Drupal - Get the public path directory

According to realpath API, you can Get it with :

\Drupal::service('file_system')->realpath(file_default_scheme() . "://");

In case you want to get just "sites/default/files" or "sites/your-multisite/files" Then you should use

// Add to the top of the file.
use Drupal\Core\StreamWrapper\PublicStream;

// In your method.
print PublicStream::basePath();