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_controlmatches if the method-under-test yields, regardless of whether or not arguments are yielded.yield_with_argsmatches 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_argsmatches if the method-under-test yields with no arguments.yield_successive_argsis 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.yield_twice_with(*args)
2.times { yield *args }
end
def self.raw_yield
yield
end
def self.dont_yield
end
end
The yield_control matcher
Given a file named “yieldcontrolspec.rb” with:
require './my_class'
RSpec.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 }
specify { expect { |b| MyClass.yield_twice_with(1, &b) }.to yield_control.twice }
specify { expect { |b| MyClass.yield_twice_with(1, &b) }.to yield_control.exactly(2).times }
specify { expect { |b| MyClass.yield_twice_with(1, &b) }.to yield_control.at_least(1) }
specify { expect { |b| MyClass.yield_twice_with(1, &b) }.to yield_control.at_most(3).times }
# 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 }
specify { expect { |b| MyClass.yield_once_with(1, &b) }.to yield_control.at_least(2).times }
specify { expect { |b| MyClass.yield_twice_with(1, &b) }.not_to yield_control.twice }
specify { expect { |b| MyClass.yield_twice_with(1, &b) }.not_to yield_control.at_least(2).times }
specify { expect { |b| MyClass.yield_twice_with(1, &b) }.not_to yield_control.at_least(1) }
specify { expect { |b| MyClass.yield_twice_with(1, &b) }.not_to yield_control.at_most(3).times }
end
When I run rspec yield_control_spec.rb
Then the output should contain all of these:
| 13 examples, 7 failures |
| expected given block to yield control |
| expected given block not to yield control |
| expected given block not to yield control at least twice |
| expected given block not to yield control at most 3 times |
The yield_with_args matcher
Given a file named “yieldwithargs_spec.rb” with:
require './my_class'
RSpec.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 |
The yield_with_no_args matcher
Given a file named “yieldwithnoargsspec.rb” with:
require './my_class'
RSpec.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”] |
The yield_successive_args matcher
Given a file named “yieldsuccessiveargs_spec.rb” with:
def array
[1, 2, 3]
end
def array_of_tuples
[[:a, :b], [:c, :d]]
end
RSpec.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(Integer, Integer, Integer) }
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(Integer, Integer, Integer) }
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 |