QueryCache class Caches

Every query of one QueryClient, stored under its QueryKey.

A client owns exactly one query cache, and usually creates it itself: QueryClient() makes an empty one. Queries are added as observers and fetches first use a key, and leave again when they are garbage-collected (after gcTime without observers) or removed. Most code never touches the cache directly — the client's methods (getQueryData, invalidateQueries, ...) work through it.

Reasons to reach for it:

• Cache-wide hooks. Pass onSuccess, onError or onSettled to the constructor, and the cache to the client, to handle every query's outcome in one place — the usual home of a global error report.
• Events. subscribe delivers a QueryCacheEvent for every query that is added, removed or updated — for logging, devtools or a persistence layer.
• Lookup. find, findAll and queries return the Query entries themselves, with their full state.

final client = QueryClient(
  queryCache: QueryCache(
    onError: (error, stackTrace, query) {
      // Only background refreshes: a first load shows its own error.
      if (query.state.hasData) {
        showSnackBar('Could not refresh ${query.queryKey}: $error');
      }
    },
  ),
);