如何通过objective-c使用UiWebView创建持久缓存？

#import <WebKit/WebKit.h>

// 创建UiWebView对象
UIWebView *webView = [[UIWebView alloc] initWithFrame:self.view.bounds];
[self.view addSubview:webView];

// 获取共享URL缓存对象
NSURLCache *URLCache = [[NSURLCache alloc] initWithMemoryCapacity:4 * 1024 * 1024
                                                     diskCapacity:20 * 1024 * 1024
                                                         diskPath:nil];

// 设置UiWebView的缓存策略
[NSURLCache setSharedURLCache:URLCache];

// 创建UiWebView的请求对象
NSURL *url = [NSURL URLWithString:@"https://example.com"];
NSURLRequest *request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReturnCacheDataElseLoad timeoutInterval:60];

// 加载请求
[webView loadRequest:request];

// 清除缓存
[[NSURLCache sharedURLCache] removeAllCachedResponses];