transform_spec.rb

# the point is to run the costly operation just once yet still be able to use
# convenient "should change by" syntax.

def transform
  # sleep 2
  $x = 11
  $y = 22
  $z = 33
end

describe 'transform' do
  before do
    $x = 1
    $y = 2
    $z = 3
  end

  context 'redundant' do
    it do
      $x.should == 1
      $y.should == 2
      $z.should == 3

      transform

      $x.should == 11
      $y.should == 22
      $z.should == 33
    end
  end

  context 'slow' do
    it { expect { transform }.to change { $x }.by(10) }
    it { expect { transform }.to change { $y }.by(20) }
    it { expect { transform }.to change { $z }.by(30) }
  end

  context 'ugly' do
    it do
      expect do
        expect do
          expect do
            transform
          end.to change { $x }.by(10)
        end.to change { $y }.by(20)
      end.to change { $z }.by(30)
    end
  end

  context 'change chain' do
    it do
      ch = ChangeChain.new { transform }
      ch.should change { $x }.by(10)
      ch.should change { $y }.by(20)
      ch.should change { $z }.by(30)
      ch.assert
    end
  end

  context 'change chain 2' do
    it do
      change_chain -> { transform },
        change { $x }.by(10),
        change { $y }.by(20),
        change { $z }.by(30)
    end
  end

  context 'perfect' do
    it do
      pending '???'
    end
  end
end

class ChangeChain
  def initialize(&block)
    @last_block = block
  end

  def should(change)
    last_block = @last_block
    @last_block = -> { -> { last_block.call }.should change }
  end

  def assert
    @last_block.call
  end
end

def change_chain(block, *changes)
  ch = ChangeChain.new(&block)
  changes.each { |change| ch.should change }
  ch.assert
end