Module: ActiveSupport::Concern
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 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 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.
Instance Method Summary
-
#class_methods(&class_methods_module_definition)
Define class methods from given block.
-
#included(base = nil, &block)
Evaluate given block in context of base class, so that you can write class macros here.
-
#prepended(base = nil, &block)
Evaluate given block in context of base class, so that you can write class macros here.
Instance Method Details
#class_methods(&class_methods_module_definition)
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 ; puts 'bar'; end
end
end
class Buzz
include Example
end
Buzz.foo # => "foo"
Buzz. # => private method 'bar' called for Buzz:Class(NoMethodError)
# File 'activesupport/lib/active_support/concern.rb', line 209
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)
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.
# File 'activesupport/lib/active_support/concern.rb', line 158
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
#prepended(base = nil, &block)
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.
# File 'activesupport/lib/active_support/concern.rb', line 175
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