Module: ActiveSupport::Concern

Included in:
ActionableError, Callbacks, Configurable, LoggerSilence, LoggerThreadSafeLevel, Rescuable, Testing::ConstantLookup, Testing::FileFixtures
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

Prepending concerns

Just like include, concerns also support prepend with a corresponding prepended do callback. module ClassMethods or class_methods do are prepended as well.

prepend is also used for any dependencies.

Defined Under Namespace

Classes: MultipleIncludedBlocks, MultiplePrependBlocks

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.extended(base) ⇒ Object

:nodoc:



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# File 'lib/active_support/concern.rb', line 123

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

Instance Method Details

#append_features(base) ⇒ Object

:nodoc:



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# File 'lib/active_support/concern.rb', line 127

def append_features(base) # :nodoc:
  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

Define class methods from given block. You can define private class methods as well.

module Example
extend ActiveSupport::Concern

class_methods do
  def foo; puts 'foo'; end

  private
    def bar; puts 'bar'; end
end
end

class Buzz
include Example
end

Buzz.foo # => "foo"
Buzz.bar # => private method 'bar' called for Buzz:Class(NoMethodError)


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# File 'lib/active_support/concern.rb', line 207

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

Evaluate given block in context of base class, so that you can write class macros here. When you define more than one included block, it raises an exception.



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# File 'lib/active_support/concern.rb', line 156

def included(base = nil, &block)
  if base.nil?
    if instance_variable_defined?(:@_included_block)
      if @_included_block.source_location != block.source_location
        raise MultipleIncludedBlocks
      end
    else
      @_included_block = block
    end
  else
    super
  end
end

#prepend_features(base) ⇒ Object

:nodoc:



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# File 'lib/active_support/concern.rb', line 140

def prepend_features(base) # :nodoc:
  if base.instance_variable_defined?(:@_dependencies)
    base.instance_variable_get(:@_dependencies).unshift self
    false
  else
    return false if base < self
    @_dependencies.each { |dep| base.prepend(dep) }
    super
    base.singleton_class.prepend const_get(:ClassMethods) if const_defined?(:ClassMethods)
    base.class_eval(&@_prepended_block) if instance_variable_defined?(:@_prepended_block)
  end
end

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

Evaluate given block in context of base class, so that you can write class macros here. When you define more than one prepended block, it raises an exception.



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# File 'lib/active_support/concern.rb', line 173

def prepended(base = nil, &block)
  if base.nil?
    if instance_variable_defined?(:@_prepended_block)
      if @_prepended_block.source_location != block.source_location
        raise MultiplePrependBlocks
      end
    else
      @_prepended_block = block
    end
  else
    super
  end
end