module ActiveSupport::Concern - activesupport-4.2.4 Documentation

# module ActiveSupport::Concern

A typical module looks like this:

    module M
      def self.included(base)
        base.extend ClassMethods
        base.class_eval do
          scope :disabled, -> { where(disabled: true) }
        end
      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

      class_methods do
        ...
      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
        def self.method_injected_by_foo
          ...
        end
      end
    end

    module Bar
      extend ActiveSupport::Concern
      include Foo

      included do
        self.method_injected_by_foo
      end
    end

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

### Public Instance Methods

append_features(base) click to toggle source
Calls superclass method

    # File lib/active_support/concern.rb, line 111
    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)
        base.class_eval(&@_included_block) if instance_variable_defined?(:@_included_block)
      end
    end

class_methods(&class_methods_module_definition) click to toggle source

    # File lib/active_support/concern.rb, line 134
    def class_methods(&class_methods_module_definition)
      mod = const_defined?(:ClassMethods, false) ?
        const_get(:ClassMethods) :
        const_set(:ClassMethods, Module.new)

      mod.module_eval(&class_methods_module_definition)
    end

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

    # File lib/active_support/concern.rb, line 124
    def included(base = nil, &block)
      if base.nil?
        raise MultipleIncludedBlocks if instance_variable_defined?(:@_included_block)

        @_included_block = block
      else
        super
      end
    end