如何设置NSURLRequest缓存过期？

Question

我正在使用AFNetworking并且需要在一个响应中缓存数据几分钟.所以我在app delegate中设置了NSUrlCache,然后在我的请求中设置它:

NSMutableURLRequest *request = //obtain request; 
request.cachePolicy = NSURLRequestReturnCacheDataElseLoad;

然后如何设置到期日期:如果数据加载时间超过n分钟,请询问服务器响应而不是磁盘响应？

编辑:

假设服务器不支持缓存,我需要在代码中管理它.

Answer 1

所以,我找到了解决方案.

想法是使用connection:willCacheResponse:方法.在缓存响应之前,它将被执行,在那里我们可以更改响应并返回new,或者返回nil,并且不会缓存响应.当我使用AFNetworking时,有一个很好的方法:

- (void)setCacheResponseBlock:(NSCachedURLResponse * (^)(NSURLConnection *connection, NSCachedURLResponse *cachedResponse))block;

添加代码:

  [operation setCacheResponseBlock:^NSCachedURLResponse *(NSURLConnection *connection, NSCachedURLResponse *cachedResponse) {
    if([connection currentRequest].cachePolicy == NSURLRequestUseProtocolCachePolicy) {
      cachedResponse = [cachedResponse responseWithExpirationDuration:60];
    }
    return cachedResponse;
  }];

其中responseWithExpirationDuration从类别:

@interface NSCachedURLResponse (Expiration)
-(NSCachedURLResponse*)responseWithExpirationDuration:(int)duration;
@end

@implementation NSCachedURLResponse (Expiration)

-(NSCachedURLResponse*)responseWithExpirationDuration:(int)duration {
  NSCachedURLResponse* cachedResponse = self;
  NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)[cachedResponse response];
  NSDictionary *headers = [httpResponse allHeaderFields];
  NSMutableDictionary* newHeaders = [headers mutableCopy];

  newHeaders[@"Cache-Control"] = [NSString stringWithFormat:@"max-age=%i", duration];
  [newHeaders removeObjectForKey:@"Expires"];
  [newHeaders removeObjectForKey:@"s-maxage"];

  NSHTTPURLResponse* newResponse = [[NSHTTPURLResponse alloc] initWithURL:httpResponse.URL
                                                               statusCode:httpResponse.statusCode
                                                              HTTPVersion:@"HTTP/1.1"
                                                             headerFields:newHeaders];

  cachedResponse = [[NSCachedURLResponse alloc] initWithResponse:newResponse
                                                            data:[cachedResponse.data mutableCopy]
                                                        userInfo:newHeaders
                                                   storagePolicy:cachedResponse.storagePolicy];
  return cachedResponse;
}

@end

因此,我们根据http/1.1在http标头中设置以秒为单位的到期. 为此我们需要设置一个标头:Expires,Cache-Control:s-maxage或max-age然后创建新的缓存响应,因为属性是只读,并返回新对象.

Answer 2

Swift相当于使用URLSession的@HotJard解决方案

extension CachedURLResponse {
    func response(withExpirationDuration duration: Int) -> CachedURLResponse {
        var cachedResponse = self
        if let httpResponse = cachedResponse.response as? HTTPURLResponse, var headers = httpResponse.allHeaderFields as? [String : String], let url = httpResponse.url{

            headers["Cache-Control"] = "max-age=\(duration)"
            headers.removeValue(forKey: "Expires")
            headers.removeValue(forKey: "s-maxage")

            if let newResponse = HTTPURLResponse(url: url, statusCode: httpResponse.statusCode, httpVersion: "HTTP/1.1", headerFields: headers) {
            cachedResponse = CachedURLResponse(response: newResponse, data: cachedResponse.data, userInfo: headers, storagePolicy: cachedResponse.storagePolicy)
            }
        }
        return cachedResponse
    }
}

然后在您的自定义类中实现URLSessionDataDelegate协议

func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, willCacheResponse proposedResponse: CachedURLResponse, completionHandler: @escaping (CachedURLResponse?) -> Void) {

    if dataTask.currentRequest?.cachePolicy == .useProtocolCachePolicy {
        let newResponse = proposedResponse.response(withExpirationDuration: 60)
        completionHandler(newResponse)
    }else {
        completionHandler(proposedResponse)
    }
}

不要忘记创建您的配置和会话，将您的自定义类作为委托引用进行传递，例如

let session = URLSession(
        configuration: URLSession.shared.configuration,
        delegate: *delegateReference*,
        delegateQueue: URLSession.shared.delegateQueue
    )
let task = session.dataTask(with: request)
task.resume()