Superlet

Lets say you are testing your app superhero in RSpec:

RSpec.describe Superhero do
  subject(:superhero) { Superhero.new }

  it { is_expected.to be_super }
end

This superhero is sad because he has no powers though. So lets try giving him x-ray vision:

RSpec.describe Superhero do
  subject(:superhero) { Superhero.new }

  it { is_expected.to be_super }
  it { is_expected.not_to have_xray_vision }

  context "when given x-ray vision" do
    subject(:superhero) { Superhero.new(:xray_vision) }
    it { is_expected.to have_xray_vision }
    it { is_expected.to be_super }
  end
end

And now we want x-ray vision and flight:

RSpec.describe Superhero do
  subject(:superhero) { Superhero.new }

  it { is_expected.to be_super }
  it { is_expected.not_to have_xray_vision }
  it { is_expected.not_to have_flight }

  context "when given x-ray vision" do
    subject(:superhero) { Superhero.new(:xray_vision) }

    it { is_expected.to be_super }
    it { is_expected.to have_xray_vision }
    it { is_expected.not_to have_flight }

    context "when given flight" do
      subject(:superhero) { Superhero.new(:xray_vision, :flight) }

      it { is_expected.to be_super }
      it { is_expected.to have_xray_vision }
      it { is_expected.to have_flight }
    end
  end
end

This test looks pretty nice, but we would like it to be dry in case we want to change the names of the powers to fight a new supervillain. First lets use the power of arrays and the splat:

RSpec.describe Superhero do
  subject(:superhero) { Superhero.new *powers }
  let(:powers) { Array.new }

  it { is_expected.to be_super }
  it { is_expected.not_to have_xray_vision }
  it { is_expected.not_to have_flight }

  context "when given x-ray vision" do
    let(:powers) { [:xray_vision] }

    it { is_expected.to be_super }
    it { is_expected.to have_xray_vision }
    it { is_expected.not_to have_flight }

    context "when given flight" do
      let(:powers) { [:xray_vision, :flight] }

      it { is_expected.to be_super }
      it { is_expected.to have_xray_vision }
      it { is_expected.to have_flight }
    end
  end
end

Next we can use the super power of lets, super():

RSpec.describe Superhero do
  subject(:superhero) { Superhero.new *powers }
  let(:powers) { Array.new }

  it { is_expected.to be_super }
  it { is_expected.not_to have_xray_vision }
  it { is_expected.not_to have_flight }

  context "when given x-ray vision" do
    let(:powers) { super() << :xray_vision }

    it { is_expected.to be_super }
    it { is_expected.to have_xray_vision }
    it { is_expected.not_to have_flight }

    context "when given flight" do
      let(:powers) { super() << :flight }

      it { is_expected.to be_super }
      it { is_expected.to have_xray_vision }
      it { is_expected.to have_flight }
    end
  end
end

Woah, now we have perfectly dried up our tests, but how on Krypton does this work?

#describe and #context are aliases and both create classes descended from RSpec::Core::ExampleGroup, but the magic is that each example group is also a subclass of the example group that it is contained in so any methods defined within an example group can be accessed by its children. The innermost example group defined above will return this if you ask for its ancestors:

[RSpec::ExampleGroups::Superhero::WhenGivenXrayVision::WhenGivenFlight, RSpec::ExampleGroups::Superhero::WhenGivenXrayVision, RSpec::ExampleGroups::Superhero, ...]

The other half of this equation is that #let defines a method on the example group it is contained within. This means that every one of the classes listed in that ancestor list has the method #powers defined on it, and through the power of inheritance you can access #powers on the parent example group with the call to super().

RSpec Advanced Subject

So, as we all know, we shouldn't use subject. But what if you really like one liner syntax?

I am working on an attempt to make subject more dynamic and allow the use of one lines in more places. I am doing this by parsing describe arguments and adding a couple new blocks in order to build the subject dynamically.

For example:

describe Object do
  describe '#nil?' do
    it { should be_false }
  end
end

Will be parsed to know that the subject under test is the following: Object.new.nil?

What about class methods and methods with arguments?

describe File do
  describe '.read' do
    when_passed 'file.txt' do
      it { should eq('contents of file.txt') }
    end
  end
end

This will be parsed as File.read('file.txt'). This works by storing the method, what the method is called on, and what is passed to the method and putting it all together within a proc that is called as the hidden subject. A period before the method indicates it is a class method and a hash indicates it is an instance method.

Finally there is a way to pass variables to the initializer:

describe Array do
  when_initialized_with 2 do
    it { should eq [nil, nil] }
  end
end

This is just the basics and I am looking for feedback and more feature ideas. Currently I am trying to figure out passing blocks and how #let fits in.

rspec-advanced_subject