can not delete cache file

Start by tracing the cacheObject.relativePath deletion path and the ExternalStorageFileSystem.createFile implementation shown in the issue. Check how the file path is produced and used, then verify that deleting a cached object removes the corresponding file in external storage without breaking the existing missing-file handling. Done means cache files created by this file system can be deleted successfully.

    if (toRemove.contains(cacheObject.id)) return;

    toRemove.add(cacheObject.id!);
    if (_memCache.containsKey(cacheObject.key)) {
      _memCache.remove(cacheObject.key);
    }
    if (_futureCache.containsKey(cacheObject.key)) {
      await _futureCache.remove(cacheObject.key);
    }
    final file = io.File(cacheObject.relativePath);

    if (file.existsSync()) {
      try {
        await file.delete();
        // ignore: unused_catch_clause
      } on PathNotFoundException catch (e) {
        // File has already been deleted. Do nothing #184
      }
    }

cacheObject.relativePath is not real abs file path ,delete file can not work
class CustomCacheManager { static const key = 'tsl'; static final fileSystem = ExternalStorageFileSystem(key); static CacheManager instance = CacheManager( Config(key, stalePeriod: const Duration(days: 30), maxNrOfCacheObjects: 999999999, fileSystem: fileSystem), ); }
`
class ExternalStorageFileSystem implements FileSystem {
final Future _fileDir;
final String _cacheKey;

ExternalStorageFileSystem(this._cacheKey)
: _fileDir = createDirectory(_cacheKey);

static Future createDirectory(String key) async {
final baseDir = await getExternalStorageDirectory();
final path = p.join(baseDir!.path, key);

const fs = LocalFileSystem();
final directory = fs.directory(path);
await directory.create(recursive: true);
return directory;

}

@override
Future createFile(String name) async {
final directory = await _fileDir;
if (!(await directory.exists())) {
await createDirectory(_cacheKey);
}
return directory.childFile(name);
}

void emptyCache() async {
final directory = await _fileDir;
if (await directory.exists()) {
await directory.delete(recursive: true);
}
}
}
`