Best-of-both-worlds example (DI + hard-coded constants) from this article: https://www.rubypigeon.com/posts/dependency_injection_containers_vs_hardcoded_constants/

hybrid-example.rb

# Usage:
# - To load default dependencies: `register_user = RegisterUser.build`
# - To inject own dependencies (for testing or reuse):
#   ```rb
#   register_admin = RegisterUser.new(validator: AdminValidator.new, repo: AdminRepository.new)
#   ```
class RegisterUser
  attr_reader :validator, :repo

  def self.build
    new(
      validator: UserValidator.new,
      repo: UserRepository.new,
    )
  end

  def initialize(validator:, repo:)
    @validator = validator
    @repo = repo
  end

  def call(params)
    if validator.validate(params)
      repo.save(params)
    else
      nil
    end
  end
end