/**webView退出点击方法*/  
- (void)closeBtnAction:(UIButton *)button{  
     _webView = nil;  
    [self cleanCacheAndCookie];  
    [self.navigationController popViewControllerAnimated:YES];  
} 

/**清除缓存和cookie*/  
- (void)cleanCacheAndCookie{  
    //清除cookies  
    NSHTTPCookie *cookie;  
    NSHTTPCookieStorage *storage = [NSHTTPCookieStorage sharedHTTPCookieStorage];  
    for (cookie in [storage cookies]){  
        [storage deleteCookie:cookie];  
    }  
   //清除UIWebView的缓存  
    [[NSURLCache sharedURLCache] removeAllCachedResponses];  
    NSURLCache * cache = [NSURLCache sharedURLCache];  
    [cache removeAllCachedResponses];  
    [cache setDiskCapacity:0];  
    [cache setMemoryCapacity:0];  
}