[iOS] Some device an not play audio 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.

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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1. Fork this repository and modify the example to reproduce the bug, then provide a link here.
2. If the unmodified official example already reproduces the bug, just write "The example".

To Reproduce (i.e. user steps, not code)
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();
  }
}

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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