NSURLRequest的缓存策略

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 = 0,

    NSURLRequestReloadIgnoringLocalCacheData = 1,
    NSURLRequestReloadIgnoringLocalAndRemoteCacheData = 4, // Unimplemented
    NSURLRequestReloadIgnoringCacheData = NSURLRequestReloadIgnoringLocalCacheData,

    NSURLRequestReturnCacheDataElseLoad = 2,
    NSURLRequestReturnCacheDataDontLoad = 3,

    NSURLRequestReloadRevalidatingCacheData = 5, // 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