Can't change wkwebview request cache policy

We are using the WKWebView for our Net page display frame,even if I set the NSURLRequest cache policy ,when it jumping ,the NSURLRequest cache policy still default to 0,how can I fix that problem?

    NSURLRequest * urlReuqest = [[NSURLRequest alloc]initWithURL:url cachePolicy:1 timeoutInterval:30.0f];
    //Before request, I set the NSURLRequest cachePolicy To 1
    if(iOS8){
        [(WKWebView *)_realWebView loadRequest:urlReuqest];
    }

when I load the page, I catch the NSURLRequest cachePolicy in WKNavigationDelegate method implement

                    - (void)webView:(WKWebView *)webView 
    decidePolicyForNavigationAction:(WKNavigationAction *)navigationAction 
                    decisionHandler:(void (^)(WKNavigationActionPolicy))decisionHandler{
                        NSLog(@"requestCachePolicy:%lu",(unsigned long)navigationAction.request.cachePolicy);
                              decisionHandler(0);

}

and the console shows requestCachePolicy:1,as I set before

however ，When I click on a link and jump to another net page,I catch the cachePolicy in the same WKNavigationDelegate method

the console shows requestCachePolicy:0 ，the cachePolicy is turn to default cachePolicy but not I set up before

How to change the WKWebview request cachePolicy When the net page jump?