Create module variables in Ruby

问题:

Is there any way to create a variable in a module in Ruby that would behave similar to a class variable? What I mean by this is that it would be able to be accessed without initializing an instance of the module, but it can be changed (unlike constants in modules).

回答1:

Ruby natively supports class variables in modules, so you can use class variables directly, and not some proxy or pseudo-class-variables:

module T
  @@foo = 'bar'

  def self.set(x)
    @@foo = x
  end

  def self.get
    @@foo
  end
end

p T.get         #=> 'bar'
T.set('fubar')
p T::get        #=> 'fubar'

回答2:

If you do not need to call it from within an instance, you can simply use an instance variable within the module body.

module SomeModule
  module_function
  def param; @param end
  def param= v; @param = v end
end

SomeModule.param
# => nil
SomeModule.param = 1
SomeModule.param
# => 1

The instance variable @param will then belong to the module SomeModule, which is an instance of the Module class.

回答3:

you can set a class instance variable in the module.

module MyModule
   class << self; attr_accessor :var; end
end

MyModule.var = 'this is saved at @var'

MyModule.var    
=> "this is saved at @var"

回答4:

You can also initialize value within module definition:

module MyModule
  class << self
    attr_accessor :my_variable
  end
  self.my_variable = 2 + 2
end

p MyModule.my_variable