fetch method Null safety

Future<T> fetch(
  1. Future<T> callback(
      )
    )

    Returns a cached value from a previous call to fetch, or runs callback to compute a new one.

    If fetch has been called recently enough, returns its previous return value. Otherwise, runs callback and returns its new return value.

    Implementation

    Future<T> fetch(Future<T> Function() callback) async {
  if (_cachedStreamSplitter != null) {
    throw StateError('Previously used to cache via `fetchStream`');
  }
  return _cachedValueFuture ??= callback()
    ..whenComplete(_startStaleTimer).ignore();
}