1> NSURLRequestUseProtocolCachePolicy = 0, 默认的缓存策略， 如果缓存不存在，直接从服务端获取。如果缓存存在，会根据response中的Cache-Control字段判断下一步操作，如: Cache-Control字段为must-revalidata, 则询问服务端该数据是否有更新，无更新的话直接返回给用户缓存数据，若已更新，则请求服务端.2> NSURLRequestReloadIgnoringLocalCacheData = 1, 忽略本地缓存数据，直接请求服务端.3> NSURLRequestIgnoringLocalAndRemoteCacheData = 4, 忽略本地缓存，代理服务器以及其他中介，直接请求源服务端.4> NSURLRequestReloadIgnoringCacheData = NSURLRequestReloadIgnoringLocalCacheData5> NSURLRequestReturnCacheDataElseLoad = 2, 有缓存就使用，不管其有效性(即忽略Cache-Control字段), 无则请求服务端.6> NSURLRequestReturnCacheDataDontLoad = 3, 死活加载本地缓存. 没有就失败. (确定当前无网络时使用)7> NSURLRequestReloadRevalidatingCacheData = 5, 缓存数据必须得得到服务端确认有效才使用(貌似是NSURLRequestUseProtocolCachePolicy中的一种情况)Tips: URL Loading System默认只支持如下5中协议: 其中只有http://和https://才有缓存策略.(1) http://(2) https://(3) ftp://(4) file://(5) data://使用方法：-(void) downloadURL:(NSURL *)paramURL{NSURLCache *urlCache = [NSURLCache sharedURLCache];[urlCache setMemoryCapacity:1*1024*1024];NSMutableURLRequest *request =[NSMutableURLRequest requestWithURL:paramURL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:30.0f];NSCachedURLResponse *response =[urlCache cachedResponseForRequest:request];if (response != nil){    FLOG(@"Cached response exists. Loading data from cache...");    [request setCachePolicy:NSURLRequestReturnCacheDataDontLoad];}UIWebView *webView = [[UIWebView alloc] initWithFrame:self.bounds];webView.autoresizingMask = UIViewAutoresizingFlexibleWidth|UIViewAutoresizingFlexibleHeight;webView.scalesPageToFit = YES;webView.backgroundColor = [UIColor whiteColor];webView.delegate = self;[webView loadRequest:request];_progressProxy = [[NJKWebViewProgress alloc] init];_progressProxy.webViewProxyDelegate = self;_progressProxy.progressDelegate = self;webView.delegate  = _progressProxy;CGFloat progressBarHeight = 2.f;CGRect barFrame = CGRectMake(0,0, kScreenWidth, progressBarHeight);_progressView = [[NJKWebViewProgressView alloc] initWithFrame:barFrame];_progressView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleTopMargin;self.webView = webView;[self addSubview:self.webView];[self addSubview:_progressView];

}