获取服务器文件的长度

实现NSURLConnectionDataDelegate的代理方法
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
    
    if([httpResponse respondsToSelector:@selector(allHeaderFields)]){
        NSDictionary *fields = [httpResponse allHeaderFields];
        int totalSize = [[fields objectForKey:@"Content-Length"]             intValue];
    }
}

totalSize就是文件的长度