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
endBy 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
endMoreover, 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
endBut 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
endUnfortunately 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
endPrepending 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 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)
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 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
#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 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