Computer Vision with ML Kit - Flutter In Focus - dart

Question

I'm trying to follow the Computer Vision with ML Kit - Flutter In Focus tutorial, where I followed the tutorial step-by step, and still didn't manage to make it work.
my code is as follow:
import 'package:flutter/material.dart';
import 'dart:io';
import 'dart:async';
import 'package:image_picker/image_picker.dart';
import 'package:firebase_ml_vision/firebase_ml_vision.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: FacePage(),
);
}
}
class FacePage extends StatefulWidget{
#override
createState() => _FacePageState();
}
class _FacePageState extends State<FacePage>{
File _imageFile;
List<Face> _faces;
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Face Detector'),
),
body: ImageAndFaces(),
floatingActionButton: FloatingActionButton(
onPressed: _getImageAndDetectFace,
tooltip: 'Pick an Image',
child: Icon(Icons.add_a_photo),
),
);
}
void _getImageAndDetectFace() async {
final imageFile = await ImagePicker.pickImage(
source: ImageSource.gallery,
);
final image = FirebaseVisionImage.fromFile(imageFile);
final faceDetector = FirebaseVision.instance.faceDetector(
FaceDetectorOptions(
mode: FaceDetectorMode.accurate,
enableLandmarks: true,
),
);
List<Face> faces = await faceDetector.detectInImage(image);
if(mounted) {
setState(() {
_imageFile = imageFile;
_faces = faces;
});
}
}
}
class ImageAndFaces extends StatelessWidget {
ImageAndFaces({this.imageFile, this.faces});
final File imageFile;
final List<Face> faces;
#override
Widget build(BuildContext context) {
return Column(
children: <Widget>[
Flexible(
flex: 2 ,
child: Container(
constraints: BoxConstraints.expand(),
child: Image.file(imageFile, fit: BoxFit.cover),
),
),
Flexible(flex: 1 ,
child: ListView(
children: faces.map<Widget>((f) => FaceCoordinates(f)).toList(),
),
),
],
);
}
}
class FaceCoordinates extends StatelessWidget {
FaceCoordinates(this.face);
final Face face;
#override
Widget build(BuildContext context) {
final pos = face.boundingBox;
return ListTile(
title: Text('(${pos.top}, ${pos.left}, ${pos.bottom}, ${pos.right})'),
);
}
}
I'm getting the following exception stack:
I/flutter ( 5077): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 5077): The following assertion was thrown building ImageAndFaces(dirty):
I/flutter ( 5077): 'package:flutter/src/painting/image_provider.dart': Failed assertion: line 532 pos 14: 'file !=
I/flutter ( 5077): null': is not true.
I/flutter ( 5077):
I/flutter ( 5077): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter ( 5077): more information in this error message to help you determine and fix the underlying cause.
I/flutter ( 5077): In either case, please report this assertion by filing a bug on GitHub:
I/flutter ( 5077): https://github.com/flutter/flutter/issues/new?template=BUG.md
I/flutter ( 5077):
I/flutter ( 5077): When the exception was thrown, this was the stack:
I/flutter ( 5077): #2 new FileImage (package:flutter/src/painting/image_provider.dart:532:14)
I/flutter ( 5077): #3 new Image.file (package:flutter/src/widgets/image.dart:254:16)
I/flutter ( 5077): #4 ImageAndFaces.build (package:visionappwork/main.dart:94:28)
I/flutter ( 5077): #5 StatelessElement.build (package:flutter/src/widgets/framework.dart:3789:28)
I/flutter ( 5077): #6 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3736:15)
.........
Does anybody know what the problem is?
I tried anything I can think of, including trying to catch the 'ImageAndFaces' class constructor as it creates an instance, without any success.
I'm new to flutter and dart, so maybe it's a stupid mistake.
Thanks a lot!

Answer 1

The reason you're having a problem is that imageFile starts out null. Since it's being passed in to Image.file(imageFile, fit: BoxFit.cover) you're seeing the failure due to the assertion that the file passed to Image.file is not null.
You need to add some logic to check whether imageFile is null and do something different if it is.