如何使用各种缓存策略（NSURLRequestCachePolicy）提高下载性能？哪些是在示例下载应用程序中使用的最佳解决方案？
答案 0 :(得分：2)
Apple put forward a neat document for the reference
它说
NSURLRequestUseProtocolCachePolicy
Specifies that the caching logic defined in the protocol implementation, if any, is used for a particular URL load request. This is the default policy for URL load requests.
Available in OS X v10.2 and later.
Declared in NSURLRequest.h.
NSURLRequestReloadIgnoringLocalCacheData
Specifies that the data for the URL load should be loaded from the originating source. No existing cache data should be used to satisfy a URL load request.
Available in OS X v10.5 and later.
Declared in NSURLRequest.h.
NSURLRequestReloadIgnoringLocalAndRemoteCacheData
Specifies that not only should the local cache data be ignored, but that proxies and other intermediates should be instructed to disregard their caches so far as the protocol allows.
Available in OS X v10.5 and later.
Declared in NSURLRequest.h.
NSURLRequestReloadIgnoringCacheData
Replaced by NSURLRequestReloadIgnoringLocalCacheData.
Available in OS X v10.2 and later.
Declared in NSURLRequest.h.
NSURLRequestReturnCacheDataElseLoad
Specifies that the existing cached data should be used to satisfy the request, regardless of its age or expiration date. If there is no existing data in the cache corresponding the request, the data is loaded from the originating source.
Available in OS X v10.2 and later.
Declared in NSURLRequest.h.
NSURLRequestReturnCacheDataDontLoad
Specifies that the existing cache data should be used to satisfy a request, regardless of its age or expiration date. If there is no existing data in the cache corresponding to a URL load request, no attempt is made to load the data from the originating source, and the load is considered to have failed. This constant specifies a behavior that is similar to an “offline” mode.
Available in OS X v10.2 and later.
Declared in NSURLRequest.h.
NSURLRequestReloadRevalidatingCacheData
Specifies that the existing cache data may be used provided the origin source confirms its validity, otherwise the URL is loaded from the origin source.
Available in OS X v10.5 and later.
Declared in NSURLRequest.h.
Declared In
NSURLRequest.h