Practical Object-Oriented Design in Ruby (by Sando Metz)

Relevant Notes for a Junior Engineer

Object-Oriented Languages (p.12)

Objects

Have behavior and may contain data,
- and this is data that the assigned object alone can access
Objects can invoke each others behavior by sending messages.
Examples of objects:
- Strings
- self.
Ruby Classes:
- Ruby is a class-based OO language
- provide the blueprint for contructing objects
- Classes define methods (definition of behavior) and attributes (definition of variables), and methods get invoked in response to messages.

Single Responsibility Principle

Classes that are easy to reuse allow for applications that are easy to change, and have few entanglements.
You want to create classes that have only one responsibility, or behavior (it does one thing), because it'll be easier to reuse that class.

Don't Repeat Yourself (DRY)

Every tiny bit of behavior should live in one place.
Behavior is captured in methods, in the form of objects, and invoked by sending messages.
Objects contain behavior and data.

Instance Variables

Data is contained in an instance variable.
Instance variables should be wrapped in an accessor method, aka. a "wrapper method" such as attr_reader, and then called in def initialize
attr_reader :chainring, :cog does this behind the scenes:
- ```
def cog
  @cog
end

def chainring
  @chainring
end
```
- then the two instance variables need to be initialized with def initialize

Namibia Torres