easy-going-rails/lib/hot_constants/hot_constants.rb

require 'dotenv'

# Hot Live Constants
module HotConstants
  HOTENV = {} # rubocop:disable Style/MutableConstant
  LISTENERS = {} # rubocop:disable Style/MutableConstant
  LOGGER = SemanticLogger[to_s]

  class << self
    def initialize
      env_sample = Dotenv.parse('.env.sample') # TODO: able to detect add/remove, then either call define_method or undef_method!
      env_sample.each do |key, value|
        dkey = key.downcase
        method = method_from_value(value)
        singleton_class.define_method(dkey) { method.call(key, value) }
        LOGGER.info("create method <#{dkey}> performing <#{method.name}> with default value <#{value}> ")
      end
      reload!
    end

    def reload!
      HOTENV.replace Dotenv.parse # TODO: detect true changes before processing block below (or yield)
      LISTENERS.each_pair do |key, block|
        old_value = nil # TODO: remember last previous value
        new_value = method(key).call
        block.call(new_value, old_value)
      end
    end

    def on_change(key, &block)
      LISTENERS[key.downcase.to_sym] = block
    end

    private

    def load_boolean(key, default) = HOTENV.fetch(key, default).to_s.downcase == 'true'
    def load_integer(key, default) = HOTENV.fetch(key, default).to_i
    def load_string(key, default) = HOTENV.fetch(key, default)

    def method_from_value(value)
      case value.downcase
      when /^(true|false)$/ then method(:load_boolean)
      when /^\d+$/ then method(:load_integer)
      else method(:load_string)
      end
    end
  end
end