来自服务器的所有 HTTP 响应都带有 header ，通知我们的应用不要缓存响应:

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

因此，如果您使用默认缓存策略“NSURLRequestUseProtocolCachePolicy”发出 NSUrlRequests，那么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 header 设置为某个时间，例如(在后端)，设置为 10 秒。但我对如何绕过此策略并将每个请求缓存 10 秒的解决方案感兴趣。

为此你需要设置共享缓存。这可能在 AppDelegate didFinishLaunchingWithOptions 中完成:

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

然后，我们需要嵌入我们的代码来强制缓存响应。如果您使用 AFHttpClient 的实例，则可以通过覆盖以下方法并将缓存手动存储到共享缓存中来完成:

- (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。再次强调，如果您使用 AFHttpClient 的实例，则可以通过覆盖以下方法来完成:

- (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所有其他时间从缓存加载响应。问题是，不清楚如何设置缓存过期时间，比如10秒。

最佳答案

您可以实现一个自定义的 NSURLCache，它只返回尚未过期的缓存响应。

例子:

#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权，那么我将使用如下所示的 startLoading 方法实现自定义 NSURLProtocol，并将其与自定义缓存结合使用。

- (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

关于ios - 绕过 http 响应头 Cache-Control : how to set cache expiration?，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/19840688/