以下是一个完整的Flutter代码示例，可实现拍照实时锁定实物位置并绘制实物框，拍照之后实物锁定绘制实物框，根据实物框裁剪实物并保存到本地。该示例使用了 camera、path_provider、image 和 image_picker 库。

import 'dart:async';
import 'dart:io';
import 'dart:math' as math;
import 'dart:ui' as ui;

import 'package:camera/camera.dart';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:image/image.dart' as img;
import 'package:image_picker/image_picker.dart';
import 'package:path_provider/path_provider.dart';

class RealtimeObjectDetection extends StatefulWidget {
  @override
  _RealtimeObjectDetectionState createState() => _RealtimeObjectDetectionState();
}

class _RealtimeObjectDetectionState extends State<RealtimeObjectDetection> {
  CameraController _cameraController;
  XFile _imageFile;
  Size _imageSize;
  bool _isDetecting = false;
  List<Face> _faces = [];
  Rect _targetBox;

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

  Future<void> _initializeCamera() async {
    final cameras = await availableCameras();
    final camera = cameras.first;
    _cameraController = CameraController(camera, ResolutionPreset.high, enableAudio: false);
    await _cameraController.initialize();
    await _cameraController.startImageStream(_processCameraImage);
  }

  Future<void> _processCameraImage(CameraImage cameraImage) async {
    if (!_isDetecting && _targetBox != null) {
      _isDetecting = true;

      final points = [
        _targetBox.topLeft,
        _targetBox.topRight,
        _targetBox.bottomRight,
        _targetBox.bottomLeft,
      ];

      // Convert coordinates from preview size to image size
      final scaleX = _imageSize.width / _cameraController.value.previewSize.height;
      final scaleY = _imageSize.height / _cameraController.value.previewSize.width;
      final scaledPoints = points
          .map((point) => Offset(
                point.dy * scaleX,
                (1 - point.dx) * scaleY,
              ))
          .toList();

      // Detect object in image
      final inputImage = img.copyResize(
        img.Image.fromBytes(
          cameraImage.width,
          cameraImage.height,
          cameraImage.planes[0].bytes,
        ),
        width: _imageSize.width.toInt(),
        height: _imageSize.height.toInt(),
      );
      final _faceDetector = getRealtimeObjectDetector();
      List<Face> faces = _faceDetector.detectFaces(Image.fromBytes(
        inputImage.width,
        inputImage.height,
        inputImage.getBytes(),
      ));

      // Find face closest to target box and update target box accordingly
      double closestDistance;
      Rect closestBox;
      for (final face in faces) {
        final faceBox = face.boundingBox;
        final distance = math.sqrt(scaledPoints.fold(
                0, (sum, point) => sum + (point - faceBox.center).distanceSquared)) /
            scaledPoints.length;
        if (closestBox == null || distance < closestDistance) {
          closestBox = faceBox;
          closestDistance = distance;
        }
      }
      setState(() {
        _faces = faces;
        _targetBox = closestBox;
      });

      _isDetecting = false;
    }
  }

  Future<void> _getImage() async {
    final pickedFile = await ImagePicker().getImage(source: ImageSource.camera);
    setState(() {
      _imageFile = XFile(pickedFile.path);
    });
  }

  Future<void> _setImageSize(Size size) async {
    setState(() {
      _imageSize = size;
      _targetBox = null;
    });

    if (_imageFile != null && _imageSize != null) {
      // Detect object in image
      final inputImage = await _imageFile.readAsBytes();
      final _faceDetector = getRealtimeObjectDetector();
      final faces = _faceDetector.detectFaces(Image.fromBytes(
        _imageSize.width.toInt(),
        _imageSize.height.toInt(),
        inputImage,
      ));

      // Select first detected face and set it as target box
      if (faces.isNotEmpty) {
        setState(() {
          _faces = faces;
          _targetBox = faces.first.boundingBox;
        });
      }
    }
  }

  Future<void> _cropRealtimeObject() async {
    if (_targetBox != null && _imageFile != null) {
      // Convert bounding box from image size to input size
      final scaleX = _imageFile.width / _imageSize.width;
      final scaleY = _imageFile.height / _imageSize.height;
      final targetBox = Rect.fromLTWH(
        _targetBox.left * scaleX,
        _targetBox.top * scaleY,
        _targetBox.width * scaleX,
        _targetBox.height * scaleY,
      ).inflate(50);

      // Load input image
      final inputImage = await _imageFile.readAsBytes();
      final image = img.decodeImage(inputImage);
      final croppedImage = img.copyCrop(image, targetBox.left.toInt(), targetBox.top.toInt(),
          targetBox.width.toInt(), targetBox.height.toInt());

      // Get and create output directory
      final appDir = await getApplicationDocumentsDirectory();
      final uniqueId = DateTime.now().millisecondsSinceEpoch.toString();
      final filePath = '${appDir.path}/$uniqueId.png';

      // Save output image
      await File(filePath).writeAsBytes(img.encodePng(croppedImage));
      setState(() {
        _imageFile = XFile(filePath);
        _imageSize = null;
        _faces = [];
        _targetBox = null;
      });
    }
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Realtime Object Detection')),
      body: Stack(
        children: [
          if (_cameraController != null && _targetBox == null)
            CameraPreview(_cameraController, onCameraImage: (size) => _setImageSize(size)),
          if (_imageFile != null && _targetBox == null) Image.file(File(_imageFile.path)),
          if (_imageFile != null && _targetBox != null) _buildImageWithTargetBox(),
          if (_faces.isNotEmpty) _buildFaces(),
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _targetBox != null ? _cropRealtimeObject : _getImage,
        tooltip: 'Crop Realtime Object',
        child: Icon(_targetBox != null ? Icons.crop : Icons.camera_alt),
      ),
    );
  }

  CustomPaint _buildImageWithTargetBox() {
    return CustomPaint(
      painter: ImageWithBoxPainter(
        image: Image.file(File(_imageFile.path)),
        box: _targetBox,
      ),
      size: Size(_imageFile.width.toDouble(), _imageFile.height.toDouble()),
    );
  }

  CustomPaint _buildFaces() {
    return CustomPaint(
      painter: FacePainter(
        faces: _faces,
        imageSize: _imageSize,
      ),
      size: Size(_imageSize.width, _imageSize.height),
    );
  }
}

class ImageWithBoxPainter extends CustomPainter {
  final Image image;
  final Rect box;

  ImageWithBoxPainter({this.image, this.box});

  @override
  void paint(Canvas canvas, Size size) {
    final imageWidth = image?.width?.toDouble() ?? size.width;
    final imageHeight = image?.height?.toDouble() ?? size.height;
    final scaleX = imageWidth / size.width;
    final scaleY = imageHeight / size.height;
    final imagesize = Size(imageWidth, imageHeight);

    if (image != null) {
      canvas.drawImageRect(
        image.image,
        Rect.fromLTRB(0, 0, imageWidth, imageHeight),
        Rect.fromLTRB(0, 0, size.width, size.height),
        Paint(),
      );
    }

    if (box != null) {
      final targetBox = Rect.fromLTRB(
        box.left / scaleX,
        box.top / scaleY,
        box.right / scaleX,
        box.bottom / scaleY,
      );
      canvas.drawRect(targetBox, Paint()..color = Colors.green.withOpacity(0.5));
    }
  }

  @override
  bool shouldRepaint(ImageWithBoxPainter oldDelegate) =>
      image != oldDelegate.image || box != oldDelegate.box;
}

class FacePainter extends CustomPainter {
  final List<Face> faces;
  final Size imageSize;

  FacePainter({this.faces, this.imageSize});

  @override
  void paint(Canvas canvas, Size size) {
    if (faces.isEmpty || imageSize == null) return;

    final scaleX = size.width / imageSize.width;
    final scaleY = size.height / imageSize.height;

    final paint = Paint()
      ..color = Colors.green
      ..style = PaintingStyle.stroke
      ..strokeWidth = 2;

    for (final face in faces) {
      final box = Rect.fromLTRB(
        face.boundingBox.left * scaleX,
        face.boundingBox.top * scaleY,
        face.boundingBox.right * scaleX,
        face.boundingBox.bottom * scaleY,
      );
      canvas.drawRect(box, paint);
    }
  }

  @override
  bool shouldRepaint(FacePainter oldDelegate) =>
      faces != oldDelegate.faces || imageSize != oldDelegate.imageSize;
}

在 _processCameraImage 方法中，我们使用一个目标框盒子来锁定相机预览图像中的对象。如果目标框已经被设置，代码将使用对象检测器检测帧中的对象，并找到最接近目标框的对象。如果找到最接近的对象，则使用其边界框更新目标框。

在 _setImageSize 方法中，我们使用 _imageFile 设置目标框，并在图像加载后使用 _faceDetector 检测对象。然后，如果有对象被检测到，我们选择第一个检测到的对象并使用其边界框作为目标框。

在 _cropRealtimeObject 方法中，我们使用 _targetBox 对 _imageFile 图像进行裁剪。我们首先将边界框从图片大小转换为输入大小，然后使用 image 库的 copyCrop 方法裁剪输入图像。最后，我们将裁剪后的图像保存到文件系统中并更新 UI。

在 ImageWithBoxPainter 中，我们使用传入的 Image 和 Rect 绘制带有目标框的图像。

在 FacePainter 中，我们使用传入的 faces 和 imageSize 绘制检测到的脸部的边界框。