# encoding: utf-8

module Hexx

  # Contains methods to declare and inject module dependencies.
  #
  # Allows to provide DJ framework for the gem.
  #
  # @example
  #   # Declare the dependencies for the gem root module
  #
  #   # lib/my_gem.rb
  #   module MyGem
  #     extend Hexx::Dependencies
  #     self.depends_on :get_item, :add_item, :delete_item
  #   end
  #
  #   # Inject dependencies (as a constant or its name)
  #
  #   # config/my_gem.rb
  #   MyGem.configure do |config|
  #     config.get_item = AnotherGem::Services::Get
  #     config.add_item = "AnotherGem::Services::Add"
  #   end
  #
  #   # Use dependency in the module code (models, services, etc.)
  #   # When the dependency hasn't been set, fails with +NotImplementedError+
  #
  #   MyGem.get_item     # => AnotherGem::Services::Get
  #   MyGem.add_item     # => AnotherGem::Services::Add
  #   MyGem.delete_item  # raises #<NotImplementedError ... >
  module Dependencies

    # Yields a block and gives it self
    #
    # @example
    #   module MyGem
    #     extend Hexx::Dependencies
    #   end
    #
    #   MyGem.configure do |config|
    #     c # => MyGem
    #   end
    #
    # @yield The block.
    # @yieldparam [Dependencies] +self+.
    def configure
      yield(self)
    end

    # Declares dependency getter and setter.
    #
    # The getter constants, strings and symbols. The setter constantize the
    # assigned value.
    #
    # This allows setting dependency before definition of a corresponding
    # constant.
    #
    # @example
    #   module MyGem
    #     extend Hexx::Dependencies
    #     self.depends_on :some_class
    #   end
    #
    #   MyGem.some_class = String
    #   # => "String"
    #   MyGem.some_class # => String
    #
    #   MyGem.some_class = "String"
    #   # => "String"
    #   MyGem.some_class # => String
    #
    #   MyGem.some_class = :String
    #   # => "String"
    #   MyGem.some_class # => String
    #
    # @param  [String, Symbol] name The name for dependency.
    def depends_on(*names)
      names.each do |name|
        DependencyName.new(name).validate
        add_dependency_setter name
        add_dependency_getter name
      end
    end

    private

    # @api hide
    # Checks the name of the dependency.
    class DependencyName < Struct.new(:name)

      # @api hide
      # Runs validations and fails in case of any error.
      def validate
        check_type
        check_value
      end

      private

      def check_type
        return if [String, Symbol].include? name.class
        fail TypeError.new "#{ name.inspect } is neither a string nor symbol."
      end

      def check_value
        return unless name.to_s == ""
        fail ArgumentError.new "The dependency name should not be blank."
      end
    end

    # @api hide
    # Checks the value of the dependency.
    class Dependency < Struct.new(:name, :value)

      # @api hide
      # Runs validations and fails in case of any error.
      def validate
        check_type
        check_value
      end

      private

      def check_type
        return if value.is_a? String
        return if value.is_a? Symbol
        return if value.is_a? Module
        fail TypeError.new "#{ inspect } is not a string, symbol or module."
      end

      def check_value
        return unless value.to_s == ""
        fail ArgumentError.new "The dependency #{ name } should not be blank."
      end

      def inspect
        value.inspect
      end
    end

    def add_dependency_setter(name)
      instance_eval "
        def #{ name }=(value);
          Dependency.new(:#{ name }, value).validate;
          @#{ name }= value.to_s;
        end"
    end

    def add_dependency_getter(name)
      instance_eval "
        def #{ name };
          value = @#{ name };
          fail NotImplementedError unless value;
          value.constantize;
        end"
    end
  end
end