Flutter - Isolate : NoSuchMethodError: The method 'toRawHandle' was called on null - flutter

Question

Goal
I'm trying to perform real-time object detection with a Flutter app, using Tensorflow 2, with a SSD Mobilenet V2 model
I managed to get this work, using this git repo
However, I am encountering some latencies on the camera output display, so I decided to call the detection method inside a Isolate
What I've tried
The recognition method is Tflite.detectObjectOnFrame() from tflite plugin
As far as I understand, I have to use a particular Isolate in order to use plugin methods in another thread, so I'm using the isolate_handler plugin
The Code
Camera.dart
import 'dart:async';
import 'dart:typed_data';
import 'package:flutter/material.dart';
import 'package:camera/camera.dart';
import 'package:tflite/tflite.dart';
import 'package:isolate_handler/isolate_handler.dart';
import 'dart:math' as math;
typedef void Callback(List<dynamic> list, int h, int w);
class Camera extends StatefulWidget {
final List<CameraDescription> cameras;
final Callback setRecognitions;
Camera(this.cameras, this.setRecognitions);
#override
_CameraState createState() => new _CameraState();
}
class _CameraState extends State<Camera> {
CameraController controller;
bool isDetecting = false;
final isolates = IsolateHandler();
int startTime;
#override
void initState() {
super.initState();
if (widget.cameras == null || widget.cameras.length < 1) {
print('No camera is found');
} else {
controller = new CameraController(
widget.cameras[0],
ResolutionPreset.high,
);
controller.initialize().then((_) {
if (!mounted) {
return;
}
setState(() {});
controller.startImageStream((CameraImage img) {
if (!isDetecting) {
isDetecting = true;
startTime = new DateTime.now().millisecondsSinceEpoch;
//HERE IS THE ISOLATE
isolates.spawn<List<dynamic>>(
entryPoint,
name: 'object_detection',
onInitialized: () => isolates.send(img, to: 'object_detection'));
}
}
);
});
}
}
#override
void dispose() {
controller?.dispose();
super.dispose();
}
#override
Widget build(BuildContext context) {
if (controller == null || !controller.value.isInitialized) {
return Container();
}
var tmp = MediaQuery.of(context).size;
var screenH = math.max(tmp.height, tmp.width);
var screenW = math.min(tmp.height, tmp.width);
tmp = controller.value.previewSize;
var previewH = math.max(tmp.height, tmp.width);
var previewW = math.min(tmp.height, tmp.width);
var screenRatio = screenH / screenW;
var previewRatio = previewH / previewW;
return OverflowBox(
maxHeight:
screenRatio > previewRatio ? screenH : screenW / previewW * previewH,
maxWidth:
screenRatio > previewRatio ? screenH / previewH * previewW : screenW,
child: CameraPreview(controller),
);
}
void entryPoint(Map<String, dynamic> context) {
final messenger = HandledIsolate.initialize(context);
messenger.listen((img) async {
// HERE IS THE METHOD I USE FROM TFLITE PLUGIN
var recognitions = Tflite.detectObjectOnFrame(
bytesList: img.planes.map((plane) {
return plane.bytes;
}).toList(),
model: "SSDMobileNet",
imageHeight: img.height,
imageWidth: img.width,
imageMean: 127.5,
imageStd: 127.5,
numResultsPerClass: 1,
threshold: 0.4,
);
messenger.send(recognitions);
});
}
void setRecognitions(List<dynamic> recognitions) {
int endTime = new DateTime.now().millisecondsSinceEpoch;
print("Detection took ${endTime - startTime}");
widget.setRecognitions(recognitions, /*img.height, img.width*/1280, 720);
isDetecting = false;
isolates.kill('object_detection');
}
}
The error I get
E/flutter ( 7960): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)] Unhandled Exception: NoSuchMethodError: The method 'toRawHandle' was called on null.
E/flutter ( 7960): Receiver: null
E/flutter ( 7960): Tried calling: toRawHandle()
E/flutter ( 7960): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
E/flutter ( 7960): #1 FlutterIsolate.spawn (package:flutter_isolate/flutter_isolate.dart:25:55)
E/flutter ( 7960): #2 HandledIsolate._init (package:isolate_handler/src/handled_isolate.dart:174:37)
E/flutter ( 7960): #3 new HandledIsolate (package:isolate_handler/src/handled_isolate.dart:108:5)
E/flutter ( 7960): #4 IsolateHandler.spawn (package:isolate_handler/isolate_handler.dart:167:22)
E/flutter ( 7960): #5 _CameraState.initState.<anonymous closure>.<anonymous closure> (package:flutter_realtime_detection/camera.dart:56:22)
E/flutter ( 7960): #6 CameraController.startImageStream.<anonymous closure> (package:camera/camera.dart:412:20)
E/flutter ( 7960): #7 _rootRunUnary (dart:async/zone.dart:1198:47)
E/flutter ( 7960): #8 _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter ( 7960): #9 _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
E/flutter ( 7960): #10 _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:357:11)
E/flutter ( 7960): #11 _DelayedData.perform (dart:async/stream_impl.dart:611:14)
E/flutter ( 7960): #12 _StreamImplEvents.handleNext (dart:async/stream_impl.dart:730:11)
E/flutter ( 7960): #13 _PendingEvents.schedule.<anonymous closure> (dart:async/stream_impl.dart:687:7)
E/flutter ( 7960): #14 _rootRun (dart:async/zone.dart:1182:47)
E/flutter ( 7960): #15 _CustomZone.run (dart:async/zone.dart:1093:19)
E/flutter ( 7960): #16 _CustomZone.runGuarded (dart:async/zone.dart:997:7)
E/flutter ( 7960): #17 _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1037:23)
E/flutter ( 7960): #18 _rootRun (dart:async/zone.dart:1190:13)
E/flutter ( 7960): #19 _CustomZone.run (dart:async/zone.dart:1093:19)
E/flutter ( 7960): #20 _CustomZone.runGuarded (dart:async/zone.dart:997:7)
E/flutter ( 7960): #21 _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1037:23)
E/flutter ( 7960): #22 _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
E/flutter ( 7960): #23 _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
Any idea of what I am doing wrong ? Thank you.

Answer 1

Isolates don't share memory between them, which means you can only use primitive values and static/top-level functions. The entryPoint is not a static/top-level function in your case.
Check out the easy_isolate plugin, it provides an easy way to work with isolates with well-explained documentation. But doesn't call platform plugins.
https://pub.dev/packages/easy_isolate