Use a custom key for caching

How can I use a custom key for caching instead of the url?
When I use the CachedNetworkImage widget, I want to use the customized cache manager. My thought was to extend CacheManager and override its getSingleFile and other methods. But it doesn't seem to work.

Start with the CachedNetworkImage widget and the custom CacheManager entry point mentioned in the issue, then trace getSingleFile and the other overridden methods. Determine whether the current API supports a custom cache key instead of the URL; document the supported usage or clearly define the required change, with completion shown by a working custom-key example.

Written by the indexing model from the issue text.