Debounced search field

import 'dart:async';
import 'package:flutter/material.dart';

typedef SearchFn<T> = Future<List<T>> Function(String query);

class DebouncedSearch<T> extends StatefulWidget {
  const DebouncedSearch({
    super.key,
    required this.search,
    required this.resultBuilder,
    this.delay = const Duration(milliseconds: 350),
    this.minLength = 2,
  });
  final SearchFn<T> search;
  final Widget Function(BuildContext, T) resultBuilder;
  final Duration delay;
  final int minLength;

  @override
  State<DebouncedSearch<T>> createState() => _DebouncedSearchState<T>();
}

class _DebouncedSearchState<T> extends State<DebouncedSearch<T>> {
  final _controller = TextEditingController();
  Timer? _timer;
  int _ticket = 0; // increases for each request; only the newest may apply
  List<T> _results = [];
  bool _busy = false;

  @override
  void dispose() {
    _timer?.cancel();
    _controller.dispose();
    super.dispose();
  }

  void _onChanged(String raw) {
    _timer?.cancel();
    final q = raw.trim();
    if (q.length < widget.minLength) {
      _ticket++; // invalidate anything still in flight
      setState(() { _results = []; _busy = false; });
      return;
    }
    _timer = Timer(widget.delay, () => _run(q));
  }

  Future<void> _run(String q) async {
    final mine = ++_ticket;
    setState(() => _busy = true);
    try {
      final rows = await widget.search(q);
      if (mounted && mine == _ticket) setState(() => _results = rows);
    } finally {
      if (mounted && mine == _ticket) setState(() => _busy = false);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Padding(
          padding: const EdgeInsets.all(12),
          child: SearchBar(
            controller: _controller,
            hintText: 'Search',
            leading: const Icon(Icons.search),
            trailing: [if (_busy) const SizedBox(width: 18, height: 18, child: CircularProgressIndicator(strokeWidth: 2))],
            onChanged: _onChanged,
          ),
        ),
        Expanded(
          child: ListView(children: [for (final r in _results) widget.resultBuilder(context, r)]),
        ),
      ],
    );
  }
}