# module ActiveSupport::Concern

A typical module looks like this:

    module M
      def self.included(base)
        base.extend ClassMethods
        scope :disabled, where(:disabled => true)
      end

      module ClassMethods
        ...
      end
    end

By using `ActiveSupport::Concern` the above module could instead be written as:

    require 'active_support/concern'

    module M
      extend ActiveSupport::Concern

      included do
        scope :disabled, where(:disabled => true)
      end

      module ClassMethods
        ...
      end
    end

Moreover, it gracefully handles module dependencies. Given a `Foo` module and a `Bar` module which depends on the former, we would typically write the following:

    module Foo
      def self.included(base)
        base.class_eval do
          def self.method_injected_by_foo
            ...
          end
        end
      end
    end

    module Bar
      def self.included(base)
        base.method_injected_by_foo
      end
    end

    class Host
      include Foo # We need to include this dependency for Bar
      include Bar # Bar is the module that Host really needs
    end

But why should `Host` care about `Bar`‘s dependencies, namely `Foo`? We could try to hide these from `Host` directly including `Foo` in `Bar`:

    module Bar
      include Foo
      def self.included(base)
        base.method_injected_by_foo
      end
    end

    class Host
      include Bar
    end

Unfortunately this won’t work, since when `Foo` is included, its `base` is the `Bar` module, not the `Host` class. With `ActiveSupport::Concern`, module dependencies are properly resolved:

    require 'active_support/concern'

    module Foo
      extend ActiveSupport::Concern
      included do
        class_eval do
          def self.method_injected_by_foo
            ...
          end
        end
      end
    end

    module Bar
      extend ActiveSupport::Concern
      include Foo

      included do
        self.method_injected_by_foo
      end
    end

    class Host
      include Bar # works, Bar takes care now of its dependencies
    end

### Public Class Methods

extended(base) click to toggle source

    # File lib/active_support/concern.rb, line 101
    def self.extended(base)
      base.instance_variable_set("@_dependencies", [])
    end

### Public Instance Methods

append_features(base) click to toggle source

    # File lib/active_support/concern.rb, line 105
    def append_features(base)
      if base.instance_variable_defined?("@_dependencies")
        base.instance_variable_get("@_dependencies") << self
        return false
      else
        return false if base < self
        @_dependencies.each { |dep| base.send(:include, dep) }
        super
        base.extend const_get("ClassMethods") if const_defined?("ClassMethods")
        if const_defined?("InstanceMethods")
          base.send :include, const_get("InstanceMethods")
          ActiveSupport::Deprecation.warn "The InstanceMethods module inside ActiveSupport::Concern will be "              "no longer included automatically. Please define instance methods directly in #{self} instead.", caller
        end
        base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block")
      end
    end

included(base = nil, &block) click to toggle source

    # File lib/active_support/concern.rb, line 123
    def included(base = nil, &block)
      if base.nil?
        @_included_block = block
      else
        super
      end
    end