Module: ActiveSupport::Concern

Included in:
Callbacks, Configurable, Rescuable, Testing::Performance, Testing::SetupAndTeardown
Defined in:
lib/active_support/concern.rb

Overview

A typical module looks like this:

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

module ClassMethods
  ...
end

module InstanceMethods
  ...
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

module InstanceMethods
  ...
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

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.extended(base) ⇒ Object 



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# File 'lib/active_support/concern.rb', line 108

def self.extended(base)
  base.instance_variable_set("@_dependencies", [])
end

Instance Method Details

#append_features(base) ⇒ Object 



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# File 'lib/active_support/concern.rb', line 112

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.send :include, const_get("InstanceMethods") if const_defined?("InstanceMethods")
    base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block")
  end
end

#included(base = nil, &block) ⇒ Object 



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# File 'lib/active_support/concern.rb', line 126

def included(base = nil, &block)
  if base.nil?
    @_included_block = block
  else
    super
  end
end