Module: ActiveSupport::Concern

Included in:
Callbacks, Configurable, LoggerThreadSafeLevel, Rescuable, Testing::ConstantLookup, Testing::FileFixtures, Testing::SetupAndTeardown, LoggerSilence
Defined in:
lib/active_support/concern.rb

Overview

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

Defined Under Namespace

Classes: MultipleIncludedBlocks

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.extended(base) ⇒ Object

:nodoc:



109
110
111
 
# File 'lib/active_support/concern.rb', line 109

def self.extended(base) #:nodoc:
  base.instance_variable_set(:@_dependencies, [])
end

Instance Method Details

#append_features(base) ⇒ Object 



113
114
115
116
117
118
119
120
121
122
123
124
 
# File 'lib/active_support/concern.rb', line 113

def append_features(base)
  if base.instance_variable_defined?(:@_dependencies)
    base.instance_variable_get(:@_dependencies) << self
    false
  else
    return false if base < self
    @_dependencies.each { |dep| base.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) ⇒ Object 



136
137
138
139
140
141
142
 
# File 'lib/active_support/concern.rb', line 136

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) ⇒ Object 



126
127
128
129
130
131
132
133
134
 
# File 'lib/active_support/concern.rb', line 126

def included(base = nil, &block)
  if base.nil?
    raise MultipleIncludedBlocks if instance_variable_defined?(:@_included_block)

    @_included_block = block
  else
    super
  end
end