Use this page to learn about the AssetBundle compression formats and cache types that Unity supports.
默认情况下，Unity 通过 LZMA 压缩来创建 AssetBundle，然后通过 LZ4 压缩将其缓存。本部分描述以上的两种压缩格式。
Unity’s AssetBundle build pipeline creates AssetBundles with LZMA compression. This compression format is a stream of data representing the entire AssetBundle, which means that if you need to read an Asset from these archives, you must decompress the entire stream. This is the preferred format for AssetBundles downloaded from a Content Delivery Network (CDN), because the file sizes are smaller than those with LZ4 compression.
LZ4 compression is a chunk-based compression algorithm. If Unity needs to access an Asset from an LZ4 archive, it only needs to decompress and read the chunks that contain bytes of the requested Asset. This is the compression method that Unity uses in both of its AssetBundle caches. Use the BuildAssetBundleOptions.ChunkBasedCompression value when building AssetBundles to force LZ4(HC) compression.
Uncompressed AssetBundles that Unity builds when you use BuildAssetBundleOptions.UncompressedAssetBundle require no decompression, but occupy more disk space. Uncompressed AssetBundles are 16-byte aligned.
To optimize the fetching, recompressing, and versioning of LZMA AssetBundles using WWW or UnityWebRequest (UWR), Unity maintains two caches:
The Memory Cache stores AssetBundles in UncompressedRuntime format in RAM.
磁盘缓存将提取的 AssetBundle 以下文描述的压缩格式存储在可写介质中。
将 AssetBundle 加载到内存缓存中会耗用大量的内存。除非您特别希望频繁且快速地访问 AssetBundle 的内容，否则内存缓存的性价比可能不高。因此，应改用磁盘缓存。
If you provide a version parameter to the UWR API, Unity stores your AssetBundle data in the Disk Cache. If you don’t provide a version parameter, Unity uses the Memory Cache. The version parameter can be either a version number or a hash. If Caching.compressionEnabled is set to true, Unity applies LZ4 compression when it writes AssetBundles to disk for all subsequent downloads. It doesn’t compress existing uncompressed data in the cache. If Caching.compressionEnabled is false, Unity applies no compression when it writes AssetBundles to disk.
最初加载缓存的 LZMA AssetBundle 所花费的时间更长，因为 Unity 必须将存档重新压缩为目标格式。随后的加载将使用缓存版本。
AssetBundle.LoadFromFile or AssetBundle.LoadFromFileAsync always use the Memory Cache for LZMA AssetBundles, so you should use the UWR API. If it’s not possible to use the UWR API, you can use AssetBundle.RecompressAssetBundleAsync to rewrite an LZMA AssetBundle on disk.
Note: WebGL doesn’t support LZMA compression for AssetBundles. Use LZ4 compression with AssetBundles on WebGL platforms. For more information, see Reduce load times with AssetBundles.
Internal testing shows significant difference in RAM usage between using the Disk Cache instead of the Memory Cache. You must weigh the trade-off between memory impact versus added disk space requirements and Asset instantiation time for your application.