[iOS] Some device can not activate session directly in the background

[ininmm]

On iPhone 8 (iOS 16), I found that audio cannot be played in the background directly. It has to be played in the foreground first before it can work in the background.

Below is my code:

class AudioUtil {
  factory AudioUtil() => _instance;

  AudioUtil._();

  static final AudioUtil _instance = AudioUtil._();

  late AudioPlayer player;

  Future<void> init() async {
    player = AudioPlayer();
    final AudioSession session = await AudioSession.instance;
    await session.configure(const AudioSessionConfiguration(
      avAudioSessionCategory: AVAudioSessionCategory.playback,
      avAudioSessionCategoryOptions: AVAudioSessionCategoryOptions.mixWithOthers,
      avAudioSessionMode: AVAudioSessionMode.defaultMode,
      androidAudioAttributes: AndroidAudioAttributes(
        contentType: AndroidAudioContentType.music,
        usage: AndroidAudioUsage.media,
      ),
      androidAudioFocusGainType: AndroidAudioFocusGainType.gain,
    ));
    await session.setActive(true);
  }

  Future<void> playNotification() async {
    String asset;
    if (Platform.isAndroid) {
      asset = 'assets/audio/mediumRinging.mp3';
    } else {
      asset = 'assets/audio/mediumRinging.caf';
    }
    await AudioUtil().player.setAsset(asset);
    await AudioUtil().player.setVolume(1.0);
    await AudioUtil().player.play();
  }
}

On iPhone XR and iPhone 12, audio can be played directly in the background.
Additionally, I tested background audio playback directly in Xcode using AVAudioSession with the same session configuration, and iPhone 8 worked fine in that setup.

Minimal reproduction project
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To Reproduce (i.e. user steps, not code)
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Error messages

┌───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ PlatformException(560557684, Session activation failed, null, null)
├───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ #0   StandardMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:651)
│ #1   MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:334)
│ #2   <asynchronous suspension>
│ #3   AVAudioSession.setActive (package:audio_session/src/darwin.dart:150)
│ #4   <asynchronous suspension>
│ #5   AudioSession.setActive (package:audio_session/src/core.dart:237)
│ #6   <asynchronous suspension>
│ #7   AudioPlayer.play (package:just_audio/just_audio.dart:948)
├───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

Expected behavior
iPhone 8 should play audio in the background just like other iOS devices.

Screenshots
If applicable, add screenshots to help explain your problem.

• iPhone 6
• OS: [e.g. iOS8.1]

Flutter SDK version

Doctor summary (to see all details, run flutter doctor -v):
[!] Flutter (Channel [user-branch], 3.19.6, on macOS 15.2 24C101 darwin-arm64, locale zh-Hant-TW)
[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 16.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2024.2)
[✓] VS Code (version 1.96.2)
[✓] Connected device (4 available)
[✓] Network resources

Additional context
Using package:

• audio_session: 0.1.21
• just_audio: 0.9.42

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