如何将另一个模块的类方法导入我的ruby类/模块？

Question

我知道我可以导入instance_methods，但是可以导入类方法吗？如何导入？

Answer 1

A common idiom is this:

module Bar
  # object model hook. It's called when module is included. 
  # Use it to also bring class methods in by calling `extend`.
  def self.included base
    base.send :include, InstanceMethods
    base.extend ClassMethods
  end

  module InstanceMethods
    def hello
      "hello from instance method"
    end
  end

  module ClassMethods
    def hello
      "hello from class method"
    end
  end
end

class Foo
  include Bar
end

Foo.hello # => "hello from class method"
Foo.new.hello # => "hello from instance method"

What's with that InstanceMethods module?

When I need module to include both instance and class methods to my class, I use two submodules. This way the methods are neatly grouped and, for example, can be easily collapsed in code editor.

It also feels more "uniform": both kinds of methods are injected from self.included hook.

Anyway, it's a matter of personal preference. This code works exactly the same way:

module Bar
  def self.included base
    base.extend ClassMethods
  end

  def hello
    "hello from instance method"
  end

  module ClassMethods
    def hello
      "hello from class method"
    end
  end
end

Answer 2

The short answer is: no, you cannot cause methods of the module object itself ("class" methods of the module) to be in the inheritance chain for another object. @Sergio's answer is a common workaround (by defining the "class" methods to be part of another module).

You may find the following diagram instructive (click for full-size or get the PDF ):

_{(source: phrogz.net )}

_{Note: this diagram has not yet been updated for Ruby 1.9, where there are additional core objects like BasicObject that slightly change the root flow.}