Groovy's delegate method, implemented for Ruby Procs

delegate_proc.rb

# The idea here is to emulate Groovy's delegate= pattern for closures.

# We define a builder module. This will receive method calls
# from our builder. It could be an instance of an object if we wanted.
module FamilyBuilder
  def self.parent(name)
    puts "Parent: #{name}"
  end
  
  def self.child(name)
    puts "Child: #{name}"
  end
end

class Proc
  # Sets the delegate for this Proc to be the given
  # object, or clears the delegate if nil is given.
  def delegate=(object)
    @delegate = object
    @delegated = !!(object)
  end
  
  alias_method :original_call, :call
  def call
    if @delegated
      # Evaluate ourselves, but against the delegate
      # rather than the original binding.
      @delegate.instance_eval(&self)
    else
      original_call
    end
  end
  
  # A helper to temporarily set the delegate, execute,
  # and then reset the delegate
  def evaluate_against(object)
    original_delegate = @delegate
    self.delegate = object
    self.call
    self.delegate = original_delegate
  end
end

build = Proc.new do
  parent "Bill"
  parent "Jane"
  child "Jack"
end

# Use the delegate setter explicitly, as in Groovy.
build.delegate = FamilyBuilder
build.call

# =================

# An example with an object instance:
greeting = "Hello James!"
Proc.new do
  gsub! "James", "Phil"
end.evaluate_against(greeting)

puts greeting

Output:

Parent: Bill
Parent: Jane
Child: Jack
Hello Phil!