Logo: Relish

  1. Sign in

Project: RSpec Mocks 3.9

Publisher: RSpec

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
Scenarios
  1. spec passes in isolation
  2. spec passes with dependencies loaded and method implemented
  3. spec fails with dependencies loaded and method unimplemented
  4. spec fails with dependencies loaded and incorrect arity
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."

Last published about 2 years ago by Jon Rowe.