Animated live-tracking marker

import 'dart:math' as math;
import 'package:flutter/material.dart';

class GeoPoint {
  const GeoPoint(this.lat, this.lng);
  final double lat, lng;

  /// Initial compass bearing in degrees from this point to [o].
  double bearingTo(GeoPoint o) {
    final p1 = lat * math.pi / 180, p2 = o.lat * math.pi / 180;
    final dl = (o.lng - lng) * math.pi / 180;
    final y = math.sin(dl) * math.cos(p2);
    final x = math.cos(p1) * math.sin(p2) - math.sin(p1) * math.cos(p2) * math.cos(dl);
    return (math.atan2(y, x) * 180 / math.pi + 360) % 360;
  }
}

class _Pose {
  const _Pose(this.point, this.bearing);
  final GeoPoint point;
  final double bearing;
}

class _PoseTween extends Tween<_Pose> {
  _PoseTween({required _Pose begin, required _Pose end}) : super(begin: begin, end: end);

  @override
  _Pose lerp(double t) {
    final a = begin!, b = end!;
    // Rotate along the shortest arc (350 -> 10 turns 20 degrees, not 340).
    final diff = ((b.bearing - a.bearing + 540) % 360) - 180;
    return _Pose(
      GeoPoint(a.point.lat + (b.point.lat - a.point.lat) * t,
          a.point.lng + (b.point.lng - a.point.lng) * t),
      a.bearing + diff * t,
    );
  }
}

/// Animates between successive positions; [project] maps a point to pixels.
class TrackingMarker extends StatefulWidget {
  const TrackingMarker({super.key, required this.position, required this.project, this.size = 36});
  final GeoPoint position;
  final Offset Function(GeoPoint) project;
  final double size;

  @override
  State<TrackingMarker> createState() => _TrackingMarkerState();
}

class _TrackingMarkerState extends State<TrackingMarker> {
  late _Pose _from = _Pose(widget.position, 0);
  late _Pose _to = _from;

  @override
  void didUpdateWidget(TrackingMarker old) {
    super.didUpdateWidget(old);
    if (old.position.lat == widget.position.lat && old.position.lng == widget.position.lng) return;
    _from = _to; // start from the last target; TweenAnimationBuilder eases the rest
    _to = _Pose(widget.position, old.position.bearingTo(widget.position));
  }

  @override
  Widget build(BuildContext context) {
    return TweenAnimationBuilder<_Pose>(
      key: const ValueKey('marker'),
      tween: _PoseTween(begin: _from, end: _to),
      duration: const Duration(seconds: 2), // roughly the GPS update interval
      curve: Curves.linear,
      builder: (context, pose, _) {
        final o = widget.project(pose.point);
        return Positioned(
          left: o.dx - widget.size / 2,
          top: o.dy - widget.size / 2,
          child: Transform.rotate(
            angle: pose.bearing * math.pi / 180,
            child: Icon(Icons.navigation, size: widget.size),
          ),
        );
      },
    );
  }
}