//1、设置好已下载的m3u8文件路径    
    final String fold = (await FileUtils.getPackageDirectory(subDir: 'video')).path;
    final String host = url.substring(0, url.lastIndexOf('/'));
    final String m3u8FileName = url.substring(url.lastIndexOf('/'));
    final String m3u8Path = '$fold/$m3u8FileName';
    final String m3u8Name = m3u8FileName.split('.').first;
    //2、获取key的下载地址
    final String key = '$fold/../../video_enc.key';

3、下载好m3u8Path文件、key文件后，
4、解析m3u8文件
playList = await HlsPlaylistParser.create().parse(Uri.parse(url), await File(m3u8Path).readAsLines());
5、下载每个ts视频片段
6、使用FFmpegKit 合并ts视频片段
关键核心代码

// #EXTM3U
// #EXT-X-VERSION:3
// #EXT-X-TARGETDURATION:11
// #EXT-X-MEDIA-SEQUENCE:0
// #EXT-X-PLAYLIST-TYPE:VOD
// #EXT-X-KEY:METHOD=AES-128,URI="../../video_enc.key",IV=0xxxxxxxxxxx
// #EXTINF:10.560000,
// xxx_0000.ts
// #EXTINF:10.720000,
// xxx_0001.ts
// #EXTINF:6.760000,
// xxx_0002.ts
// #EXT-X-ENDLIST

/**
 * https://www.jianshu.com/p/b3a99aa7b154
 */
class DownloadUtil {
  DownloadUtil._();

  static late final DownloadUtil instance = DownloadUtil._();
  List<String> downLoadUrl = [];
  Map<String, String> downLoadUrlMap = {};

  Future<void> downloadM3u8(String url) async {
    debugPrint('DownloadUtil, _url=$url');
    // if (downLoadUrl.contains(url)) {
    //   debugPrint('DownloadUtil, 已进入下载队列 _url=$url, path=${downLoadUrlMap[url]}');
    //   return;
    // }
    final String fold = (await FileUtils.getPackageDirectory(subDir: 'video')).path;
    downLoadUrl.add(url);
    final String host = url.substring(0, url.lastIndexOf('/'));
    final String m3u8FileName = url.substring(url.lastIndexOf('/'));
    final String m3u8Path = '$fold/$m3u8FileName';
    final String m3u8Name = m3u8FileName.split('.').first;
    final String key = '$fold/../../video_enc.key';

    final Response response = await HttpManager().downloadFile(url, m3u8Path);
    final Response responseKey = await HttpManager().downloadFile('http://xxxx.com/video_enc.key', key);
    if (response.statusCode != 200) {
      debugPrint('DownloadUtil, 下载 m3u8 失败 _url=$url');
      downLoadUrl.remove(url);
      return;
    }
    if (responseKey.statusCode != 200) {
      debugPrint('DownloadUtil, 下载 key 失败 _url=$url');
      downLoadUrl.remove(url);
      return;
    }
    debugPrint('DownloadUtil, 下载m3u8成功 m3u8Path=$m3u8Path,key=$key');

    Uri playlistUri;
    List<String> lines;
    HlsPlaylist? playList;
    try {
      playList = await HlsPlaylistParser.create().parse(Uri.parse(url), await File(m3u8Path).readAsLines());
    } on ParserException catch (e) {
      print(e);
    }

    if (playList is HlsMasterPlaylist) {
      // master m3u8 file
    } else if (playList is HlsMediaPlaylist) {
      final key = playList.segments.map((it) => it.fullSegmentEncryptionKeyUri);

      debugPrint('DownloadUtil, 下载  key=$key');

      // media m3u8 file
      final mediaPlaylistUrls = playList.segments.map((it) => it.url);
      for (var value in mediaPlaylistUrls) {
        String tsUrl = '$host/$value';
        File file = File('$fold/$value');
        if (!file.existsSync()) {
          file.create();
        }
        debugPrint('DownloadUtil, 下载tsUrl tsUrl=$tsUrl， file.path=${file.path}');
        final Response response = await HttpManager().downloadFile(tsUrl,
            file.path /*, onReceiveProgress: (int count, int total){
          debugPrint('DownloadUtil, onReceiveProgress tsUrl=$tsUrl， count=$count,total=$total,process=${count/total}');
        }*/
            );
        // if(response.statusCode != 200){
        //   return;
        // }
        debugPrint('DownloadUtil, 下载tsUrl tsUrl=$tsUrl， file.path=${file.path}, 完成=========');
      }
      downLoadUrlMap[url] = m3u8Path;
      debugPrint('DownloadUtil, 全部ts视频下载完成===');
      // String cmd = '-i \"/storage/emulated/0/Android/data/com.azhansy.video/files/video/hevc.mp4\" -vcodec h264 \"/storage/emulated/0/Android/data/com.azhansy.video/files/video/hevc_h264.mp4\"';
      // String cmd = '-i \"/storage/emulated/0/Android/data/com.azhansy.video/files/video/hhh.mp4\" \"/storage/emulated/0/Android/data/com.azhansy.video/files/video/hhh111i.mp4\"';
      //-protocol_whitelist "file,http,crypto,tcp"
      '下载完成'.toast();
      String cmd =
          '-allowed_extensions ALL -i $m3u8Path  \"/storage/emulated/0/Android/data/com.azhansy.video/files/video/$m3u8Name.mp4\"';

      ///合并ts为mp4
      FFmpegKit.executeAsync(cmd, (FFmpegSession session) {
        cLogInfo('DownloadUtil, FFmpegKit session completeCallback cmd=${session.getCommand()}');
      }, (Log log) {
        debugPrint('DownloadUtil, FFmpegKit log===${log.getMessage()}');
      }, (Statistics statistics) {
        debugPrint('DownloadUtil, FFmpegKit statistics===${statistics.getTime()}');
      });
    }
  }
}

class DialogHeading extends StatelessWidget {
  const DialogHeading({Key? key, required this.text}) : super(key: key);

  final String text;

  @override
  Widget build(BuildContext context) => Padding(
        child: Text(
          text,
          style: Theme.of(context).textTheme.headline6,
        ),
        padding: EdgeInsets.only(top: 24, bottom: 8),
      );
}

用到的插件：

https://pub.dev/packages/flutter_hls_parser

flutter_hls_parser: ^2.0.0
ffmpeg_kit_flutter: ^4.5.1