要实现基于高德地图的Flutter地理围栏功能，可以使用flutter_map插件结合高德地图API。首先，在高德开放平台注册并获取Key。

1. 添加依赖：在pubspec.yaml中加入flutter_map和location库。
2. 初始化地图：创建一个Flutter Map Widget，并加载高德底图。
3. 设置围栏区域：定义矩形或圆形围栏区域，记录其坐标范围。
4. 获取当前位置：通过location插件实时获取用户位置。
5. 检测进出围栏：比较用户位置与围栏区域的关系，当位置变化跨越围栏边界时触发事件。
6. 通知用户：使用SnackBar或弹窗告知用户进入或离开围栏状态。

示例代码：

import 'package:flutter/material.dart';
import 'package:flutter_map/flutter_map.dart';
import 'package:latlong2/latlong.dart';
import 'package:location/location.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: GeoFencePage(),
    );
  }
}

class GeoFencePage extends StatefulWidget {
  @override
  _GeoFencePageState createState() => _GeoFencePageState();
}

class _GeoFencePageState extends State<GeoFencePage> {
  final Location location = Location();
  bool insideGeoFence = false;

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

  Future<void> startLocationService() async {
    bool _serviceEnabled;
    PermissionStatus _permissionGranted;

    _serviceEnabled = await location.serviceEnabled();
    if (!_serviceEnabled) {
      _serviceEnabled = await location.requestService();
      if (!_serviceEnabled) {
        return;
      }
    }

    _permissionGranted = await location.hasPermission();
    if (_permissionGranted == PermissionStatus.denied) {
      _permissionGranted = await location.requestPermission();
      if (_permissionGranted != PermissionStatus.granted) {
        return;
      }
    }

    location.onLocationChanged.listen((locationData) {
      double latitude = locationData.latitude!;
      double longitude = locationData.longitude!;
      print('Lat: $latitude, Long: $longitude');
      checkGeoFence(latitude, longitude);
    });
  }

  void checkGeoFence(double lat, double long) {
    // 定义围栏区域
    double fenceLat = 39.9096046;
    double fenceLng = 116.3972285;
    double radius = 100; // 单位：米

    double distance = calculateDistance(lat, long, fenceLat, fenceLng);
    if (distance <= radius && !insideGeoFence) {
      setState(() {
        insideGeoFence = true;
        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text("已进入围栏")));
      });
    } else if (distance > radius && insideGeoFence) {
      setState(() {
        insideGeoFence = false;
        ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text("已离开围栏")));
      });
    }
  }

  double calculateDistance(double lat1, double lon1, double lat2, double lon2) {
    // 使用Haversine公式计算两点间距离
    const int R = 6371; // 地球半径，单位：公里
    var dLat = degToRad(lat2 - lat1);
    var dLon = degToRad(lon2 - lon1);
    var a =
        sin(dLat / 2) * sin(dLat / 2) +
        cos(degToRad(lat1)) * cos(degToRad(lat2)) *
            sin(dLon / 2) * sin(dLon / 2);
    var c = 2 * atan2(sqrt(a), sqrt(1 - a));
    return R * c * 1000; // 转换为米
  }

  double degToRad(double deg) => deg * (pi / 180);
}