ruby-on-rails - 如何在子类中包含行为？

Question

我担心 included block 。在此示例中，它设置了一个类实例变量。

require 'active_support/concern'

module Mod1
  extend ActiveSupport::Concern

  included do
    p "#{self}: included Mod1"
    @foo = "foo is set"
  end

  class_methods do
    def foo
      @foo
    end
  end
end

如果我包含它，我将获得方法 foo 并设置包含的变量。

class Parent
  include Mod1
end

p Parent.foo # "Parent: included Mod1" "foo is set"

但是如果我从 Parent 派生子类，我继承方法 foo，但包含的 block 不会运行。

class Subclass < Parent
end

p Subclass.foo # nil

即使我包含模块，包含的 block 也不会运行。我期望是因为 Subclass.include?(Mod1) 是真的。

class Subclass < Parent
  include Mod1
end

p Subclass.foo # nil
p Subclass.include?(Mod1) # true

如何编写关注点，使其 included block 甚至在子类上运行？

Answer 1

ActiveSupport::Concern#included实际上只是 Module#included 的语法糖所以它只会在模块包含在父类中时触发。您要找的 Hook 是Class#inherited每当创建当前类的子类时都会调用它。

require 'active_support/concern'

module Mod1
  extend ActiveSupport::Concern

  included do
    set_foo!
  end

  class_methods do
    def foo
      @foo
    end

    def set_foo!
      p "Setting foo"
      @foo = "foo is set"
    end

    def inherited(child_class)
      puts "inherited!"
      child_class.set_foo!
    end
  end
end

class Parent
  include Mod1
end

class Subclass < Parent; end

require 'minitest/autorun'
class Mod1Test < Minitest::Test
  def test_class_ivar_set_in_subclass
    assert_equal("foo is set", Subclass.foo) # passes
  end
end

"Setting foo"
inherited!
"Setting foo"
Run options: --seed 35149

# Running:

.

Finished in 0.000999s, 1000.7185 runs/s, 1000.7185 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips