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"SOLID Object-Oriented Design" - Sandi Metz @ GoRuCo 2009
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solid-oo-design-by-sandi-metz-at-goruco-2009.md

"SOLID Object-Oriented Design" by Sandi Metz @ GoRuCo 2009

Date: May 30, 2009

Video on confreaks.net

Notes

  • Design is all about dependencies

    • If you refer to something, you depend on it.
    • When the things you depend on change, you must change.

  • Resistance is a resource => Listen to what the pain is telling you. Listen to what the code smells are telling you. Embrace the friction. Fix the problem.

  • On assessing the design:

    • When you get to the refactor stage of red-green-refactor, ask yourself ...
      • Is it DRY?
      • Does it have one responsibility?
      • Does everything change at the same rate?
      • Does it depend on things that change less than it does?
    • When the answer to all of these things is 'yes', the design is probably in good shape.

  • "Triangle of Responsibility" Refactoring

    1. Refactor
    2. Extract - Pull functionality out where necessary
    3. Inject - Inject that new dependency into place from which it was extracted

  • What if I don't know where I want this refactoring to take me?

    • That's OK. In fact, that's typical.
    • "Refactor, not because you know the abstraction, but because you want to find it."
    • "You don't have to know where you're going to successfully refactor."
    • When you see someone's code and think it's beautiful and you wonder how they thought of it, they didn't. They evolved it to that point.

  • When injecting dependencies into a class, do so only via arguments to the #initialize method

      def intialize(downloader=FTPDownloader.new)
    @downloader = downloader
  end

  • When you need to inject a few dependencies, you can use an options hash to remove the dependency on the order of the arguments

      def intialize(opts)
    @env     = opts[:env] || Rails.env
    filename = opts[:filename]
  end

  • How to assess: "Does each object depend on things that change less than it does?"

    • Line up the objects from left to right
      • Left = lower likelihood of change
      • Right = higher likelihood of change
    • Only depend on things on your left

  • Recommended reading

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