Test rake tasks with rspec

foo_spec.rb

require 'spec_helper'
require 'rake'

describe 'foo tasks' do
  before :all do
    Rake::Task.clear
    Rake.application.rake_require 'tasks/foo'
    Rake::Task.define_task(:environment)
  end

  before do
    Rake.application.tasks.map(&:reenable)
  end

  after :all do
    Rake::Task.clear
  end

  describe 'bar' do
    let(:quux) { double(Quux, frob: nil) }

    before do
      allow(Rake.application).to receive(:invoke_task)
      allow(Rake.application).to receive(:invoke_task).with('foo:bar').and_call_original
      allow(Quux).to receive(:new).with('a').and_return(quux)

      Rake.application.invoke_task 'foo:bar'
    end

    it "frobs the quuxs" do
      expect(quux).to have_received(:frob).with(no_args)
    end
  end
end