Ruby Metaprogramming Cheat Sheet

class Product  ATTRS = [:name, :price, :sku]  ATTRS.each do |attr|    define_method(attr) { instance_variable_get("@#{attr}") }    define_method("#{attr}=") { |val| instance_variable_set("@#{attr}", val) }  endendp = Product.newp.name = "Widget"p.name # => "Widget"

class DynamicProxy  def initialize(data)    @data = data  end  def method_missing(name, *args)    key = name.to_s    if key.end_with?('=')      @data[key.chomp('=')] = args.first    elsif @data.key?(key)      @data[key]    else      super # important: fall back for truly unknown methods    end  end  def respond_to_missing?(name, include_private = false)    @data.key?(name.to_s.chomp('=')) || super  endendobj = DynamicProxy.new({'title' => 'Ruby'})obj.title      # => "Ruby"obj.respond_to?(:title) # => true

class Base  def self.inherited(subclass)    puts "#{subclass} inherited from #{self}"  endendmodule Trackable  def self.included(base)    puts "Trackable included in #{base}"    base.extend(ClassMethods)  end  module ClassMethods    def track(attr)      define_method("track_#{attr}") { send(attr) }    end  endendString.class_eval do  def shout    upcase + "!"  endend"hi".shout # => "HI!"

class Router  def initialize(&block)    @routes = []    instance_eval(&block) if block_given?  end  def get(path, to:)    @routes << { verb: :get, path: path, handler: to }  end  def routes    @routes  endendrouter = Router.new do  get '/users', to: 'users#index'  get '/users/:id', to: 'users#show'endrouter.routes # => [{verb: :get, path: "/users", ...}, ...]