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.”.

Created with the assistance of Ninefold