Fix Flutter iOS AudioVisualizerWidget missing plugin exception

Some Flutter developers using the LiveKit Flutter SDK have reported an issue where the AudioVisualizerWidget (specifically SoundWaveformWidget) works fine on Android, but fails on iOS. The error typically looks like this:

MissingPluginException(No implementation found for method listen on channel io.livekit.audio.visualizer/eventChannel-XXXX)

This indicates that Flutter is unable to find the native iOS implementation for the event channel used by the audio visualizer.

Symptoms

• The waveform visualization does not appear on iOS
• Android builds function correctly
• Error appears in logs when enabling microphone:

  MissingPluginException(No implementation found for method listen on channel io.livekit.audio.visualizer/eventChannel-TR_abc123)

Root Cause

This issue was traced to a conflict with the flutter_background_service plugin.

On iOS, this plugin interferes with event channel handling when its service is initialized. As a result, LiveKit's audio visualizer event channel cannot properly register its listener, leading to the MissingPluginException.

Steps Attempted (but not root cause)

Users initially tried:

• Checking LiveKit plugin registration in AppDelegate.swift
• Ensuring microphone permissions (NSMicrophoneUsageDescription) in Info.plist
• Cleaning/rebuilding Flutter project (flutter clean && flutter pub get)
• Upgrading LiveKit dependencies

These steps did not resolve the problem, as the conflict originated from another plugin.

Solution

The problem was resolved by adjusting how the flutter_background_service plugin is initialized.

Specifically, its initializeService() call was causing interference:

Future<void> initializeService() async {
  final service = FlutterBackgroundService();

  await service.configure(
    iosConfiguration: IosConfiguration(
      autoStart: true,
      onForeground: onStart,
      onBackground: onIosBackground,
    ),
    androidConfiguration: AndroidConfiguration(
      autoStart: true,
      onStart: onStart,
      isForegroundMode: false,
      autoStartOnBoot: true,
    ),
  );
}

Workarounds:

1. Disable or remove flutter_background_service if it's not required.

2. Modify service configuration so that iOS background handling does not conflict with LiveKit's audio visualizer. For example, try removing or adjusting the onBackground handler in IosConfiguration.

3. Initialize flutter_background_service only when needed rather than at app startup.

After these changes, the audio visualizer began working correctly on iOS.

Recommendations

• If you need both LiveKit's audio visualizer and background services in the same Flutter app, test carefully on iOS to ensure compatibility.
• When encountering MissingPluginException, check for other plugins that register or override event channels.
• Keep LiveKit Flutter SDK and related dependencies updated to the latest versions for the most stable interop.