#Ruby: Class Methods & Context

Prerequisites

• ruby-enumerable-comparable

Preparation

1. Fork and clone this repository. FAQ
2. Create a new branch, training, for your work.
3. Checkout to the training branch.
4. Install dependencies with bundle install.

Objectives

Developers should be able to:

• Determine runtime context using self.
• Invoke a getter or setter method using self.
• Define and use class instance variables and class methods.

Introduction

You may have noticed that not all methods we use with Ruby follow the Object.new(*args) pattern we introduced with object-oriented programming (OOP) in Ruby. We're already familiar with defining and using setters and getters on instances of classes, but what does it mean when we call a method directly on a class instead of an instance?

For example, let's say we have a model named Person. We can create new objects by using Person.new.

jeff = Person.new(given_name: "Jeffrey")
jeff.given_name #=> "Jeffrey"

The method #given_name is called on an instance, and hence is known as an instance method. The methods ::new and ::find_by are called directly on the class, and hence are known as class methods.

Sometimes we'd like to create our own class methods, or use setters and getters already provided by the parent (super) class to work on our objects. In order to accomplish these goals, we first need to understand context.

Context

Just like JavaScript's this keyword, self in Ruby is a reference to the runtime context of your program. We can use self instead of referring to particular instances to get or set data on these objects. Think of self as a pronoun for any object in our system.

Just like pronouns, self changes depending on the context in which it is used. At times self is going to refer to a particular object. Later, self could refer to a different object. In fact, at some times in a running program self may point to a class.

At every point in time when your program is running there is one and only one class or object that self is referencing or pointing to.

At any time during the life of your program the context may change. You may be running code inside of an instance method and self would point to the instance that invoked the method, or you could be inside a class definition and self would point to the class itself.

self will point to one of three runtime contexts: global context, object context, or class context.

Global

For all methods invoked without a class or object self will be an instance of the Object class, main.

puts "In the global scope, self is #{self}"

def example
  puts "In a method attached to the global scope, self is #{self}."
end

example

Object

For all methods invoked on an object self will point to that object.

class Person
  def whoami
    puts "In an instance method, self is #{self}."
  end
end

Person.new.whoami

Class

For all methods invoked in a class self will point to that class.

class Person
  puts "In a class,  self is #{self}."
end

Lab: Getters and Setters

Have a look at lib/person.rb. Notice how our custom setter has a side effect? Take a moment to read the code, and write comments with what you expect the code to do. Then load the file in a console and then try running the snippets below. pry >>> require_relative 'lib/person.rb'

Does each line do what you expected? What does self refer to?

person = Person.new
person.status = "ready"
person.log_in
person.log_out

Lab: Using Self

Experiment with the code provided in lib/method_chainer.rb

Think about the following:

• What is the return value if self is the last line of the instance method?
• What is the return value of the instance method without self?
• Try chaining methods, does commenting out self have an effect?

Best Practices

It's best practice to use self.foo and self.foo = instead of @foo and @foo =.

Implicit Receiver

self is always the implicit receiver inside method definitions. That means when we're inside an instance method and we want to work on the instance, we don't have to use self to refer to it.

Combined with the implicit returns, this makes Ruby code very concise, but it also means we have to pay attention to where we are when we're writing code!

License

1. All content is licensed under a CC­BY­NC­SA 4.0 license.
2. All software code is licensed under GNU GPLv3. For commercial use or alternative licensing, please contact legal@ga.co.