ios 目标 c：如何让 NSURLSession 从服务器返回 Content-Length（http 标头）答案

【问题标题】：ios objective c : How to get NSURLSession to return Content-Length(http header) from serverios 目标 c：如何让 NSURLSession 从服务器返回 Content-Length（http 标头）
【发布时间】：2016-12-02 06:46:59
【问题描述】：

我已经尝试过了
How to get NSURLSession to return Content-Length(http header) from server. Objective-c, ios

- (long long) getContentLength
{
    NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:config];

    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:url]];
    request.HTTPMethod = @"HEAD";
    [request addValue:@"identity" forHTTPHeaderField:@"Accept-Encoding"];

    NSURLSessionDownloadTask *uploadTask
    = [session downloadTaskWithRequest:request
                     completionHandler:^(NSURL *url,NSURLResponse *response,NSError *error) {
                         NSLog(@"handler size: %lld", response.expectedContentLength);
                       totalContentFileLength = [[NSNumber alloc] initWithFloat:response.expectedContentLength].longLongValue;

                     }];
    NSLog(@"content length=%lld", totalContentFileLength);
    [uploadTask resume];
    return totalContentFileLength;
}

我总是从返回值中得到0。

【问题讨论】：

标签： ios objective-c nsurlsession nsurlsessiondownloadtask

【解决方案1】：

对于 Swift 3.0：（使用普通的老式函数..）

func getContentLengthOf(urlString: String,
                        completionHandler: @escaping (Int64?) -> ()  ) {

    let url = URL(string: urlString)
    var request = URLRequest(url: url!)
    request.httpMethod = "HEAD"
    request.addValue( "identity", forHTTPHeaderField: "Accept-Encoding")

    let session = URLSession(configuration: URLSessionConfiguration.default)

    let task = session.dataTask(with: request, completionHandler: {(data, response, error) -> Void in
        if let response = response as? HTTPURLResponse , 200...299 ~= response.statusCode {
            let contentLength : Int64 = response.expectedContentLength
            completionHandler(contentLength)

        } else {
            completionHandler(nil)
        }
    })

    task.resume()
}

然后这样调用：

....
        let URLString = "https://lh4.googleusercontent.com/-KdgJnz1HIdQ/AAAAAAAAAAI/AAAAAAAAA8s/31PBnNCL-qs/s0-c-k-no-ns/photo.jpg"

        getContentLengthOf(urlString: URLString) { (contentLength: Int64?) in
            if let contentLength = contentLength {
                print("size is: \(contentLength)")
            }else{
                print("error getting contentLength")
            }
        }

在回调中你会得到一个可选的 Int64。

【讨论】：

【解决方案2】：

这是因为你在completion handler之外返回函数的值，所以你的函数在得到服务器响应之前就返回了值。现在你不能从completion handler 返回值。因此，您需要创建具有自定义 completion handler 作为参数的方法，例如，

 - (void) getContentLength : (void(^)(long long returnValue))completionHandler
 {


NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *session = [NSURLSession sessionWithConfiguration:config];

NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:url]];
request.HTTPMethod = @"HEAD";
[request addValue:@"identity" forHTTPHeaderField:@"Accept-Encoding"];

NSURLSessionDownloadTask *uploadTask
= [session downloadTaskWithRequest:request
                 completionHandler:^(NSURL *url,NSURLResponse *response,NSError *error) {

                     NSLog(@"handler size: %lld", response.expectedContentLength);
                     totalContentFileLength = [[NSNumber alloc] initWithFloat:response.expectedContentLength].longLongValue;

                     completionHandler(totalContentFileLength);

                 }];
NSLog(@"content length=%lld", totalContentFileLength);
[uploadTask resume];

}

你可以像这样调用这个方法，

   [self getContentLength:^(long long returnValue) {

    NSLog(@"your content length : %lld",returnValue);

}];

【讨论】：