Imagine you are writing a program to help your local cat animal shelter, so you start by defining a Cat Class in Ruby 😺
class Cat def initialize(name) @name = name end def name @name end end
Now you go ahead and make a call to create a new Cat with a name:
cat = Cat.new cat.name = "Telemakhos" #=> NoMethodError
You run into
NoMethodError 🙀. Well, that is expected since we haven't written a method to define a cat's name. We've just written the getter method. We also need to write a _setter_method.
Getter methods are used to retrieve the value of an instance variable from an object and Setter methods are used to set the value of an instance variable. In many languages, you can read and write the value of an instance variable directly but in Ruby you have to specify a getter and setter.
Let's have a second pass at our Cat Class. Say you create a new instance of Cat attached to a variable
class Cat def initialize(name) @name = name end def name #getter method @name end def name=(name) #setter method @name = name end end tabby = Cat.new('Telemakhos') puts tabby.name #=> "Telemakhos"
With our new getter and setter methods, we can now get and set the name of a cat 😼
Now imagine if you had more properties to a Cat than just
class Cat def initialize(name, age, breed, weight) @name = name @age = age @breed = breed @weight = weight end end
If you wanted to get or set any of those properties, it would require eight more getter and setter methods! 😿
attr_writer are Ruby methods that can be used to replace getter and setter methods and mimic the accessing of instance variables seen in other languages such as TypeScript. Using
attr_writer is especially useful when you want to read and manipulate the values of a Class that has many instance variables.
Here is an example of it's utility in depth:
# cat.rb class Cat attr_reader :name, :age, :breed, :weight attr_writer :name, :age, :breed, :weight def initialize(name, age, breed, weight) @name = name @age = age @breed = breed @weight = weight end end cat = Cat.new("Julie", 3, "Tabby", 8) puts cat.name # => "Julie" puts cat.age # => 3 puts cat.breed # => "Tabby" puts cat.weight # => 8 cat.weight = 10 puts cat.weight # => 10
attr_writer can replace a bunch of getter and setter methods in just a single line! 😻