Sample spec for https://twitter.com/miyohide/status/1060727244719501312

mock_spec.rb

RSpec.describe do
  example do
    dbl = double
    allow(dbl).to receive(:x) do |*args|
      case args.size
      when 1 then 'foo'
      when 2 then 'bar'
      end
    end
    expect(dbl.x(10)).to eq 'foo'
    expect(dbl.x(10, 20)).to eq 'bar'
  end

  example do
    dbl = double
    allow(dbl).to receive(:x) do |arg|
      case arg
      when 1 then 'foo'
      when 2 then 'bar'
      end
    end
    expect(dbl.x(1)).to eq 'foo'
    expect(dbl.x(2)).to eq 'bar'
  end
end