Class variables in Ruby are not the same as instance variables of a
class. They're particularly tricky when it comes to inheritance and
modules.
In the case of the Measurable module, for example, using a class
variable will make all classes including the Measurable module share
the same value. However, that's not what we want; we want the variable
to be different for every class. And that's accomplished using a class
instance variable.
Although in this case it would probably be better to remove the caching
variable. I don't think these methods are called more than once during a
request, and even if they did it's highly unlikely the would become a
bottleneck.
We were very inconsistent regarding these rules.
Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.
The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.
[1] https://rubystyle.guide/#empty-lines-around-bodies
