功能
First-level memory cache using NSCache.
Second-level LRU disk cache using the file system.
Asynchronous and synchronous image retrieval.
Background image resizing and file reading.
Thread-safe.
Automatic cache eviction on memory warnings or disk capacity reached.
Preloading images from the disk cache into memory on startup.
Zero-config UIImageView category to use the cache, optimized for UITableView and UICollectionView cell reuse.
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