获取文件的大小

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{

    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;

    if(httpResponse && [httpResponse respondsToSelector:@selector(allHeaderFields)]){

        NSDictionary *httpResponseHeaderFields = [httpResponse allHeaderFields];

        

        total_ = [[httpResponseHeaderFields objectForKey:@"Content-Length"] longLongValue];

        

        //NSLog(@"%lld", total_);

    }

}