Auction countdown chip

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

class CountdownChip extends StatefulWidget {
  const CountdownChip({
    super.key,
    required this.endsAt,
    this.serverOffset = Duration.zero,
    this.onEnded,
  });
  final DateTime endsAt;
  final Duration serverOffset; // serverNow - deviceNow, measured once per session
  final VoidCallback? onEnded;

  @override
  State<CountdownChip> createState() => _CountdownChipState();
}

class _CountdownChipState extends State<CountdownChip> {
  Timer? _timer;
  late Duration _left = _remaining();

  Duration _remaining() {
    final left = widget.endsAt.difference(DateTime.now().add(widget.serverOffset));
    return left.isNegative ? Duration.zero : left;
  }

  @override
  void initState() {
    super.initState();
    _schedule();
  }

  @override
  void didUpdateWidget(CountdownChip old) {
    super.didUpdateWidget(old);
    if (old.endsAt != widget.endsAt) {
      _timer?.cancel();
      _left = _remaining();
      _schedule(); // a late bid extended the auction
    }
  }

  void _schedule() {
    if (_left == Duration.zero) return;
    // Wake up just after the next whole second so the digits never lag.
    final wait = Duration(milliseconds: 1000 - (_left.inMilliseconds % 1000) + 20);
    _timer = Timer(wait, () {
      if (!mounted) return;
      setState(() => _left = _remaining());
      if (_left == Duration.zero) {
        widget.onEnded?.call();
      } else {
        _schedule();
      }
    });
  }

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

  String get _label {
    if (_left == Duration.zero) return 'Ended';
    final d = _left.inDays, h = _left.inHours % 24, m = _left.inMinutes % 60, s = _left.inSeconds % 60;
    String two(int n) => n.toString().padLeft(2, '0');
    return d > 0 ? '${d}d ${two(h)}h' : '${two(h)}:${two(m)}:${two(s)}';
  }

  @override
  Widget build(BuildContext context) {
    final scheme = Theme.of(context).colorScheme;
    final urgent = _left > Duration.zero && _left < const Duration(minutes: 5);
    final ended = _left == Duration.zero;
    return Chip(
      avatar: Icon(ended ? Icons.lock_clock : Icons.timer_outlined, size: 18,
          color: urgent ? scheme.onErrorContainer : null),
      label: Text(_label, style: const TextStyle(fontFeatures: [FontFeature.tabularFigures()])),
      backgroundColor: urgent ? scheme.errorContainer : (ended ? scheme.surfaceContainerHighest : null),
    );
  }
}