Project: RSpec Expectations 2.10

Publisher: RSpec

yield matchers

There are four related matchers that allow you to specify whether
or not a method yields, how many times it yields, whether or not
it yields with arguments, and what those arguments are.

yield_control matches if the method-under-test yields, regardless of whether or not arguments are yielded.
yield_with_args matches if the method-under-test yields with arguments. If arguments are provided to this matcher, it will only pass if the actual yielded arguments match the expected ones using === or ==.
yield_with_no_args matches if the method-under-test yields with no arguments.
yield_successive_args is designed for iterators, and will match if the method-under-test yields the same number of times as arguments passed to this matcher, and all actual yielded arguments match the expected ones using === or ==.

Note: your expect block must accept an argument that is then passed on to
the method-under-test as a block. This acts as a "probe" that allows the matcher
to detect whether or not your method yields, and, if so, how many times and what
the yielded arguments are.

Background

Given

a file named "my_class.rb" with:

class MyClass
  def self.yield_once_with(*args)
    yield *args
  end

  def self.raw_yield
    yield
  end

  def self.dont_yield
  end
end

Scenarios

yield_control matcher
yield_with_args matcher
yield_with_no_args matcher
yield_successive_args matcher

yield_control matcher

Given

a file named "yield_control_spec.rb" with:

require './my_class'

describe "yield_control matcher" do
  specify { expect { |b| MyClass.yield_once_with(1, &b) }.to yield_control }
  specify { expect { |b| MyClass.dont_yield(&b) }.not_to yield_control }

  # deliberate failures
  specify { expect { |b| MyClass.yield_once_with(1, &b) }.not_to yield_control }
  specify { expect { |b| MyClass.dont_yield(&b) }.to yield_control }
end

When

I run rspec yield_control_spec.rb

Then

the output should contain all of these:

4 examples, 2 failures

expected given block to yield control

expected given block not to yield control

yield_with_args matcher

Given

a file named "yield_with_args_spec.rb" with:

require './my_class'

describe "yield_with_args matcher" do
  specify { expect { |b| MyClass.yield_once_with("foo", &b) }.to yield_with_args }
  specify { expect { |b| MyClass.yield_once_with("foo", &b) }.to yield_with_args("foo") }
  specify { expect { |b| MyClass.yield_once_with("foo", &b) }.to yield_with_args(String) }
  specify { expect { |b| MyClass.yield_once_with("foo", &b) }.to yield_with_args(/oo/) }

  specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.to yield_with_args("foo", "bar") }
  specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.to yield_with_args(String, String) }
  specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.to yield_with_args(/fo/, /ar/) }

  specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.not_to yield_with_args(17, "baz") }

  # deliberate failures
  specify { expect { |b| MyClass.yield_once_with("foo", &b) }.not_to yield_with_args }
  specify { expect { |b| MyClass.yield_once_with("foo", &b) }.not_to yield_with_args("foo") }
  specify { expect { |b| MyClass.yield_once_with("foo", &b) }.not_to yield_with_args(String) }
  specify { expect { |b| MyClass.yield_once_with("foo", &b) }.not_to yield_with_args(/oo/) }
  specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.not_to yield_with_args("foo", "bar") }
  specify { expect { |b| MyClass.yield_once_with("foo", "bar", &b) }.to yield_with_args(17, "baz") }
end

When

I run rspec yield_with_args_spec.rb

Then

the output should contain all of these:

14 examples, 6 failures

expected given block not to yield with arguments, but did

expected given block not to yield with arguments, but yielded with expected arguments

expected given block to yield with arguments, but yielded with unexpected arguments

yield_with_no_args matcher

Given

a file named "yield_with_no_args_spec.rb" with:

require './my_class'

describe "yield_with_no_args matcher" do
  specify { expect { |b| MyClass.raw_yield(&b) }.to yield_with_no_args }
  specify { expect { |b| MyClass.dont_yield(&b) }.not_to yield_with_no_args }
  specify { expect { |b| MyClass.yield_once_with("a", &b) }.not_to yield_with_no_args }

  # deliberate failures
  specify { expect { |b| MyClass.raw_yield(&b) }.not_to yield_with_no_args }
  specify { expect { |b| MyClass.dont_yield(&b) }.to yield_with_no_args }
  specify { expect { |b| MyClass.yield_once_with("a", &b) }.to yield_with_no_args }
end

When

I run rspec yield_with_no_args_spec.rb

Then

the output should contain all of these:

6 examples, 3 failures

expected given block not to yield with no arguments, but did

expected given block to yield with no arguments, but did not yield

expected given block to yield with no arguments, but yielded with arguments: ["a"]

yield_successive_args matcher

Given

a file named "yield_successive_args_spec.rb" with:

def array
  [1, 2, 3]
end

def array_of_tuples
  [[:a, :b], [:c, :d]]
end

describe "yield_successive_args matcher" do
  specify { expect { |b| array.each(&b) }.to yield_successive_args(1, 2, 3) }
  specify { expect { |b| array_of_tuples.each(&b) }.to yield_successive_args([:a, :b], [:c, :d]) }
  specify { expect { |b| array.each(&b) }.to yield_successive_args(Fixnum, Fixnum, Fixnum) }
  specify { expect { |b| array.each(&b) }.not_to yield_successive_args(1, 2) }

  # deliberate failures
  specify { expect { |b| array.each(&b) }.not_to yield_successive_args(1, 2, 3) }
  specify { expect { |b| array_of_tuples.each(&b) }.not_to yield_successive_args([:a, :b], [:c, :d]) }
  specify { expect { |b| array.each(&b) }.not_to yield_successive_args(Fixnum, Fixnum, Fixnum) }
  specify { expect { |b| array.each(&b) }.to yield_successive_args(1, 2) }
end

When

I run rspec yield_successive_args_spec.rb

Then

the output should contain all of these:

8 examples, 4 failures

expected given block not to yield successively with arguments, but yielded with expected arguments

expected given block to yield successively with arguments, but yielded with unexpected arguments

Last published almost 5 years ago by dchelimsky.

yield matchers

RSpec Expectations settings