用于离线视图的Swift iOS Cache WKWebView内容

We're trying to save the content (HTML) of WKWebView in a persistent storage (NSUserDefaults, CoreData or disk file). The user can see the same content when he re-enters the application with no internet connection. WKWebView doesn't use NSURLProtocol like UIWebView (see post We're trying to save the content (HTML) of WKWe