Question

I’m trying to DRY up some code, and I feel like Ruby’s variable assignment must provide a way to simplify this. I have a class with a number of different instance variables defined. Some of these are intended to be hidden (or read-only), but many are public, with read/write access.

For all of the variables with public write-access, I want to perform a certain method after each assignment. I know that, in general, I can do this:

def foo=(new_foo)
    @foo = new_foo
    post_process(@foo)
end

def bar=(new_bar)
    @bar = new_bar
    post_process(@foo)
end

However, it seems that there should be a nice way to DRY this up, since I’m doing essentially the same thing after each assignment (ie, running the same method, and passing the newly-assigned variable as a parameter to that method). Since I have a number of such variables, it would be great to have a general-purpose solution.

Answer 1

Simpler solution

If you assign those variables in batch, you can do something like this:

kv_pairs = {:foo => new_foo_value,
  :bar => new_bar_value}

kv_pairs.each do |k, v|
  self.send(k.to_s + '=', v)
  post_process(v)
end

Metaprogramming

Here’s some ruby magic 🙂

module PostProcessAssignments
  def hooked_accessor( *symbols )
    symbols.each { | symbol |
      class_eval( "def #{symbol}() @#{symbol}; end" )
      class_eval( "def #{symbol}=(val) @#{symbol} = val; post_process('#{symbol}', val); end" )
    }
  end
end

class MyClass
  extend PostProcessAssignments

  hooked_accessor :foo

  def post_process prop, val
    puts "#{prop} was set to #{val}"
  end


end

mc = MyClass.new
mc.foo = 4
puts mc.foo

Outputs:

foo was set to 4
4