Flutter - How correctly pause camera when user moved to other (preview) screen?

Question

I need to pause camera when I move to another screen on the navigator tree in order to save battery and performance.

I tried to dispose() cameraController, but flutter doesn't re-initialize the state when it returns from another screen (which is obvious, though).

My main code to work with a camera:

 @override
  void initState() {
    super.initState();
    availableCameras().then((cameras) {
      setState(() {
        _firstCamera = cameras.first;
        _controller = CameraController(_firstCamera, ResolutionPreset.high);

        _initializeControllerFuture = _controller.initialize();
      });
    });
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _scaffoldKey,
      body: Stack(
        children: <Widget>[
          FutureBuilder<void>(
            future: _initializeControllerFuture,
            builder: (context, snapshot) {

              if (snapshot.connectionState == ConnectionState.done) {
                return Stack(
                  alignment: FractionalOffset.center,
                  children: <Widget>[
                    new Positioned.fill(
                      child: _getCameraPreview(context),
                    ),
                    ...                    
                  ],
                );
              } else {
                return Center(child: CircularProgressIndicator());
              }
            },
          ),
          Align(
            alignment: Alignment.bottomCenter,
            child: BottomAppBar(
              color: Color.fromARGB(0, 0, 0, 0),
              child: _getBottomAppBarRow(context),
            ),
          ),
        ],
      ),
    );
  }

  _getCameraPreview(BuildContext context) {
    final size = MediaQuery.of(context).size;
    final deviceRatio = size.width / size.height;
    return Transform.scale(
      scale: _controller.value.aspectRatio / deviceRatio,
      child: Center(
        child: AspectRatio(
          aspectRatio: _controller.value.aspectRatio,
          child: CameraPreview(_controller),
        ),
      ),
    );
  }

Answer 1

Suppose the camera controller for an instance of the camera package is defined as such:

List<CameraDescription> cameras = [];
controller = CameraController(
  cameras[0],
  ResolutionPreset.high,
  enableAudio: false,
);

This can be used to pause the camera:

controller.pausePreview();

This can be used to resume the camera:

controller.resumePreview();

Answer 2

Have a variable like _cameraOn = true. Show CameraPreview when it is true and not when it is false. While navigating to another screen set it to false

You could have the camera related functionality in a separate widget. So every time it is displayed it is initialized, and when it is not it's disposed.

A simple working example

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

List<CameraDescription> cameras;

Future<void> main() async {
  cameras = await availableCameras();
  runApp(MaterialApp(
    home: CameraApp(),
  ));
}

class CameraApp extends StatefulWidget {
  @override
  _CameraAppState createState() => _CameraAppState();
}

class _CameraAppState extends State<CameraApp> {
  bool _cameraOn = true;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: <Widget>[
          Expanded(
            child: _cameraOn ? Camera() : Container(),
          ),
          FlatButton(
            onPressed: () {
              setState(() {
                _cameraOn = false;
              });
              Navigator.push(
                  context,
                  MaterialPageRoute(
                      builder: (BuildContext context) => Post())).then((res) {
                setState(() {
                  _cameraOn = true;
                });
              }).catchError((err) {
                print(err);
              });
            },
            child: Text("NEXT PAGE"),
          ),
        ],
      ),
    );
  }
}

class Camera extends StatefulWidget {
  @override
  _CameraState createState() => _CameraState();
}

class _CameraState extends State<Camera> {
  CameraController controller;

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

  @override
  Widget build(BuildContext context) {
    if (!controller.value.isInitialized) {
      return Container();
    }

    return AspectRatio(
      aspectRatio: controller.value.aspectRatio,
      child: CameraPreview(controller),
    );
  }

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

class Post extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Text("Post"),
    );
  }
}