Advanced-Ruby.md

Presenter: Anthony Lewis

Bio

This is a training session @ 2013 Lone Star Ruby Conference

Abstract

This training covers the following topics:

• Ruby Object model
• Modules/ Mixing
• Metaprogramming

Summary

• In Ruby, everything is an object, you can define methods on class level and instance level.
• inlcude is for instance methods and extend is for class methods, you can also use include to do extend
• learn how to use prepend to effeciently do Fiboniacci method
• method missing is powerful and dangerous (see example)

Memorable Quotes

• "The wifi really sucks..."
• "Usually people don't do live coding as a presentation, but I'd like to try to get my hands dirty."

Notes

Ruby Object Model

send : calling a method is also sending a message

Person.new.send :greet

Singleton Class (Eigenclass): a singleton method is a method defined on an instance

superman = Person.new("Clark")
def superman.fly
  puts "Up, up, and high"
end
superman.singleton_class
> #<Class:#<Person:0x007fa0eb03c6c0>>

Modules/Mixins

include: instance methods, extend class methods. Prep a module: include module methods before methods defined in the class

module Memoize
  def calc(n)
    @@memo || = {}
    @@memo[n] || = super
  end
end

class Fib
  # without prepend, it's not fast.
  def calc(n)
    return n if n < 2
    return calc(n-1) + calc(n-2)
  end
end

Metaprogramming

class_eval: creates instance method, evaulate a code string (type is String) as if it were typed directly into the class definition

  > p.name
  > NoMethodError: undefined method 'name'

  class Class
    def get_attr(attr)
      self.class_eval "
        def #{attr}
          @#{attr}
        end
      "
    end
  end

  class Person
    get_attr :name
  end

define_method: create a method at runtime

  class Person
    define_method :punch do |arg|
      "Ouch!"*arg
    end
  end

method_missing: invoked when a method is not found class method_missing(method, *args) # do something end

Rails attributes, Rails dynmaic finders

  def method_missing(meth, *args)
    if metho.to_s =~ /^find_by_(.+)$/
      find_by_attributes(attr, *args)
    else
      super
    end
  end

  def find_by_attributes(attr, *args)
    conditions = Hash[attr.map {|a| [a, args[attr.index(a)]]}]
    where(condition, args)
  end

Method decoractors / class decorators

  module TrackMethod
    def track(meth)
      self.class_eval do
        alias_method "old_#{meth}", meth
        define_mthod meth do |*args|
          puts "logging #{meth} with #{args.join(',')}"
          self.send "old_#{meth}", args
        end
      end
    end
  end

  # in the class where you want to use the tracking
  class Person
    extend TrackMethod

    track :greeting

    def greeting
      # something
    end
  end
````

Show me only the instance_methods defined

  > p = Person.new
  > p.class.instance.instance_methods(false)

###Use Ruby Set require 'set'

class User
  def initialize
    @features = Set.new
  end

  def method_missing(meth, *args)
    if meth.to_s =~ /^can_(.*)\?$/
      @features.include? $1.to_s
    elsif meth.to_s =~ /^can_(.*)$/
      @features << $1.to_s
    else
      super
    end
  end
end

Discussion

Define new instance method as we go..

# define a new method inside that class
def method_missing(meth, *args)
  if meth.to_s =~ /^can_(.*)\?$/
    self.class.class_eval do
      define_method meth.to_sym do
        @features.include? $1.to_s
      end
    end
  elsif meth.to_s =~ /^can_(.*)$/
    self.class.class_eval do
      define_meth meth.to_sym do
        @features << $1.to_s
      end
    end
  else
    super
  end
end

External Links

• Some related website