freezer/lib/api/audio_sources/offline_audio_source.dart

import 'dart:io';

import 'package:freezer/api/definitions.dart';
import 'package:just_audio/just_audio.dart';

class OfflineAudioSource extends StreamAudioSource {
  final File file;
  final StreamInfoCallback? onStreamObtained;
  OfflineAudioSource(this.file, {this.onStreamObtained});

  @override
  Future<StreamAudioResponse> request([int? start, int? end]) async {
    // determine content type
    final first = await file.openRead(0, 4).join();
    bool isFlac = false;
    if (first == 'fLaC') isFlac = true;

    final length = await file.length();
    final stream = file.openRead(start, end);

    onStreamObtained?.call(StreamQualityInfo(
        format: isFlac ? Format.FLAC : Format.MP3,
        source: Source.offline,
        quality: null,
        size: length));

    return StreamAudioResponse(
        sourceLength: length,
        contentLength: (end ?? length) - (start ?? 0),
        offset: start,
        stream: stream,
        contentType: isFlac ? 'audio/flac' : 'audio/mpeg');
  }
}