Test Doubles

Test double is a generic term for any object that stands in for a real object during a test (think "stunt double"). You create one using the double method. Doubles are "strict" by default -- any message you have not allowed or expected will trigger an error -- but you can switch a double to being "loose". When creating a double, you can allow messages (and set their return values) by passing a hash.

Once you have a test double, you can allow or expect messages on it.

We recommend you use verifying doubles whenever possible.

Doubles are strict by default

Given a file named "double_spec.rb" with:

RSpec.describe "A test double" do
  it "raises errors when messages not allowed or expected are received" do
    dbl = double("Some Collaborator")
    dbl.foo
  end
end

When I run rspec double_spec.rb

Then it should fail with:

#<Double "Some Collaborator"> received unexpected message :foo with (no args)

A hash can be used to define allowed messages and return values

Given a file named "double_spec.rb" with:

RSpec.describe "A test double" do
  it "returns canned responses from the methods named in the provided hash" do
    dbl = double("Some Collaborator", :foo => 3, :bar => 4)
    expect(dbl.foo).to eq(3)
    expect(dbl.bar).to eq(4)
  end
end

When I run rspec double_spec.rb

Then the examples should all pass.