[技巧]Include与Extend的比较

2gua

你可以用include或extend来MixIn（混入）module的方法到class。
它俩的不同在于：

    include使得module的方法被类实例使用；
    extend使得module的方法被类本身使用。

举例：

module Greetings
  def say_hello
    puts "Hello!"
  end
end

class Human
  include Greetings
end

Human.new.say_hello # => "Hello!"
Human.say_hello     # NoMethodError

class Robot
  extend Greetings
end

Robot.new.say_hello # NoMethodError
Robot.say_hello     # => "Hello!"

如果你想了解更多关于include和extend比较的信息，推荐你看看以下资源：

• What is the difference between include and extend in Ruby? on Stack Overflow.
• Include vs Extend in Ruby by John Nunemaker
• Ruby extend and include by Jay Fields