QueryCancelToken class Queries

Handed to a query function, as QueryFunctionContext.signal, so it can abort work that is no longer wanted.

Dart has no ecosystem-wide cancellation primitive, so onCancel is the universal interop point — forward it to whatever your HTTP client uses:

queryFn: (context) {
  final cancelToken = dio.CancelToken();
  context.signal.onCancel(cancelToken.cancel);
  return dio.get('/tasks', cancelToken: cancelToken);
}

For a client that takes a future instead, pass whenCancelled. package:http 1.5 and later aborts an AbortableRequest when its abortTrigger completes, and the request then fails with RequestAbortedException:

queryFn: (context) async {
  final request = http.AbortableRequest(
    'GET',
    Uri.parse('https://api.example.com/tasks'),
    abortTrigger: context.signal.whenCancelled,
  );
  final response = await http.Response.fromStream(
    await client.send(request),
  );
  return parseTasks(response.body);
}

A client with no cancellation at all simply never registers a callback; the request then runs to completion and its result is discarded, as in TanStack Query. For long loops, poll isCancelled or call throwIfCancelled between steps, or await whenCancelled.

Reading signal matters: a query whose function never read it is not cancelled when its last observer leaves — the request cannot be stopped, so it is allowed to finish and its result is cached. Once the signal has been read, the fetch is cancelled and the query reverts instead.

The library creates one token per fetch, shared by its retries, and cancels it; a query function never calls cancel. Tests that call a query function directly create their own with the default constructor, QueryCancelToken(), and pass it to the QueryFunctionContext they build.