Skip to main content

Using an instance double

An instance_double is the most common type of verifying double. It takes a class name or object as its first argument, then verifies that any methods being stubbed would be present on an instance of that class. In addition, when it receives messages, it verifies that the provided arguments are supported by the method signature, both in terms of arity and allowed or required keyword arguments, if any. The same argument verification happens when you constrain the arguments using with.

For methods handled by method_missing, see dynamic classes.

Background​

Given a file named "app/models/user.rb" with:

class User < Struct.new(:notifier)
  def suspend!
    notifier.notify("suspended as")
  end
end

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

$LOAD_PATH.unshift("app/models")

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

require 'unit_helper'

require 'user'
require 'console_notifier'

RSpec.configure do |config|
  config.mock_with :rspec do |mocks|

    # This option should be set when all dependencies are being loaded
    # before a spec run, as is the case in a typical spec helper. It will
    # cause any verifying double instantiation for a class that does not
    # exist to raise, protecting against incorrectly spelt names.
    mocks.verify_doubled_constant_names = true

  end
end

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

require 'unit_helper'

require 'user'

RSpec.describe User, '#suspend!' do
  it 'notifies the console' do
    notifier = instance_double("ConsoleNotifier")

    expect(notifier).to receive(:notify).with("suspended as")

    user = User.new(notifier)
    user.suspend!
  end
end

Spec passes in isolation​

When I run rspec spec/unit/user_spec.rb

Then the examples should all pass.

Spec passes with dependencies loaded and method implemented​

Given a file named "app/models/console_notifier.rb" with:

class ConsoleNotifier
  def notify(msg)
    puts message
  end
end

When I run rspec -r./spec/spec_helper spec/unit/user_spec.rb

Then the examples should all pass.

Spec fails with dependencies loaded and method unimplemented​

Given a file named "app/models/console_notifier.rb" with:

class ConsoleNotifier
end

When I run rspec -r./spec/spec_helper spec/unit/user_spec.rb

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

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

Spec fails with dependencies loaded and incorrect arity​

Given a file named "app/models/console_notifier.rb" with:

class ConsoleNotifier
  def notify(msg, color)
    puts color + message
  end
end

When I run rspec -r./spec/spec_helper spec/unit/user_spec.rb

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

And the output should contain "Wrong number of arguments.".