AFNetWorking是如何进行数据缓存的?之AFImageCache & NSURLCache 详解

原文－Tim Brandt《How Does Caching Work in AFNetworking? : AFImageCache & NSUrlCache Explained》

  如果你是一个正在使用由Matt Thompson开发的网络库 AFNetWorking(如果你还没有使用,那你还在等什么?)的iOS开发者,也许你一直很好奇和困惑它的缓存机制,并且想要了解如何更好地充分利用它?

AFNetworking实际上利用了两套单独的缓存机制:

1. AFImagecache : 继承于NSCache,AFNetworking的图片内存缓存的类.
2. NSURLCache : NSURLConnection的默认缓存机制,用于存储NSURLResponse对象:一个默认缓存在内存,并且可以通过一些配置操作可以持久缓存到磁盘的类.

AFImageCache是如何工作的?

AFImageCache属于UIImageView+AFNetworking的一部分,继承于NSCache,以URL(从NSURLRequest对象中获取)字符串作为key值来存储UIImage对象. AFImageCache的定义如下:(这里我们声明了一个2M内存和100M磁盘空间的NSURLCache对象.)

@interface AFImageCache : NSCache <AFImageCache>

// singleton instantiation :

+ (id <AFImageCache>)sharedImageCache {
    static AFImageCache *_af_defaultImageCache = nil;
    static dispatch_once_t oncePredicate;
    dispatch_once(&oncePredicate, ^{
        _af_defaultImageCache = [[AFImageCache alloc] init];

// clears out cache on memory warning :

    [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationDidReceiveMemoryWarningNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification * __unused notification) {
        [_af_defaultImageCache removeAllObjects];
    }];
});

// key from [[NSURLRequest URL] absoluteString] :

static inline NSString * AFImageCacheKeyFromURLRequest(NSURLRequest *request) {
    return [[request URL] absoluteString];
}

@implementation AFImageCache

// write to cache if proper policy on NSURLRequest :

- (UIImage *)cachedImageForRequest:(NSURLRequest *)request {
    switch ([request cachePolicy]) {
        case NSURLRequestReloadIgnoringCacheData:
        case NSURLRequestReloadIgnoringLocalAndRemoteCacheData:
            return nil;
        default:
            break;
    }

    return [self objectForKey:AFImageCacheKeyFromURLRequest(request)];
}

// read from cache :

- (void)cacheImage:(UIImage *)image
        forRequest:(NSURLRequest *)request {
    if (image && request) {
        [self setObject:image forKey:AFImageCacheKeyFromURLRequest(request)];
    }
}

AFImageCache是NSCache的私有实现,它把所有可访问的UIImage对象存入NSCache中,并控制着UIImage对象应该在何时释放,如果UIImage对象释放的时候你希望去做一些监听操作,你可以实现NSCacheDelegate的 cache:willEvictObject 代理方法.Matt Thompson已经谦虚的告诉我在AFNetworking2.1版本中可通过setSharedImageCache方法来配置AFImageCache，这里是 AFN2.2.1中的UIImageView+AFNetworking文档.

NSURLCache

AFNetworking使用了NSURLConnection,它利用了iOS原生的缓存机制,并且NSURLCache缓存了服务器返回的NSURLRespone对象.NSURLCache 的shareCache方法是默认开启的,你可以利用它来获取每一个NSURLConnection对象的URL内容.让人不爽的是,它的默认配置是缓存到内存而且并没有写入到磁盘.为了tame the beast(驯服野兽？不太懂),增加可持续性,你可以在AppDelegate中简单地声明一个共享的NSURLCache对象,像这样:

NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:2 * 1024 * 1024
                                              diskCapacity:100 * 1024 * 1024
                                              diskPath:nil];
[NSURLCache setSharedURLCache:sharedCache];

设置NSURLRequest对象的缓存策略

NSURLCache 将对每一个NSURLRequest对象遵守缓存策略(NSURLRequestCachePolicy)，策略如下所示:

 NSURLRequestUseProtocolCachePolicy                //默认的缓存策略，对特定的URL请求使用网络协议中实现的缓存逻辑
 NSURLRequestReloadIgnoringLocalCacheData          //忽略本地缓存，重新请请求
 NSURLRequestReloadIgnoringLocalAndRemoteCacheData //忽略本地和远程缓存，重新请求
 NSURLRequestReturnCacheDataElseLoad               //有缓存则从中加载，如果没有则去请求
 NSURLRequestReturnCacheDataDontLoad               //无网络状态下不去请求，一直加载本地缓存数据无论其是否存在
 NSURLRequestReloadRevalidatingCacheData           //默从原始地址确认缓存数据的合法性之后，缓存数据才可使用，否则请求原始地址

用NSURLCache缓存数据到磁盘

###Cache-Control HTTP Header  Cache-Controlheader或Expires header存在于服务器返回的HTTP response header中,来用于客户端的缓存工作(前者优先级要高于后者),这里面有很多地方需要注意,Cache-Control可以拥有被定义为类似max-age的参数（在更新响应之前要缓存多长时间), public/private 访问或者是non－cache(不缓存响应数据),这里对HTTP cache headers进行了很好的介绍.

###继承并控制NSURLCache 如果你想跳过Cache-Control,并且想要自己来制定规则读写一个带有NSURLResponse对象的NSURLCache,你可以继承NSURLCache.下面有个例子,使用 CACHE_EXPIRES来判断在获取源数据之前对缓存数据保留多长时间.(感谢 Mattt Thompson的回复).

@interface CustomURLCache : NSURLCache

static NSString * const CustomURLCacheExpirationKey = @"CustomURLCacheExpiration";
static NSTimeInterval const CustomURLCacheExpirationInterval = 600;

@implementation CustomURLCache

+ (instancetype)standardURLCache {
   static CustomURLCache *_standardURLCache = nil;
   static dispatch_once_t onceToken;
   dispatch_once(&onceToken, ^{
       _standardURLCache = [[CustomURLCache alloc]
                                initWithMemoryCapacity:(2 * 1024 * 1024)
                                diskCapacity:(100 * 1024 * 1024)
                                diskPath:nil];
   }

   return _standardURLCache;
}

#pragma mark - NSURLCache

- (NSCachedURLResponse *)cachedResponseForRequest:(NSURLRequest *)request {
   NSCachedURLResponse *cachedResponse = [super cachedResponseForRequest:request];

   if (cachedResponse) {
       NSDate* cacheDate = cachedResponse.userInfo[CustomURLCacheExpirationKey];
       NSDate* cacheExpirationDate = [cacheDate dateByAddingTimeInterval:CustomURLCacheExpirationInterval];
       if ([cacheExpirationDate compare:[NSDate date]] == NSOrderedAscending) {
           [self removeCachedResponseForRequest:request];
           return nil;
       }
   }
}

   return cachedResponse;
}

- (void)storeCachedResponse:(NSCachedURLResponse *)cachedResponse
                forRequest:(NSURLRequest *)request
{
   NSMutableDictionary *userInfo = [NSMutableDictionary dictionaryWithDictionary:cachedResponse.userInfo];
   userInfo[CustomURLCacheExpirationKey] = [NSDate date];

   NSCachedURLResponse *modifiedCachedResponse = [[NSCachedURLResponse alloc] initWithResponse:cachedResponse.response data:cachedResponse.data userInfo:userInfo storagePolicy:cachedResponse.storagePolicy];

   [super storeCachedResponse:modifiedCachedResponse forRequest:request];
}
@end

现在你有了属于自己的NSURLCache的子类,不要忘了在AppDelegate中初始化并且使用它.

###在缓存之前重写NSURLResponse -connection:willCacheResponse 代理方法是在被缓存之前用于截断和编辑由NSURLConnection创建的NSURLCacheResponse的地方. 对NSURLCacheResponse进行处理并返回一个可变的拷贝对象(代码来自NSHipster blog).

- (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];
}

// If you do not wish to cache the NSURLCachedResponse, just return nil from the delegate function:

- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
                  willCacheResponse:(NSCachedURLResponse *)cachedResponse {
    return nil;
}

###禁用NSURLCache 不想使用NSURLCache? 不为所动? 好吧,你可以禁用NSURLCache,只需要将内存和磁盘空间设置为0就行了.

NSURLCache *sharedCache = [[NSURLCache alloc] initWithMemoryCapacity:0
                                              diskCapacity:0
                                              diskPath:nil];
[NSURLCache setSharedURLCache:sharedCache];

总结

我写这篇博客的目的是为iOS社区贡献绵薄之力,并总结了我是如何来处理关于AFNetworking缓存问题的.我们有个内部App在加载了大量图片后,出现了内存和性能问题,而我的主要职责是诊断这个App的缓存行为,在研究过程中,我在网上搜索了很多资料并且做了很多调试,在我汇总之后就写到了这篇博客中,我希望这篇文章可以为开发者使用AFNetworking时提供一些帮助,真心希望对你们有用!