celluloid_spec.rb

require 'celluloid'

RSpec.describe "Celluloid" do
  it 'can mock a method' do
    foo = Celluloid::AsyncProxy.new([], :bar)
    expect(foo).to receive(:bar).once
    foo.bar
  end
end

Gemfile

# A sample Gemfile
source "https://rubygems.org"

gem 'celluloid'
gem 'rspec'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    celluloid (0.15.2)
      timers (~> 1.1.0)
    diff-lcs (1.2.5)
    rspec (3.0.0)
      rspec-core (~> 3.0.0)
      rspec-expectations (~> 3.0.0)
      rspec-mocks (~> 3.0.0)
    rspec-core (3.0.2)
      rspec-support (~> 3.0.0)
    rspec-expectations (3.0.2)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.0.0)
    rspec-mocks (3.0.2)
      rspec-support (~> 3.0.0)
    rspec-support (3.0.2)
    timers (1.1.0)

PLATFORMS
  ruby

DEPENDENCIES
  celluloid
  rspec