keys abstract method
Returns every stored key, expired or not, and removes nothing.
Reading must not delete. Both built-in backends used to purge expired
entries while walking, which made a listing destructive in a way its name
gave no hint of — and destructive of the wrong thing: an expired entry is
exactly what networkFirst serves when the network is gone, and what
cacheFirst serves while it revalidates. Calling getCacheKeys() to see
what was cached therefore threw away the offline fallback, silently,
leaving a cache miss where there had been data.
Use CacheInterceptor.evictExpired() when removing them is the intent.
Implementation
Future<List<String>> keys();