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Ruby - 用 Instance_eval & Class_eval 自己加 Method!

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有時候會發現有些 object，沒有設定 setter & getter ，剛好又需要用到，這時就可以派上用場拉。

instance_eval & class_eval

instance_eval Use ClassName.instance_eval to define a class method (one associated with the class object but not visible to instances).

class_eval Use ClassName.class_eval to define an instance method (one that applies to all of the instances of ClassName).

簡單的來說，就是自己定義 class or instance 的 method

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class MyClass
  def initialize(num)
    @num = num
  end
end

a = MyClass.new(1)
b = MyClass.new(2)

錯誤，因為沒有 getter or setter methods

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a.num
#=>NoMethodError: undefined method `num' for #<MyClass:0x007fba5c02c858 @num="1">

自行加 instance method

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a.instance_eval { @num }
#=> 1

也可以直接定義 method，這樣就可以一直重複使用

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a.instance_eval do
   def num
     @num
   end
end

a.num
#=> 1
b.num
#NoMethodError: undefined method `num' for #<MyClass:0x007fba5c08e5f8 @num="2">

上述指定義了 a 的 instance method 所以 b 會錯誤。 因此可以直接定義在 class instance

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MyClass.class_eval do
  def num
      @num
  end
end

a.num
#=> 1
b.num
#=> 2

另外由於 class 也是 Class 的 instance 因此也可以直接去做定義，class method

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class A
end

A.instance_eval do
  def hi
      'hi'
  end
end

A.hi
#=> hi

Example

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class Tweet
  def initialize(user)
    @user = user
    @tweet = []
  end

  def submit_to_twitter
    tweet_text = @tweet.join(' ')
    puts "#{@user}: #{tweet_text}"
  end

  def text(str)
    @tweet << str
  end

  def hashtag(str)
    @tweet << "#" + str
    self
  end

  def mention(*users)
    users.each do |user|
      @tweet << "@" + user
    end
    self
  end

  def text(str)
    @tweet << str
    self
  end
end

def tweet_as(user, text = nil, &block)
  tweet = Tweet.new(user)
  tweet.text(text) if text
  tweet.instance_eval(&block) if block_given?
  tweet.submit_to_twitter
end

tweet_as 'markkendall' do
  mention 'foo', 'bar'
  text 'I made a DSL!'
  hashtag 'hooray'
end

參考文件：

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