第一.NSURLRequest的缓存设置
+(id)initWithURL:(NSURL*)URLcachePolicy:(NSURLRequestCachePolicy)cachePolicytimeoutInterval:(NSTimeInterval)timeoutInterval;
cachePolicy表示缓存策略
1、 NSURLRequestUseProtocolCachePolicy = 0,
默认策略，使用缓存
2.NSURLRequestReloadIgnoringLocalCacheData = 1,
忽略本地缓存
3.NSURLRequestReturnCacheDataElseLoad = 2,
如果有缓存，不管过期时间优先使用本地缓存，如果没有本地缓存，才从原地址下载
4. NSURLRequestReturnCacheDataDontLoad = 3
只使用缓存，如果没有匹配的缓存则报告离线模式，而不会从网上load数据
第二、NSURLCache
1. 初始化相关的几个方法：
sharedURLCache；setSharedURLCache；initWithMemoryCapacity
sharedURLCache方法返回一个NSURLCache实例。
默认情况下，内存是4M，4* 1024 *
1024；Disk为20M，20 * 1024 ＊
1024；路径在(NSHomeDirectory)/Library/Caches/(current application name,
[[NSProcessInfo processInfo] processName])
setSharedURLCache可以通过这个方法来改变默认的NSURLCache。通过initWithMemoryCapacity来定制自己的NSURLCache。
2.常用的几个函数：
//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
系统缓存对象，保持了缓存对象的个性和特性。
1. NSURLCacheStoragePolicy 缓存策略有三种
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设备上NSURLCache默认只能进行内存缓存。
2. 构造方法
- (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;
