public function add(array $assets)
    {
        if (! file_exists($this->file)) {
            throw new RuntimeException("Could not add assets, `app.json` file do not exists.");
        }

        $packages = json_decode(file_get_contents($this->file), true);

        $packages['assets'] = $assets + $packages['assets'];

        ksort($packages['assets']);

        file_put_contents($this->file, json_encode($packages, JSON_UNESCAPED_SLASHES | JSON_PRETTY_PRINT).PHP_EOL);
    }