Ruby: automatically call a method when attribute is set

Question

I am new to Ruby, and to object oriented programming more generally.

I think what I want is exactly a Ruby translation of this. That is, I want to be able to initialize an instance of the class, and then as soon as a certain attribute is set call a method on it to convert it.

For example, say I had a Weather class with a temperature attribute that I would like converted from Fahrenheit to Celsius as soon as it is set. Here is the way I would like this to behave.

today = Weather.new
today.temp = 32
today.temp     # => 0

How do I do that?

max pleaner · Accepted Answer · 2017-01-12 23:12:23Z

1

You can write your own attr_writer-style method to accomplish this. Feel free to comment if this is unclear.

class Weather
  attr_reader :temp
  def temp=(val)
    @temp = f_to_c(val)
  end
  def f_to_c(temp)
    (temp - 32).to_f * (5.0 / 9.0) 
  end
end

answered Jan 12, 2017 at 23:12

max pleaner

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2 Comments

Katie Over a year ago

I just translated it to my actual context and it works great, thanks. In terms of clarity, I think I don't quite understand what attr_writer is and does, and I need to wrestle with that some. This will help, I think. It definitely helps to see the correct implementation of what I was trying to accomplish.

Jörg W Mittag Over a year ago

Module#attr_writer is a method which generates an attribute writer method. Its implementation looks (roughly) like this:

class Module; def attr_writer(*methods) methods.each do |method| define_method(:"#{method}=") do |val| instance_variable_set(:"@#{method}", val) end end end end

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