第一.NSURLRequest的緩存設置
+(id)initWithURL:(NSURL*)URLcachePolicy:(NSURLRequestCachePolicy)cachePolicytimeoutInterval:(NSTimeInterval)timeoutInterval;
cachePolicy表示緩存策略
1、 NSURLRequestUseProtocolCachePolicy = 0,
默認策略，使用緩存
2.NSURLRequestReloadIgnoringLocalCacheData = 1,
忽略本地緩存
3.NSURLRequestReturnCacheDataElseLoad = 2,
如果有緩存，不管過期時間優先使用本地緩存，如果沒有本地緩存，才從原地址下載
4. NSURLRequestReturnCacheDataDontLoad = 3
只使用緩存，如果沒有匹配的緩存則報告離線模式，而不會從網上load數據
第二、NSURLCache
1. 初始化相關的幾個方法：
sharedURLCache；setSharedURLCache；initWithMemoryCapacity
sharedURLCache方法返回一個NSURLCache實例。
默認情況下，內存是4M，4* 1024 *
1024；Disk為20M，20 * 1024 ＊
1024；路徑在(NSHomeDirectory)/Library/Caches/(current application name,
[[NSProcessInfo processInfo] processName])
setSharedURLCache可以通過這個方法來改變默認的NSURLCache。通過initWithMemoryCapacity來定制自己的NSURLCache。
2.常用的幾個函數：
//Returns the NSCachedURLResponse stored in the cache with the given request.
(NSCachedURLResponse *)cachedResponseForRequest:(NSURLRequest *)request;
//Stores the given NSCachedURLResponse in the cache using the given request.
- (void)storeCachedResponse:(NSCachedURLResponse *)cachedResponse forRequest:(NSURLRequest *)request;
// Removes the NSCachedURLResponse from the cache that is stored using the given request.
(void)removeCachedResponseForRequest:(NSURLRequest *)request;
//Clears the given cache, removing all NSCachedURLResponse objects that it stores.
- (void)removeAllCachedResponses;
3. property方法
- (NSUInteger)memoryCapacity;
- (NSUInteger)diskCapacity;
- (void)setMemoryCapacity:(NSUInteger)memoryCapacity;
第三、NSCachedURLResponse
系統緩存對象，保持了緩存對象的個性和特性。
1. NSURLCacheStoragePolicy 緩存策略有三種
enum{
NSURLCacheStorageAllowed,
NSURLCacheStorageAllowedInMemoryOnly,
NSURLCacheStorageNotAllowed,
};
NSURLCacheStorageAllowed
Specifies that storage in NSURLCache is allowed without restriction.
Important:iOSprior to version 5 ignores this cache policy, and instead treats it asNSURLCacheStorageAllowedInMemoryOnly.
可以看出，iOS設備上NSURLCache默認只能進行內存緩存。
2. 構造方法
- (id)initWithResponse:(NSURLResponse *)response data:(NSData *)data;
-(id)initWithResponse:(NSURLResponse *)response data:(NSData *)data
userInfo:(NSDictionary *)userInfo
storagePolicy:(NSURLCacheStoragePolicy)storagePolicy;
3. Open API
- (NSURLResponse *)response;
- (NSData *)data;
- (NSDictionary *)userInfo;
- (NSURLCacheStoragePolicy)storagePolicy;
