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[self]

  class << self
    def initialize
      @old_definitions = {}
      @hot_definitions = {}

      set_definitions
      load_values
    end

    def set_definitions
      LOGGER.info('--set_definitions')

      new_definitions = Dotenv.parse('.env.sample')

      definitions_to_actualize = {}
      @old_definitions.merge(new_definitions) do |key, old_value, new_value|
        definitions_to_actualize[key] = new_value if new_value != old_value
      end
      definitions_to_actualize.each { |key, value| define_method(:actualize, key, value) }

      @old_definitions = new_definitions

      definitions_to_remove = @hot_definitions.except(*new_definitions.keys)
      definitions_to_remove.each_pair do |key, dkey|
        LOGGER.info("remove method <#{dkey}>")
        singleton_class.undef_method(dkey)
        @hot_definitions.delete(key)
      end

      definitions_to_add = new_definitions.except(*@hot_definitions.keys)
      definitions_to_add.each { |key, value| define_method(:add, key, value) }
    end

    def load_values
      LOGGER.info('--load_values')

      new_env = Dotenv.parse
      constants_to_delete = HOTENV.except(*new_env.keys)
      constants_to_delete.each do |constant|
        LOGGER.info("constant <#{constant}> reverts to default value <#{}>")
      end

      constants_to_add = new_env.except(*HOTENV.keys)
      constants_to_add.each do |constant|
        LOGGER.info("constant to add <#{constant}>")
      end

      HOTENV.replace new_env
      # LISTENERS.each_pair { |k, b| perform_change(k, b) }
    end

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

    private

    def define_method(mode, key, value)
      dkey = key.downcase
      method = infer_method_from_value(value)

      inferred_type = method.name.to_s.split('_')[1]
      LOGGER.info("#{mode} method <#{dkey}> of type <#{inferred_type}> with default value <#{value}> ")

      singleton_class.define_method(dkey) { method.call(key, value) }
      @hot_definitions.store(key, dkey)
    end

    def perform_change(key, block)
      old_value = nil # TODO: remember last previous value
      new_value = method(key).call
      block.call(new_value, old_value)
    end

    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 infer_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

  initialize # done once on first require, cause this is just a module (not a class!)
end