/// Schedule heavy tab work off the first paint, yielding so nav stays responsive.
library;

import 'dart:async';

/// Runs [compute] after a frame, cancellable via [cancel] / generation bumps.
class DeferredTabWork<T> {
  int _gen = 0;
  Object? _loadingKey;
  T? value;
  Object? valueKey;

  bool readyFor(Object key) => value != null && valueKey == key;

  bool get isLoading => _loadingKey != null;

  void cancel() {
    _gen++;
    _loadingKey = null;
  }

  /// Starts work for [key] unless already ready or in flight for that key.
  void ensure({
    required Object key,
    required Future<T> Function(Future<void> Function() yieldUi) compute,
    required void Function() onUpdate,
    required bool Function() mounted,
  }) {
    if (readyFor(key) || _loadingKey == key) return;
    _loadingKey = key;
    final gen = ++_gen;

    unawaited(() async {
      await Future<void>.delayed(Duration.zero);
      if (!mounted() || gen != _gen) return;

      Future<void> yieldUi() async {
        await Future<void>.delayed(Duration.zero);
      }

      final result = await compute(yieldUi);
      if (!mounted() || gen != _gen) return;

      value = result;
      valueKey = key;
      if (_loadingKey == key) _loadingKey = null;
      onUpdate();
    }());
  }
}