessionDownloadTask *)downloadTask
     didWriteData:(int64_t)bytesWritten
totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    //to see response header
     NSLog(@"downloadTask.response:%@\n", downloadTask.response);

//    { status code: 200, headers {
//        "Cache-Control" = "no-cache";
//        "Content-Disposition" = "attachment; filename=Directory.zip";
//        "Content-Encoding" = gzip;
//        "Content-Length" = 33666264;
//        "Content-Type" = "application/octet-stream";
//        Date = "Tue, 27 Oct 2015 15:50:01 GMT";
//        Expires = "-1";
//        Pragma = "no-cache";
//        Server = "Microsoft-IIS/8.5";
//        "X-AspNet-Version" = "4.0.30319";
//        "X-Powered-By" = "ASP.NET";
//    } }

    NSDictionary *responseHeaders = ((NSHTTPURLResponse *)downloadTask.response).allHeaderFields;
    NSString * contentLengthString = responseHeaders[@"Content-Length"];
    double contentLengthDouble = 0.0f;

    if (contentLengthString) {
        NSNumberFormatter *f = [[NSNumberFormatter alloc] init];
        NSNumber *contentLengthNumber = [f numberFromString:contentLengthString];
        contentLengthDouble = [contentLengthNumber doubleValue];
    }else{

    }
    NSLog(@"contentLengthString:[%@]", contentLengthString);
    //You can get progress her

    NSLog(@"bytesWritten:%lld", bytesWritten);
    NSLog(@"totalBytesWritten:%lld", totalBytesWritten);

    //DONT USE CAN BE ALWAYS -1 for Gzip
    NSLog(@"totalBytesExpectedToWrite:%lld", totalBytesExpectedToWrite);

    //avoid DIV by 0
    if (contentLengthDouble > 0.0) {
        double percentage1 = (totalBytesWritten / contentLengthDouble);
        double percentage = percentage1 * 100.0;
        NSLog(@"PERCENTAGE DOWNLOADED:[%f%%]", percentage);
    }else{
        NSLog(@"PERCENTAGE DOWNLOADED:[contentLengthDouble is 0]");
    }

    NSLog(@"=========");
}

以下是下载 zip 时反复输出的内容。

但 totalBytesExpectedToWrite:-1

所以你需要在 downloadTask.response 中检查 Content-Length

2015-10-27 16:04:18.580 ClarksonsDirectory[89873:15495901] downloadTask.response:<NSHTTPURLResponse: 0x7f9eabaae750> { URL: http://asset10232:50/api/1/dataexport/ios/?lastUpdatedDate=01012014000000 } { status code: 200, headers {
    "Cache-Control" = "no-cache";
    "Content-Disposition" = "attachment; filename=Directory.zip";
    "Content-Encoding" = gzip;
    "Content-Length" = 33666264;
    "Content-Type" = "application/octet-stream";
    Date = "Tue, 27 Oct 2015 16:03:55 GMT";
    Expires = "-1";
    Pragma = "no-cache";
    Server = "Microsoft-IIS/8.5";
    "X-AspNet-Version" = "4.0.30319";
    "X-Powered-By" = "ASP.NET";
} }

 contentLengthString:[33666264]
 bytesWritten:47278
 totalBytesWritten:33606690
 totalBytesExpectedToWrite:-1
 PERCENTAGE DOWNLOADED:[99.823045%]

【讨论】：