debian-mirror-gitlab/doc/development/utilities.md

GitLab utilities

We developed a number of utilities to ease development.

MergeHash

• Deep merges an array of hashes:

  Gitlab::Utils::MergeHash.merge(
    [{ hello: ["world"] },
     { hello: "Everyone" },
     { hello: { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] } },
      "Goodbye", "Hallo"]
  )
  

  Gives:

  [
    {
      hello:
        [
          "world",
          "Everyone",
          { greetings: ['Bonjour', 'Hello', 'Hallo', 'Dzien dobry'] }
        ]
    },
    "Goodbye"
  ]
  

• Extracts all keys and values from a hash into an array:

  Gitlab::Utils::MergeHash.crush(
    { hello: "world", this: { crushes: ["an entire", "hash"] } }
  )
  

  Gives:

  [:hello, "world", :this, :crushes, "an entire", "hash"]
  

Override

• This utility could help us check if a particular method would override another method or not. It has the same idea of Java's @Override annotation or Scala's override keyword. However we only do this check when ENV['STATIC_VERIFICATION'] is set to avoid production runtime overhead. This is useful to check:

  • If we have typos in overriding methods.
  • If we renamed the overridden methods, making original overriding methods overrides nothing.

  Here's a simple example:

  class Base
    def execute
    end
  end
  
  class Derived < Base
    extend ::Gitlab::Utils::Override
  
    override :execute # Override check happens here
    def execute
    end
  end
  

  This also works on modules:

  module Extension
    extend ::Gitlab::Utils::Override
  
    override :execute # Modules do not check this immediately
    def execute
    end
  end
  
  class Derived < Base
    prepend Extension # Override check happens here, not in the module
  end
  

StrongMemoize

• Memoize the value even if it is nil or false.

  We often do @value ||= compute, however this doesn't work well if compute might eventually give nil and we don't want to compute again. Instead we could use defined? to check if the value is set or not. However it's tedious to write such pattern, and StrongMemoize would help us use such pattern.

  Instead of writing patterns like this:

  class Find
    def result
      return @result if defined?(@result)
  
      @result = search
    end
  end
  

  We could write it like:

  class Find
    include Gitlab::Utils::StrongMemoize
  
    def result
      strong_memoize(:result) do
        search
      end
    end
  end
  

• Clear memoization

  class Find
    include Gitlab::Utils::StrongMemoize
  end
  
  Find.new.clear_memoization(:result)