Flutter_fft: Why is the argument type 'String' can't be assigned to the parameter type 'int'?

Question

I've started working on a project with Flutter and Dart. The app should display a spectrometer based on the signal received by the microphone. I used flutter_fft to handle the signals and syncfusion to make the chart.

While coding the startListening() function i've stumbled across this weird error:

The argument type 'String' can't be assigned to the parameter type 'int'. How can I resolve this error? 'freq' or 'frequency? should be the keys to extract the audio. I tried everything, even if I am an absolute beginner in Android coding, but neither AI seems to know the answer.

this is the whole code:

import 'package:flutter/material.dart';
import 'package:flutter_fft/flutter_fft.dart';
import 'package:intl/intl.dart'; 
import 'package:permission_handler/permission_handler.dart';
import 'package:syncfusion_flutter_charts/charts.dart';

class Spettrometro extends StatefulWidget {
  @override
  _SpettrometroState createState() => _SpettrometroState();
}

class _SpettrometroState extends State<Spettrometro> {
  FlutterFft flutterFft = FlutterFft();
  List<DataPoint> data = [];
  bool isRecording = false;

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

  Future<void> requestPermission() async {
    var status = await Permission.microphone.request();
    if (status.isGranted) {
      startListening();
    } else {
      print("Permesso microfono negato");
    }
  }

  void startListening() async {
    await flutterFft.startRecorder();
    setState(() => isRecording = true);

    flutterFft.onRecorderStateChanged.listen((event) {
      final dynamic frequencyValue = event['freq']; // This is the problematic line of code
      double? freq;

      // Conversione robusta a double
      if (frequencyValue != null) {
        freq = double.tryParse(frequencyValue.toString());
      }

      if (freq != null && freq > 0) {
        setState(() {
          data.add(DataPoint(DateTime.now(), freq!)); 
          if (data.length > 100) data.removeAt(0);
        });
      }
    });
  }

  void stopListening() {
    flutterFft.stopRecorder();
    setState(() => isRecording = false);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Spettrometro Audio")),
      body: Column(
        children: [
          Expanded(
            child: SfCartesianChart(
              primaryXAxis: DateTimeAxis(),
              primaryYAxis: NumericAxis(
                numberFormat:
                    NumberFormat.decimalPattern(),
              ),
              series: <LineSeries<DataPoint, DateTime>>[
                LineSeries<DataPoint, DateTime>(
                  dataSource: data,
                  xValueMapper: (DataPoint point, _) => point.time,
                  yValueMapper: (DataPoint point, _) => point.frequency,
                ),
              ],
            ),
          ),
          ElevatedButton(
            onPressed: isRecording ? stopListening : startListening,
            child: Text(isRecording ? "Ferma" : "Avvia"),
          ),
        ],
      ),
    );
  }
}

class DataPoint {
  final DateTime time;
  final double frequency;
  DataPoint(this.time, this.frequency);
}

I tried to force that line of code using

double? freq = event["freq"] is num ? (event["freq"] as num).toDouble() : double.tryParse(event["freq"].toString());

but it doesn't work

YektaDev · Accepted Answer · 2025-03-13 16:40:03Z

1

I managed to fix the problem by forcing the variable to be an integer:

final dynamic frequencyValue = event[0];
double? freq;
int? intValue = int.tryParse(event[1].toString());

For some reason, it now works as intended.

edited Mar 13 at 16:40

YektaDev

5443 gold badges11 silver badges32 bronze badges

answered Mar 8 at 14:11

KF_2677

214 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

galad · Accepted Answer · 2025-03-08 02:44:07Z

0

You are trying to get a value from event as if it is a Map<String, dynamic> on this listener:

flutterFft.onRecorderStateChanged.listen((event)...

Update: based on documentation, event is a List<Object> so you need to get it by index and cast it to double. From the docs you can see frequency data is supposed to be on index 1. So yours would be:

frequency = event[1] as double;

See example on https://pub.dev/documentation/flutter_fft/latest/

Original comment:

Are you sure event is a Map? Maybe it's a List? Or maybe it is a Map, but not a Map<String, dynamic>

I'm not familiar with flutter fft, but I imagine their API docs will have some info for you.

edited Mar 8 at 2:44

answered Mar 7 at 4:39

galad

1161 gold badge1 silver badge7 bronze badges

Collectives™ on Stack Overflow

Flutter_fft: Why is the argument type 'String' can't be assigned to the parameter type 'int'?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related