绕过http响应头Cache-Control：如何设置缓存过期？

问题描述

来自服务器的所有http响应都带有标题，告知我们的应用不要缓存响应：

All http responses from a server come with the headers that inform our app not to cache the responses:

Cache-Control: no-cache
Pragma: no-cache
Expires: 0

所以，如果您使用默认缓存策略NSURLRequestUseProtocolCachePolicy制作NSUrlRequests，则应用程序将始终从服务器加载数据。但是，我们需要缓存响应，显而易见的解决方案是将这些标头设置为某个时间（例如在后端），设置为10秒。但是我对如何绕过这个策略并将每个请求缓存10秒的解决方案感兴趣。

So, if you are making NSUrlRequests with default cache policy "NSURLRequestUseProtocolCachePolicy" then the app will always load data from the server. However, we need to cache the responses and the obvious solution would be to set these headers to some time for example (at backend side), set to 10 seconds. But I'm interested in a solution how to bypass this policy and cache every request for 10 seconds.

为此您需要设置共享缓存。这可以在AppDelegate中完成didFinishLaunchingWithOptions：

For that you need to setup shared cache. That might be done in AppDelegate didFinishLaunchingWithOptions:

NSURLCache *URLCache = [[NSURLCache alloc] initWithMemoryCapacity:4 * 1024 * 1024
                                             diskCapacity:20 * 1024 * 1024
                                               diskPath:nil];
[NSURLCache setSharedURLCache:URLCache];

然后，我们需要嵌入代码以强制缓存响应。如果您使用AFHttpClient的实例，则可以通过覆盖以下方法并手动将缓存存储到共享缓存中来完成：

Then, we need to embed our code to force to cache a response. If you use an instance of AFHttpClient then it can be done by overriding the method below and manually storing the cache into the shared cache:

- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
              willCacheResponse:(NSCachedURLResponse *)cachedResponse {

  NSMutableDictionary *mutableUserInfo = [[cachedResponse userInfo] mutableCopy];
  NSMutableData *mutableData = [[cachedResponse data] mutableCopy];
  NSURLCacheStoragePolicy storagePolicy = NSURLCacheStorageAllowedInMemoryOnly;

  // ...

  return [[NSCachedURLResponse alloc] initWithResponse:[cachedResponse response]
                                                data:mutableData
                                            userInfo:mutableUserInfo
                                       storagePolicy:storagePolicy];
}

最后一件事是为请求设置cachePolicy。在我们的例子中，我们希望为所有请求设置相同的缓存策略。所以再次，如果您使用AFHttpClient的实例，那么可以通过覆盖以下方法来完成：

And the last thing is to set cachePolicy for the requests. In our case we want to set the same cache policy to all requests. So again, if you use an instance of AFHttpClient then it can be done by overriding the method below:

- (NSMutableURLRequest *)requestWithMethod:(NSString *)method path:(NSString *)path parameters:(NSDictionary *)parameters {

  NSMutableURLRequest *request = [super requestWithMethod:method path:path parameters:parameters];
  request.cachePolicy = NSURLRequestReturnCacheDataElseLoad;

  return request;
}

到目前为止一切顺利。 NSURLRequestReturnCacheDataElseLoad使得第一次执行请求并在其他时间从缓存加载响应。问题是，目前还不清楚如何设置缓存过期时间，例如10秒。

So far so good. "NSURLRequestReturnCacheDataElseLoad" makes to perform request first time and load response from cache all other times. The problem is, it's unclear how to set cache expiration time, for example 10 seconds.

推荐答案

您可以实现仅返回尚未过期的缓存响应的自定义NSURLCache。

You can implement a custom NSURLCache that only returns cached responses that has not expired.

示例：

#import "CustomURLCache.h"

NSString * const EXPIRES_KEY = @"cache date";
int const CACHE_EXPIRES = -10;

@implementation CustomURLCache

// static method for activating this custom cache
+(void)activate {
    CustomURLCache *urlCache = [[CustomURLCache alloc] initWithMemoryCapacity:(2*1024*1024) diskCapacity:(2*1024*1024) diskPath:nil] ;
    [NSURLCache setSharedURLCache:urlCache];
}

-(NSCachedURLResponse *)cachedResponseForRequest:(NSURLRequest *)request {
    NSCachedURLResponse * cachedResponse = [super cachedResponseForRequest:request];
    if (cachedResponse) {
        NSDate* cacheDate = [[cachedResponse userInfo] objectForKey:EXPIRES_KEY];
        if ([cacheDate timeIntervalSinceNow] < CACHE_EXPIRES) {
            [self removeCachedResponseForRequest:request];
            cachedResponse = nil;
        }
    }

    return cachedResponse;
}

- (void)storeCachedResponse:(NSCachedURLResponse *)cachedResponse forRequest:(NSURLRequest *)request {
    NSMutableDictionary *userInfo = cachedResponse.userInfo ? [cachedResponse.userInfo mutableCopy] : [NSMutableDictionary dictionary];
    [userInfo setObject:[NSDate date] forKey:EXPIRES_KEY];
    NSCachedURLResponse *newCachedResponse = [[NSCachedURLResponse alloc] initWithResponse:cachedResponse.response data:cachedResponse.data userInfo:userInfo storagePolicy:cachedResponse.storagePolicy];

    [super storeCachedResponse:newCachedResponse forRequest:request];
}

@end

如果这不能给你足够的控制然后我将使用如下的 startLoading 方法实现自定义NSURLProtocol，并将其与自定义缓存一起使用。

If this does not give you enough control then I would implement a custom NSURLProtocol with a startLoading method as below and use it in conjunction with the custom cache.

- (void)startLoading
{
    NSMutableURLRequest *newRequest = [self.request mutableCopy];
    [NSURLProtocol setProperty:@YES forKey:@"CacheSet" inRequest:newRequest];

    NSCachedURLResponse *cachedResponse = [[NSURLCache sharedURLCache] cachedResponseForRequest:self.request];
    if (cachedResponse) {
        [self connection:nil didReceiveResponse:[cachedResponse response]];
        [self connection:nil didReceiveData:[cachedResponse data]];
        [self connectionDidFinishLoading:nil];
    } else {
        _connection = [NSURLConnection connectionWithRequest:newRequest delegate:self];
    }
}

一些链接：

• Useful info on NSURLCache
• Creating a custom NSURLProtocol

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