A typical module looks like this:
module M def self.included(base) base.extend ClassMethods scope :disabled, where(:disabled => true) 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 module ClassMethods ... 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
Methods
- A
- E
- I
Class Public methods
Instance Public methods
# File activesupport/lib/active_support/concern.rb, line 105 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") if const_defined?("InstanceMethods") base.send :include, const_get("InstanceMethods") ActiveSupport::Deprecation.warn "The InstanceMethods module inside ActiveSupport::Concern will be " "no longer included automatically. Please define instance methods directly in #{self} instead.", caller end base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block") end end