How to implement Tap To Focus in Flutter Camera Plugin?

You have to use the camera controller method to set the focus manually :

controller.setFocusPoint(offset)

See the official Api documention here for learn about method and camera functions.

The method need a point to focus. The x,y point have to be > 0 and < 1 where 0 is top left point of the camera and 1 the bottom right.

Then you have to retrieve your camera size and create proportions.

Here is a full sample code that you can copy and run

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

late List<CameraDescription> cameras;

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  cameras = await availableCameras();
  runApp(const MaterialApp(home: CameraApp()));
}

class CameraApp extends StatefulWidget {
  const CameraApp({Key? key}) : super(key: key);

  @override
  _CameraAppState createState() => _CameraAppState();
}

class _CameraAppState extends State<CameraApp> {
  late CameraController controller;
  bool showFocusCircle = false;
  double x = 0;
  double y = 0;

  @override
  void initState() {
    super.initState();
    controller = CameraController(cameras[0], ResolutionPreset.max);
    controller.initialize().then((_) {
      if (!mounted) {
        return;
      }
      setState(() {});
    });
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    if (!controller.value.isInitialized) {
      return Container();
    }
    return  GestureDetector(
      onTapUp: (details) {
        _onTap(details);
      },
        child: Stack(
          children: [
            Center(
                child: CameraPreview(controller)
            ),
            if(showFocusCircle) Positioned(
                top: y-20,
                left: x-20,
                child: Container(
              height: 40,
              width: 40,
              decoration: BoxDecoration(
                shape: BoxShape.circle,
                border: Border.all(color: Colors.white,width: 1.5)
              ),
            ))
          ],
        )
    );
  }


  Future<void> _onTap(TapUpDetails details) async {
    if(controller.value.isInitialized) {
      showFocusCircle = true;
      x = details.localPosition.dx;
      y = details.localPosition.dy;

      double fullWidth = MediaQuery.of(context).size.width;
      double cameraHeight = fullWidth * controller.value.aspectRatio;

      double xp = x / fullWidth;
      double yp = y / cameraHeight;

      Offset point = Offset(xp,yp);
      print("point : $point");

      // Manually focus
      await controller.setFocusPoint(point);
      
      // Manually set light exposure
      //controller.setExposurePoint(point);
      
      setState(() {
        Future.delayed(const Duration(seconds: 2)).whenComplete(() {
          setState(() {
            showFocusCircle = false;
          });
        });
      });
    }
  }
}

Be aware to add bounds for the x,y point value to avoid crash if the tap is beyond the camera preview :

if (point != null &&
      (point.dx < 0 || point.dx > 1 || point.dy < 0 || point.dy > 1)) {
    throw ArgumentError(
        'The values of point should be anywhere between (0,0) and (1,1).');
  }

Besides, the focus point is not available for the IOS camera front as far as I know, so allow the focus tap only on back camera lens.