NSURLSessionDownloadTask中的totalBytesExpectedToWrite为-1

Question

我遇到了一个奇怪的问题。我使用NSURLSession和NSURLSessionDownloadTask从互联网上加载文件。以下是代码

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];

我的类被声明为NSURLSessionDownloadDelegate，并且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，我没有能力向用户显示进度，因为我不知道下载文件的大小。

你能提示我哪里弄错了吗？

Answer 1

-1是NSURLSessionTransferSizeUnknown，这意味着http服务器没有提供"Content-Length“报头(数据是使用"Transfer-Encoding: chunked”发送的)。

你能做的可能不多。如果https://stackoverflow.com/a/12599242/1187415的变通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"];

Answer 2

web服务可能没有在报头字段Content-Length中提供总大小。

如果没有提供总大小，你的应用程序就无法知道长度，这会提供一个进度条。

使用Charles Proxy之类的分析器检查来自web服务器的内容。

Answer 3

Content-Length可以为非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时反复输出的结果。

但totalBytesExpectedToWrite:-1

因此，您需要检查downloadTask.response中的内容长度

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%]