模块是方法和常量的集合，模块和类一样，其中的可以包括两种方法：实例方法(Instance Method)、模块方法(Module Method)。
当一个类include(Mixin)一个模块时，模块中的实例方法会成为该类的实例方法，expand(Mixin)时,模块中的实例方法会成为该类的类方法。两种情况下模块方法都会被忽略。
同时实现类方法和实例方法的混入看起来是鱼和熊掌的问题，要放弃吗？显然是不可能的，我们都是贪婪的。
一个解决办法
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module ClassMethods
end
module InstanceMethods
end
class Foo
extend ClassMethods
include InstanceMethods
end
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看起来可以正常工作，但是不那么优雅，粒度太大。
聪明的程序员总是能想到解决办法
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module Mod
def self.included(base)
base.extend(ClassMothods)
end
module ClassMethods
# 类方法定义
end
#实例方法定义
end
class M
include Mod
end
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嗯，看起来不错，比第一种方法优雅多了。但是用着用着问题就来
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module Foo
def self.included(base)
base.extend ClassMethods
end
module ClassMethods
def say
puts 'say'
end
end
end
module Bar
include Foo
def self.included(base)
base.say
end
end
class Host
include Bar
end
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上面代码会抛出NoMethodError: undefined method 'say' for Host:Class异常
原因在于include Bar的时候Foo中included(base)被执行,此时base是Bar
一个解决办法
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module Foo
def self.included(base)
base.extend ClassMethods
end
module ClassMethods
def say
puts 'say'
end
end
end
module Bar
def self.included(base)
base.say
end
end
class Host
include Foo
include Bar
end
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问题虽然解决的，但是引入了一个新的问题，所有使用Bar的地方都要知道它依赖Foo,需要分出额外的精力来维护这种依赖关系，我们应该把他们的依赖关系隐藏起来。
看来我们还是太挑剔
ActiveSupport::Concern
本文重点来了
为了解决这种依赖关系，rails中增加了ActiveSupport::Concern(源码)这个工具。
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module ActiveSupport
# 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 <tt>ActiveSupport::Concern</tt> 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 <tt>base</tt>
# is the +Bar+ module, not the +Host+ class. With <tt>ActiveSupport::Concern</tt>,
# 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 # works, Bar takes care now of its dependencies
# end
module Concern
def self.extended(base) #:nodoc:
base.instance_variable_set("@_dependencies", [])
end
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")
base.class_eval(&@_included_block) if instance_variable_defined?("@_included_block")
end
end
def included(base = nil, &block)
if base.nil?
@_included_block = block
else
super
end
end
end
end
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实现代码相当简短，使用也非常简单
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module M
extend ActiveSupport::Concern
included do
self.send(:do_host_something)
end
module ClassMethods
def wo
# do something
end
end
module InstanceMethods
def ni
# do something
end
end
end
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终极版
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require 'active_support/concern'
module Foo
extend ActiveSupport::Concern
module ClassMethods
def say
puts 'say'
end
end
end
module Bar
extend ActiveSupport::Concern
include Foo
included do
self.say
end
end
class Host
include Bar
end
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洗洗睡觉