NSURLCache不caching

我正在使用Xcode 6.1（6A1030），iOS7和iOS8模拟器。

NSURLCache似乎没有caching任何东西。 我使用“caching控制”标题。 我的服务器以'max-age = 6000'的forms返回Cache-Control头。

在这个例子中，我篡改了Google的一个请求，这个请求也没有被caching：

AppDelegate中：

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool { var URLCache = NSURLCache(memoryCapacity: 500 * 1024 * 1024, diskCapacity: 500 * 1024 * 1024, diskPath: "bla") NSURLCache.setSharedURLCache(URLCache) sleep(1) return true } 

然后我开始连接：

  let urlPath: String = "http://www.google.com" var url: NSURL = NSURL(string: urlPath)! var request: NSURLRequest = NSURLRequest(URL: url, cachePolicy: NSURLRequestCachePolicy.ReturnCacheDataElseLoad, timeoutInterval: 5000) var cachedURLResponse = NSURLCache.sharedURLCache().cachedResponseForRequest(request) // cachedURLResponse is always nil var connection: NSURLConnection = NSURLConnection(request: request, delegate: self, startImmediately: true)! connection.start() 

在caching响应之前，我添加了Cache-Control头：

 func connection(connection: NSURLConnection, willCacheResponse cachedResponse: NSCachedURLResponse) -> NSCachedURLResponse? { var HTTPResponse = cachedResponse.response as NSHTTPURLResponse var modifiedHeaders = NSMutableDictionary(dictionary: HTTPResponse.allHeaderFields) modifiedHeaders["Cache-Control"] = "max-age=6000" modifiedHeaders.removeObjectForKey("Expires") var modifiedResponse = NSHTTPURLResponse(URL: HTTPResponse.URL!, statusCode: HTTPResponse.statusCode, HTTPVersion: "HTTP/1.1", headerFields: modifiedHeaders) return NSCachedURLResponse(response: modifiedResponse!, data: cachedResponse.data, userInfo: cachedResponse.userInfo, storagePolicy: cachedResponse.storagePolicy) } 

修改后的响应如下所示：

{URL： http : //www.google.com/ ?gfe_rd=cr&ei=MSBGVKe7AeeI8QepoYGgCg} {状态代码：200，标题{“Alternate-Protocol”=“80：quic，p = 0.01”; “Cache-Control”=“max-age = 6000”; “Content-Type”=“text / html; charset = windows-1255”; date=“星期二，2014年10月21日08:58:25 GMT”; 服务器= gws; “Transfer-Encoding”=身份; “X-Frame-Options”= SAMEORIGIN; “X-XSS-Protection”=“1; mode = block”; }}

然而，'cachedResponseForRequest'每次都返回nil，并且每次发送一个新的服务器请求。

难道我做错了什么？ 非常感谢你的帮助！