为什么不使用`@ messages`？

Question

以下内容来自The Rspec Book.我很奇怪,为什么messages << message在def puts不是@messages << message.我知道这||= []是memoization,我也理解这@messages是一个实例变量.那么,它应该在同一个类中使用吗？或者这是一个错字？

class Output
  def messages
    @messages ||= []
  end

  def puts(message)
    messages << message
  end
end

def output
  @output ||= Output.new
end

Given /^I am not yet playing$/ do
end

When /^I start a new game$/ do
  game = Codebreaker::Game.new(output)
  game.start
end

Then /^I should see "([^"]*)"$/ do |message|
  output.messages.should include(message)
end

Answer 1

因为

def puts(message)
  messages << message
end

然后打电话

def messages
  @messages ||= []
end

的||= []受让人空数组到@messages如果不存在它.这就是为什么puts(message)不@messages直接使用.

编辑:如果你@messages << message直接做@messages,那时可能还不存在.这就是你经历的原因messages.

Answer 2

我认为你的混淆在@messages变量和messages方法之间

class Output
  def messages
    @messages ||= []
  end

  def puts(message)
    messages << message #=> here `messages` refer to the method above
  end
end

这是一个简单的例子

class Output
  def m
    @messages ||= []
  end

  def puts(message)
    m << message #=> method `m`
  end
end

v = Output.new
v.puts "blah"
v.m #=> ["blah"]

示例2(为了更加清晰)

class Output
  def m
    @messages ||= []
  end

  def puts(message)
    m << message #=> here `m` refer to the method above
  end

  def show_at_messages_variable
    @messages
  end
end

v = Output.new
v.puts "blah"
v.show_at_messages_variable #=> ["blah"]

Answer 3

作者选择了惰性对象创建,消息不是在构造函数中创建的,而是在getter方法中创建的.只要您不直接访问@messages,就无法获得零例外.

class Output
  def messages
    @messages ||= []
  end

  def puts(message)
    # If i was to use the variable @messages at this point,
    # i would get a nil exception, the array was never allocated.
    # Using the getter ensures that the array is allocated.
    messages << message
  end
end

如果你有一个明显的不需要懒惰创建对象,你可以用以下方式编写同一个类.

class Output
  attr_reader :messages

  def initialize(message)
    @messages = []
  end

  def puts(message)
    # messages was created when this object was initialized,
    # thus it is perfectly safe to use here.
    @messages << message
  end
end