meet it

A is a that writes methods for you.

attr_accessor :name looks like a keyword, but it is an ordinary method call. It runs while Ruby reads the class body, and it writes two methods for you. You can build your own. A class method, written def self.field, belongs to the class itself rather than to each object.

Call your class method inside the class body and it becomes a class macro: one readable line that creates methods, usually with define_method. Rails is full of them, from has_many to validates. Here field writes a reader that falls back to a default value.

class Product
  def self.field(name, default)
    define_method(name) do
      @values.fetch(name, default)
    end
  end
  field :colour, "black"
  field :stock, 0
  def initialize(values)
    @values = values
  end
end
mug = Product.new({ colour: "teal" })
puts "#{mug.colour}, #{mug.stock} in stock"

fetch(name, default) looks up a key and hands back the default when that key is missing.

guess first

When does a class macro run? What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

class Report
  def self.column(name)
    puts "Adding column #{name}"
    define_method("#{name}_label") { name.to_s.capitalize }
  end
  column :price
  column :stock
end
puts "Class ready"
puts Report.new.price_label

your turn

Your turn. Give accounts a `flag` macro.

Add a flag of your own, such as verified, in the first blank. Then ask Sam about that flag in the second blank.

Your code

press run to see what happens

fix it

The class falls over while it is being read. Fix the macro.

Broken code to fix

press run to see what happens

The error says the class Product has no method field. Who is self inside a class body?
Inside the class body, self is the class, so the macro must be a class method.
Change line 2 to def self.field(name, default), then press run.

nailed it

you wrote your own class macros. :D

def self.name makes a class method.
A class macro runs while Ruby reads the class body.
Macros use define_method to write methods for you.

new words in your collection

class method
class macro

This lesson isn’t saved. A free account remembers every lesson you finish.

A class macro is a class method that writes methods for you.

When does a class macro run? What will this show?

Your turn. Give accounts a flag macro.

The class falls over while it is being read. Fix the macro.

you wrote your own class macros. :D

A is a that writes methods for you.

Your turn. Give accounts a `flag` macro.