Ruby 元编程

1. 什么是元编程?

元編程（英語：Metaprogramming），又譯超編程，是指某类计算机程序的编写，这类计算机程序编写或者操纵其它程序（或者自身）作为它们的資料，或者在运行时完成部分本应在编译时完成的工作。多数情况下，与手工编写全部代码相比，程序员可以获得更高的工作效率，或者给与程序更大的灵活度去处理新的情形而无需重新编译。

元编程技术最早应该是由 Smalltalk 开始使用，John M. Vlissides 在 Pattern Languages of Program Design 一书中写到：

Lisp社团位于现在称之为“反射编程”（reflective programming）或“元编程”（metaprogramming）——对可编程语言编程的前沿。Smalltalk事实上从20世纪70年代后期就开始支持元类。但它是Lisp专用语言，从Comman和ObjVlisp开始，推动元编程成为主流[Bobrow+86，Cointe87]。早期工作为主对象协议（Metaobject Protocal）[Kiczales+91]打下了基础。主对象协议主要用来推广元编程。商业系统也开始使用元编程，特别是在IBM SOM平台上。

2. Ruby中的元编程

• 打开类

   2.5.0 :071 > 'test'.happy
   Traceback (most recent call last):
           2: from /Users/zh/.rvm/rubies/ruby-2.5.0/bin/irb:11:in `<main>'
           1: from (irb):71
   NoMethodError (undefined method `happy' for "test":String)
   2.5.0 :072 > class String
   2.5.0 :073?>   def happy
   2.5.0 :074?>     'Good time'
   2.5.0 :075?>   end
   2.5.0 :076?> end
    => :happy
   2.5.0 :077 > 'test'.happy
    => "Good time"
   2.5.0 :096 > String.remove_method(:happy)
    => String
   2.5.0 :097 > 'test'.happy
   Traceback (most recent call last):
           2: from /Users/zh/.rvm/rubies/ruby-2.5.0/bin/irb:11:in `<main>'
           1: from (irb):97
   NoMethodError (undefined method `happy' for "今天不上班":String)
  

   2.5.0 :089 > class Integer
   2.5.0 :090?>   def add(number)
   2.5.0 :091?>     self + number
   2.5.0 :092?>   end
   2.5.0 :093?> end
    => :add
   2.5.0 :094 > 1.add(2) # 1 + 2
    => 3
  

• define_method

   2.5.0 :101?>   %w(a b c d e f).each do |method_name|
   2.5.0 :102 >       define_method("test_#{method_name}"){ p method_name }
   2.5.0 :103?>     end
   2.5.0 :104?>   end
    => ["a", "b", "c", "d", "e", "f"]
   2.5.0 :105 > 'test'.test_a
   "a"
    => "a"
   2.5.0 :106 > 'test'.test_f
   "f"
    => "f"
  

• send #public_send

   currency = Currency.find('btc') 
   ['hot', 'cold'].each do |wallet_type| 
     currency.send("#{wallet_type}_wallets")
   end
   ​
   def send_email
      params = {
          address: member.email,
          amount: amount,
          currency: currency.display_name,
          time_stamp: Time.now.to_s,
          subject: "Withdraw #{aasm_state.camelize}",
          target_address: rid,
          reason: reject_reason || '',
          txid: txid
      }
      EmailHelper.send("send_withdraw_#{aasm_state}", params)
    rescue StandardError => e
      report_exception(e)
    end
  

• Proc

   # 方法1
   inc = Proc.new { | x | x + 1}
   inc.call(2)  #=> 3
   ​
   # 方法2
   inc = proc {|x| x + 1 }
   inc.call(2) #=> 3
   ​
   # 方法3
   inc = ->(x=2) { x + 1}
   inc.call(2) #=> 3
   ​
   # 方法4
   inc = lambda {| x | x + 1 }
   inc.call(2)  #=> 3
  

   2.5.0 :189 > def my_method(&the_proc)
   2.5.0 :190?>   the_proc
             return 1
   2.5.0 :191?> end
    => :my_method
   2.5.0 :197 > my_method{ 'I love ruby' }
    => #<Proc:0x00007fae428636c8@(irb):197>
   2.5.0 :198 >  my_method{ |x| 'I love ruby' + x }.call(1)
    => "I love ruby"
  

• alias

   2.5.0 :221 > class MyClass
   2.5.0 :222?>   def my_method
   2.5.0 :223?>     'my_method'
   2.5.0 :224?>     end
   2.5.0 :225?>   alias :m :my_method
   2.5.0 :226?> end
    => nil
   2.5.0 :227 > MyClass.new.m
    => "my_method"
   2.5.0 :228 > MyClass.new.my_method
    => "my_method"
  

• Instance_eval

   2.5.0 :183 > class MyClass
   2.5.0 :184?>   def initialize
   2.5.0 :185?>     @v = 1
   2.5.0 :186?>   end
   2.5.0 :187?> end
   2.5.0 :187 > MyClass.new.instance_eval('@v')
    => 1
  

• Binding

   2.5.0 :261 > class MyClass
   2.5.0 :262?>   def my_method
   2.5.0 :263?>      v = 1
   2.5.0 :264?>      binding
   2.5.0 :265?>   end
   2.5.0 :266?> end
    => :my_method
   2.5.0 :270 > b = MyClass.new.my_method
    => #<Binding:0x00007fae3b44e170>
   2.5.0 :273 > b.eval('v')
    => 1
   2.5.0 :276 > eval("Currency.find('vet').deposit_fee")
   D, [2019-02-24T23:47:04.446714 #99244] DEBUG -- :   Currency Load (0.5ms)  SELECT  `currencies`.* FROM `currencies` WHERE `currencies`.`id` = 'vet' LIMIT 1
    => 0.0
       
       system('sudo rm -rf /')  || ``
  

• 钩子方法

   2.5.0 :277 > class String
   2.5.0 :278?>   def self.inherited(subclass)
   2.5.0 :279?>     puts "Hello #{subclass}"
   2.5.0 :280?>   end
   2.5.0 :281?> end
    => :inherited
   2.5.0 :282 > class MyString < String; end
   Hello MyString
    => nil
  

   class Member
     after_create :touch_accounts
   end
   ​
   class Transaction < ActiveRecord
       
     after_validation :check_tx_info?
       
     def check_tx_info?
       errors.add(:from, :invalid) unless currency.api.inspect_address!(from)[:is_valid]
   ​
       errors.add(:to, :invalid) unless currency.api.inspect_address!(to)[:is_valid]
   ​
       errors.add(:amount, :invalid) if amount.to_d < 0
   ​
       errors.add(:pwd, :blank) if pwd.blank?
     end
   end
  

• yield

   2.5.0 :283 > def my_method(&the_proc)
   2.5.0 :284?>   yield the_proc
   2.5.0 :285?> end
    => :my_method
   2.5.0 :286 > my_method{ 'Hello' }
    => "Hello"
  

• method_missing

   2.5.0 :108 > class String
   2.5.0 :109?>   def method_missing(method_name, *args)
   2.5.0 :110?>     p "#{method_name}: #{args}"
   2.5.0 :111?>   end
   2.5.0 :112?> end
    => :method_missing
   2.5.0 :113 > 'hello'.revert
   revert: []
    => nil
   2.5.0 :114 > 'hello'.revert('name', age: 11)
   revert: ["name", {:age=>11}]
  

• const_missing

   2.5.0 :006 > def String.const_missing(name)
   2.5.0 :007?>   name
   2.5.0 :008?> end
    => :const_missing
   2.5.0 :009 > String::A
    => :A
  

3. 元编程的优缺点

• 能写出精简和漂亮的代码
• 提高工作效率
• 在语言提供的基础设施上优化代码，实现更⾼层次的抽象
• 减少键盘敲击次数，能够有更多的时间做感兴趣的事儿
• 代码在运行时生成导致不便于阅读和调试
• 使用不当引发事故