Using a class double

class_double is provided as a complement to instance_double with the difference that it verifies class methods on the given class rather than instance methods.

In addition, it also provides a convenience method as_stubbed_const to replace concrete classes with the defined double. See mutating constants for more details.

Note: class_double can be used for modules as well. We chose to stick with the class_double terminology because the methods a class_double verifies against are commonly called "class methods", not "module methods", even when working with a module.

Background

Given a file named "lib/user.rb" with:

class User
  def suspend!
    ConsoleNotifier.notify("suspended as")
  end
end

Given a file named "lib/console_notifier.rb" with:

class ConsoleNotifier
  MAX_WIDTH = 80

  def self.notify(message)
    puts message
  end
end

Given a file named "spec/user_spec.rb" with:

require 'user'
require 'console_notifier'

RSpec.describe User, '#suspend!' do
  it 'notifies the console' do
    notifier = class_double("ConsoleNotifier").
      as_stubbed_const(:transfer_nested_constants => true)

    expect(notifier).to receive(:notify).with("suspended as")
    expect(ConsoleNotifier::MAX_WIDTH).to eq(80)

    user = User.new
    user.suspend!
  end
end

Replacing existing constants

When I run rspec spec/user_spec.rb

Then the examples should all pass.

Renaming ConsoleNotifier.notify to send_notification

Given a file named "lib/console_notifier.rb" with:

class ConsoleNotifier
  MAX_WIDTH = 80

  def self.send_notification(message)
    puts message
  end
end

When I run rspec spec/user_spec.rb

Then the output should contain "1 example, 1 failure"

And the output should contain "the ConsoleNotifier class does not implement the class method:".