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Expecting messages

Use expect(...).to receive(...) to expect a message on a test double. Unfulfilled message expectations trigger failures when the example completes. You can also use expect(...).not_to receive(...) to set a negative message expectation.

Failing positive message expectation​

Given a file named "unfulfilled_message_expectation_spec.rb" with:

RSpec.describe "An unfulfilled positive message expectation" do
  it "triggers a failure" do
    dbl = double("Some Collaborator")
    expect(dbl).to receive(:foo)
  end
end

When I run rspec unfulfilled_message_expectation_spec.rb

Then it should fail with:

  1) An unfulfilled positive message expectation triggers a failure
     Failure/Error: expect(dbl).to receive(:foo)

       (Double "Some Collaborator").foo(*(any args))
           expected: 1 time with any arguments
           received: 0 times with any arguments

Passing positive message expectation​

Given a file named "fulfilled_message_expectation_spec.rb" with:

RSpec.describe "A fulfilled positive message expectation" do
  it "passes" do
    dbl = double("Some Collaborator")
    expect(dbl).to receive(:foo)
    dbl.foo
  end
end

When I run rspec fulfilled_message_expectation_spec.rb

Then the examples should all pass.

Failing negative message expectation​

Given a file named "negative_message_expectation_spec.rb" with:

RSpec.describe "A negative message expectation" do
  it "fails when the message is received" do
    dbl = double("Some Collaborator").as_null_object
    expect(dbl).not_to receive(:foo)
    dbl.foo
  end
end

When I run rspec negative_message_expectation_spec.rb

Then it should fail with:

  1) A negative message expectation fails when the message is received
     Failure/Error: dbl.foo

       (Double "Some Collaborator").foo(no args)
           expected: 0 times with any arguments
           received: 1 time

Passing negative message expectation​

Given a file named "negative_message_expectation_spec.rb" with:

RSpec.describe "A negative message expectation" do
  it "passes if the message is never received" do
    dbl = double("Some Collaborator").as_null_object
    expect(dbl).not_to receive(:foo)
  end
end

When I run rspec negative_message_expectation_spec.rb

Then the examples should all pass.

Failing positive message expectation with a custom failure message​

Given a file named "example_spec.rb" with:

RSpec.describe "An unfulfilled positive message expectation" do
  it "triggers a failure" do
    dbl = double
    expect(dbl).to receive(:foo), "dbl never calls :foo"
  end
end

When I run rspec example_spec.rb --format documentation

Then the output should contain:

  1) An unfulfilled positive message expectation triggers a failure
     Failure/Error: expect(dbl).to receive(:foo), "dbl never calls :foo"
       dbl never calls :foo

Failing negative message expectation with a custom failure message​

Given a file named "example_spec.rb" with:

RSpec.describe "A negative message expectation" do
  it "fails when the message is received" do
    dbl = double
    expect(dbl).not_to receive(:foo), "dbl called :foo but is not supposed to"
    dbl.foo
  end
end

When I run rspec example_spec.rb --format documentation

Then the output should contain:

  1) A negative message expectation fails when the message is received
     Failure/Error: dbl.foo
       dbl called :foo but is not supposed to