配置AVAssetCache以存储AVURLAsset

Question

我正在尝试从AVURLAsset对象访问属性assetCache，但它始终为零。

文档提供了下一个信息：

/*!
  @property assetCache
  @abstract Provides access to an instance of AVAssetCache to use for 
  inspection of locally cached media data. Will be nil if an asset has 
  not been configured to store or access media data from disk.
*/
@property (nonatomic, readonly, nullable) AVAssetCache *assetCache NS_AVAILABLE(10_12, 10_0);

那么，如何配置AVURLAsset以便能够从磁盘存储或访问媒体数据？