App Academy W6D1 & D2

exams

周一为了准备周二的考试，作为review day，其实这个形式还是蛮不错的，之前完全不理解ActiveRecord返回的东西，怎么使用，现在这几天下来，基本知道怎么用了。看来这种有考试逼迫的填鸭式教学还是最有效率的。

周二上午讲了关于meta programming的一些内容，我觉得很有用，未来可能会在建立一些自己的类的时候用到。

Learning Objectives

• Define metaprogramming
• Identify self in:
• class definitions
• class methods
• instance methods
• Describe the difference between class variables and class instance variables
• Know how to use send and define_method (and method_missing if time allows) ********这个是重点。

Agenda

• What is metaprogramming? 用program来创建program

What is Metaprogramming?

• What is programming?
• The essence of programming is defining behavior
• Metaprogramming is defining behavior that will define future behavior
• A program is a set of instructions with some goal, ie a plan
• ex: an exercise program, television programming, the 12 step program
• Meta just means self-referential

Metaprogramming

• Often used to dynamically define a suite of similar methods based on undetermined future input to reduce duplicate code

Examples:

• attr_reader/attr_writer
• belongs_to/has_many
• validates 这些function都是take in一系列的method名称，然后创建了getter和setter的，本身就是meta programming的典范。

Metaprogramming

• In practice, metaprogramming in Ruby makes use of a collection of methods to help the developer define behavior

Today we’ll explore just a small sample of those methods

require 'byebug'

class Pet
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def self.add_pet
    @@total_pets ||= 0
    @@total_pets += 1
  end

  def self.total_pets
    @@total_pets
  end

end

class Dog < Pet
  puts self # Dog

  def self.doggie_things
    puts self  # Dog
  end

  def who_am_i?
    puts self # Dog instance
  end

  def self.record_dog_birth(dog)
    @all_dogs ||= []
    add_pet
    @all_dogs << dog
  end

  def self.all
    @all_dogs.each { |dog| puts dog.name }
  end

  def initialize(name)
    super(name)
    self.class.record_dog_birth(self)
    @secret = "I'm a goat in disguise"
  end

  def do_this_then_that(this, that)
    self.send(this) 这个就很有意思了，类似于javascript的call back function，可接收function 并执行。
    self.send(that)
  end

  def do_this_with_args(this, *args)
    self.send(this, *args)
  end

  def do_this_on_class(this)
    Dog.send(this)
  end

  # def sleep
  #   puts "sleeping!"
  # end

  # def eat
  #   puts "eating!"
  # end

  # def drink
  #   puts "drinking!"
  # end

  # def self.create_tricks(*trick_names)
  #   puts self # Dog class
  #   trick_names.each do |trick_name|
  #     puts self # Dog class
  #     define_method(trick_name) do |num = 1|
  #       puts self # Dog instance
  #       num.times do
  #         puts "#{trick_name}ing!!!"
  #       end
  #     end
  #   end
  # end

  def method_missing(method_name, *args)
    self.send(:define_method, method_name) do |*arguments|
      arguments[0].times do
        puts "#{method_name}ing!"
      end
      puts arguments[1]
    end
    self.send(method_name, *args)
  end

  def self.my_attr_reader(*method_names)
    # Iterate over method names
    # define methods with those names
    # grab instance variables of that name
  end

  def trick
    puts "BACK FLIP!!"
  end

  def juggle(thing, num)
    num.times do
      puts "juggling #{thing}!"
    end
  end

  private
  
  attr_reader :secret

  def tell_secret
    puts "My secret is: #{@secret}"
  end
end