RSpec Double vs Instance Double

double_vs_instance_double.rb

# RSpec Double vs Instance Double

# stub FeatureToggle using a double
def stub_feature_toggle_using_double
 double(FeatureToggle, on?: true).tap do |feature_toggle|
  allow(FeatureToggle).to receive(:new).and_return(feature_toggle)
 end
end

# In the above example, we're stubbing out the response for "on?".
# In the event we typo "on?" and write "onn?", running the test returns the error message:

# => <Double FeatureToggle> received unexpected message :on?

# stub FeatureToggle using instance double
def stub_feature_toggle_using_double
 instance_double(FeatureToggle, on?: true).tap do |feature_toggle|
  allow(FeatureToggle).to receive(:new).and_return(feature_toggle)
 end
end

# using the same example, in the event we typo "on?" and write "onn?", running the test returns the error message:

# => the FeatureToggle class does not implement the instance method: onn?

# instance doubles will verify that the method(s) being stubbed exist on the underlying object
# and provide a clearer error message when a non-existent method is being stubbed.