Question

module MyModule
    def self.my_method
        @instance_variable ||= "some string"
    end
end

考虑另一个包含OtherModule的模块MyModule。当OtherModule多次调用my_method的代码@instance_variable多次实例化时？或者只是一次？

Answer 1

更新回答：

如果委托模块调用该方法，则会出错。
如果原始模块调用该方法，则该类实例变量仅实例化1次。

如果要在模块中定义“类方法”，则无法定义self.xx，只需扩展/包含它即可。你应该“扩展”它，或者“把它包括在本征类中”，例如

module SomeModule
  def self.aim_to_be_class_method ;    end;  
  def aim_to_be_instance_method ;    end;
end

# class methods:  []
# instance methods: ["aim_to_be_instance_method"]
class SomeClassIncludingTheModule
  include SomeModule  
end

# class methods:  ["aim_to_be_instance_method"]
# instance methods: []
class SomeClassExtendingTheModule
  extend SomeModule   
end

# class methods:  ["aim_to_be_instance_method"]
# instance methods: []
class SomeClassMadeEigenClassIncludingModule
  class << self
    include SomeModule
  end
end

您的代码是“类实例变量”的示例。根据该书＆lt;＆lt;元编程ruby＆gt;＆gt;，第127页，您可以将“类实例变量”视为Java的静态字段。所以，我认为大多数情况下，它应该运行一次。

有关详细信息，请编写单元测试，看看会发生什么？我会在一段时间后用我自己的单元测试代码更新我的答案。

更新：我的单元测试和结果：

# all of the code is placed in a file: class_instance_variable_test.rb

# define a class so that we see its initialize process
class Apple
  def initialize
    puts "Apple initialized~"
  end 
end

# define an original_module that calls Apple.new
module OriginalModule
  def self.original_method
    @var ||= Apple.new
  end 
end

# define another module to call the original_module
module DelegateModule
  include OriginalModule
end

# now the test begins
require 'test/unit'
class ClassInstanceTest < Test::Unit::TestCase

  # output of this test case: 
  # NoMethodError: undefined method `original_method' for DelegateModule:Module      
  def test_if_delegate_module_could_call_original_method_by_including_original_module
    DelegateModule.original_method
  end

  # output of this test case: 
  # ----- calling from original_method, 3 times called, how many times initialized?
  # Apple initialized~
  # ------ ends
  def test_if_we_could_call_original_method_from_original_module
    puts "----- calling from original_method, 3 times called, how many times initialized? "
    OriginalModule.original_method
    OriginalModule.original_method
    OriginalModule.original_method
    puts "------ ends "
  end

end

rails lib目录中的模块

1 个答案: