listingDirectoryContent.dart

// Method 
// Initialize it into the initState

var files = <FileSystemEntity>[];
  Future<List<FileSystemEntity>> dirContents(Directory dir) async {
    var completer = Completer<List<FileSystemEntity>>();
    var lister = dir.list(recursive: false, followLinks: false);

    lister.listen((file) async {
      FileStat f = file.statSync();
      // print(f.toString());
      if (f.type == FileSystemEntityType.directory) {
        await dirContents(Directory(file.uri.toFilePath()));
      } else if (f.type == FileSystemEntityType.file &&
                 // change the extension type to list file according to desire (such as mp4 for video or pdf or mp3 or docx etc)
          file.path.endsWith('.mp3')) {
        files.add(file);
      }
    }, onDone: () {
      completer.complete(files);
      setState(() {
        //
      });
    });

    return completer.future;
  }


// initiState

  @override
  void initState() {
    requestingPermission();
    // directly passing the direcotry location
    dirContents(Directory('/storage/emulated/0'));

    super.initState();
  }

// this function is not much important 
//   getDirectory() {
//     Directory dir = Directory('/storage/emulated/0');
//     var files = dirContents(dir);
//     // print(files);
//   }


// requesting permissions initialized it in initState

  void requestingPermission() async {
    var status = await Permission.storage.status;
    if (!status.isGranted) {
      await Permission.storage.request();
    }
  }

// Permisions in AndroidMenifest.xml
//  add under after the package line ---
    <uses-permission android:name="android.permission.READ_INTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.ACCESS_MEDIA_LOCATION" />
    <uses-permission android:name="android.permission.MANAGE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.ACTION_MANAGE_WRITE_SETTINGS" />


// Widget Example

ListView.builder(
        itemCount: files.length,
        itemBuilder: (context, index) {
          return Card(
              color: AppColors.littleBlack,
              child: ListTile(
                title: Text(files[index].path.split('/').last),
                // leading: const Icon(Icons.audiotrack),
                trailing: const Icon(
                  Icons.play_arrow,
                  color: Colors.redAccent,
                ),
              ));
        });