刚刚看到了一个小知识点，拿出来与大家分享一下，也是做个笔记。
NSURLRequest有缓存策略，是这条属性：
/*!
@method cachePolicy
@abstract Returns the cache policy of the receiver.
@result The cache policy of the receiver.
*/
@property (readonly) NSURLRequestCachePolicy cachePolicy;
NSURLRequestCachePolicy 是一个枚举
typedef NS_ENUM(NSUInteger, NSURLRequestCachePolicy)
{
NSURLRequestUseProtocolCachePolicy = , NSURLRequestReloadIgnoringLocalCacheData = ,
NSURLRequestReloadIgnoringLocalAndRemoteCacheData = , // Unimplemented
NSURLRequestReloadIgnoringCacheData = NSURLRequestReloadIgnoringLocalCacheData, NSURLRequestReturnCacheDataElseLoad = ,
NSURLRequestReturnCacheDataDontLoad = , NSURLRequestReloadRevalidatingCacheData = , // Unimplemented
};
有这么几个值，分别是这些意思：
默认缓存策略：NSURLRequestUseProtocolCachePolicy
忽视本地缓存，只加载网络数据：NSURLRequestReloadIgnoringLocalCacheData
先加载本地缓存，没有的情况下网络请求数据：NSURLRequestReturnCacheDataElseLoad
不加载本地缓存，直接请求：NSURLRequestReturnCacheDataDontLoad