Rspec for Delegate

delegate.rb

# RSpec matcher to spec delegations, taken from gist at
# https://gist.github.com/joeytheman/0fe021821e4c62f552ce
#
# Usage:
#
#     describe Post do
#       it { should delegate(:name).to(:author).with_prefix } # post.author_name
#       it { should delegate(:name).to(:author).with_prefix(:any) } # post.any_name
#       it { should delegate(:month).to(:created_at) }
#       it { should delegate(:year).to(:created_at) }
#       it { should delegate(:something).to(:'@instance_var') }
#     end

module RSpec
  module Matchers
    define :delegate do |*methods|
      methods.each do |method|
        match do |delegator|
          @method = @prefix ? :"#{@prefix}_#{method}" : method
          @delegator = delegator

          if @to.to_s[0] == '@'
            # Delegation to an instance variable
            old_value = @delegator.instance_variable_get(@to)
            begin
              @delegator.instance_variable_set(@to, receiver_double(method))
              @delegator.send(@method) == :called
            ensure
              @delegator.instance_variable_set(@to, old_value)
            end
          elsif @delegator.respond_to?(@to, true)
            unless [0, -1].include?(@delegator.method(@to).arity)
              raise "#{ @delegator }'s' #{ @to } method does not have zero or -1 arity (it expects parameters)"
            end

            allow(@delegator).to receive(@to).and_return(receiver_double(method))
            @delegator.send(@method) == :called
          else
            raise "#{ @delegator } does not respond to #{ @to }"
          end
        end
      end

      description do
        "delegate :#{ @method } to its #{ @to }#{ prefix_string }"
      end

      failure_message do |text|
        "expected #{ @delegator } to delegate :#{ @method } to its #{ @to }#{ prefix_string }"
      end

      failure_message_when_negated do |text|
        "expected #{ @delegator } not to delegate :#{ @method} to its #{ @to }#{ prefix_string }"
      end

      chain(:to) { |receiver| @to = receiver }
      chain(:with_prefix) { |prefix = nil| @prefix = prefix || @to }

      def receiver_double(method)
        double('receiver').tap do |receiver|
          allow(receiver).to receive(method).and_return(:called)
        end
      end

      def prefix_string
        @prefix ? ' with prefix' : ''
      end
    end
  end
end