Ruby included Again

Ruby 的 Module#included 和 Module#append_features 肯定不陌生了, 再画一下重点.

当 C1 类 include M1 模块, 期间会调用 M1.append_features 这个私有方法; 之后, 会调用 M1.included 这个回调方法.

这句话有几个重点:

included 和 append_features 方法都是 M1 上的方法, 是 M1 的模块方法, 不是实例方法 .
included 是一个回调方法. 作用是锦上添花, 内部默认留空没有逻辑.
append_features 是一个私有方法, 在 include 的过程中自动调用, 其内部实现 include 的功能. 如果重载该方法一定记得 super, 否则 include 功能会失效.

module M1
  def included(base)
    p "included(base), base:#{base}, self:#{self}"
  end

  def self.included(base)
    p "self.included(base), base:#{base}, self:#{self}"
  end

  def self.append_features(base)
    p "append_features(base), base:#{base}, self:#{self}"

    super
  end

  def hi
    p 'Hi, this is M1 .'
  end

  def self.hello
    p 'Hello, this is M1 .'
  end
end

普通的 include

class C1
  include M1
end

begin
  obj1 = C1.new
  obj1.hi
  obj1.hello
rescue NoMethodError => e
  p e.to_s
ensure
  p '---'
end

=begin
"append_features(base), base:C1, self:M1"
"self.included(base), base:C1, self:M1"
"Hi, this is M1 ."
"undefined method `hello' for #<C1:0x00007ff8f4948c58>"

=end

调用的是 M1.included 和 M1.append_features, 没有 M1#included .

级联的 include

module M2
  include M1

  def included(base)
    p "included(base), base:#{base}, self:#{self}"
  end

  def self.included(base)
    p "self.included(base), base:#{base}, self:#{self}"
  end

  def self.append_features(base)
    p "append_features(base), base:#{base}, self:#{self}"

    super
  end
end

class C2
  include M2
end

begin
  obj2 = C2.new
  obj2.hi
  obj2.hello
rescue NoMethodError => e
  p e.to_s
ensure
  p '---'
end

=begin
"append_features(base), base:M2, self:M1"
"self.included(base), base:M2, self:M1"
"append_features(base), base:C2, self:M2"
"self.included(base), base:C2, self:M2"
"Hi, this is M1 ."
"undefined method `hello' for #<C2:0x00007ffe98014e30>"

=end

先处理 M2 include M1, 调用 M1.included 和 M1.append_features, 没有 M1#included, self 为 M1, base 为 M2 .

再处理 C2 include M2, 调用 M2.included 和 M2.append_features, 没有 M2#included, self 为 M2, base 为 C2 .

调用包含 extend 的 include

module M3
  extend M1
end

class C3
  include M3
end

begin
  obj3 = C3.new
  obj3.hi
  obj3.hello
rescue NoMethodError => e
  p e.to_s
ensure
  p '---'
end

=begin
"included(base), base:C3, self:M3"
"undefined method `hi' for #<C3:0x00007ff8f492c530>"
=end

M1#included 方法被 extend 到 M3 上, M3 就拥有了 M3.included .

C3 include 了 M3, 触发 M3.included 回调方法, 对于该 included 方法, self 为 M3, base 为 C3 .

更复杂的综合使用参看 ActiveSupport::Concern .