import 'package:meta/meta.dart';

/// Runs an action once for everyone who asks: a caller while it runs awaits
/// the same run, and one after it is done gets it done; only a run that
/// failed is run again by the next caller.
@internal
class Once {
  Future<void>? _run;

  /// Runs [action] unless it ran or runs already, and completes with that
  /// run.
  Future<void> run(Future<void> Function() action) async {
    final run = _run ??= action();
    try {
      await run;
    } catch (_) {
      if (identical(_run, run)) _run = null;
      rethrow;
    }
  }
}