ioc_container.rb

# Simple thread-safe IoC container
# Example
#
# DEPS = Container.new
# # db singleton is lazily initialized if needed
# DEPS.register(:db) do
#   require 'sequel'
#   Sequel.connect ENV.fetch('DATABASE_URL')
# end
# database-backed repo depends on :db
# DEPS.register(:db_repo) do
#   DBRepo.new(DEPS.get(:db))
# end
# in-memory repo does not depend on :db
# DEPS.register(:mem_repo) do
#   MemRepo.new
# end
# # abstract repo will initialize either :db_repo or :mem_repo
# # depending on configuration
# DEPS.register(:repo) do
#   DEPS.get(ENV.fetch(:repo_type, :mem_repo).to_sym)
# end
#
# # App will :get what it needs from DEPS
# # only used dependencies will be initialized
# app = SomeApp.new(repo: DEPS.get(:repo))
class Container
  def initialize
    @definitions = {}
    @cache = {}
    @mutex = Mutex.new
    @locked = false
  end

  def register(name, &block)
    @definitions[name] = block
  end

  def get(name)
    with_lock do
      if v = @cache[name]
        return v
      else
        d = @definitions[name]
        return nil unless d
        @cache[name] = d.call
      end
    end
  end

  private
  def with_lock(&block)
    if @locked
      yield
    else
      @mutex.synchronize {
        @locked = true
        result = yield
        @locked = false
        result
      }
    end
  end
end