Notes from Sandi Metz “Introduction to Practical Object-Oriented”

Object-Oriented Design

• Object-oriented design (OOD) is about managing dependencies
• Objects shouldn’t know too much about each other
• Design is an art of arranging code
• Write code for the feature you’re developing today, but also changeable in the future
• You can’t predict what changes will be needed in your code, merely that change will occur
• Tools for a OOD ➡️ principles and patterns

Design principles

• [S]ingle responsibility
• [O]pen-closed
• [L]iskov substitution
• [I]nterface segregation
• [D]ependency inversion
• We can look to quantify “good code”

Design patterns

• Name common problem and solve the problems in common ways
• Patterns are like a tool, it’s the user who must master the pattern/tool

Act of Design

• “Programmer’s past experience does not predict the future”
  • This especially hits home for me as I’m learning. I can be afraid of past failures defining me.

How Design fails

• When a program is easy to create, but changes become gradually impossible
• When design is separated from the act of programming
• Design relies on the feedback loop
  • Think agile software development

When to design

• Agile works because it acknowledges certainty is unattainable in advance
• Agile is an example where OOD is required, because of the way you’re iteratively implementing code

Judging design

• What’s the thing that separates a novice and senior programmer?
  • Lines of code written?
  • How frequently they are deploying?
• How much design depends on two things:
  • Your skills and timeframe
• Think of good design like good investing, don’t turn down compounding interest

Brief intro to Object-Oriented programming

• Applications are made up of objects and messages that pass between them