The wording on the Message Chains
page is very careful to
warn that receive_message_chain
makes it painlessly easy to break the Law of
Demeter. They even go on to explain that double and instance double offer
a better way of testing code with these chained method patterns.
double
and receive
in place of receive_message_chain
Using The tests in question look like this
describe Game do
subject(:game) { Game.new() }
describe '#done' do
context 'the game fairy says there is a tie' do
it 'returns false' do
allow(GameFairyGateway).to receive_message_chain(:get_fairy, :proclamation) { false }
expect(game.done?).to be false
end
end
context 'the game fairy says the player_1 is the winner' do
it 'returns true' do
allow(GameFairyGateway).to receive_message_chain(:get_fairy, :proclamation) { true }
expect(game.done?).to be true
end
end
end
end
and they are for the following code
class GrumpyGameFairy < StandardError
def initialize(msg="Outlook is cloudy, try again later")
super
end
end
class GameFairy
def proclamation
if mystic_visions == 'The game is in a stalemate!'
false
elsif mystic_visions == 'The game is won!'
true
else
raise GrumpyGameFairy
end
end
private
def mystic_visions
"Leave me alone!"
end
end
class GameFairyGateway
def self.get_fairy
game_fairy = GameFairy.new()
end
end
class Game
def done?
return GameFairyGateway.get_fairy.proclamation
end
end
Again, we want to avoid using the receive_message_chain
helper method. In its place we want to use a double for our
GameFairy
. To that end we'll add game_fairy
as a helper methods via let.
describe '#done' do
let(:game_fairy) { double }
This gives us a double to use in place of the real game fairy, but we still need to stub out the methods being used by
the Game
class. We'll do this with allow
and receive
. We'll put those stubs into a before block in the root
decribe so that both contexts can use it, and we'll add another helper method to dictate what value should be returned
by proclamation
.
before do
allow(game_fairy).to receive(:proclamation).and_return( proclamation_output )
allow(GameFairyGateway).to receive(:get_fairy).and_return(game_fairy)
end
Then we change our test like so.
context 'the game fairy says there is a tie' do
let(:proclamation_output) { false }
it 'returns false' do
expect(game.done?).to be false
end
end
Note that we've removed the receive_message_chain
stubbing, and we've added the proclamation_output
helper
declaration.
Conclusion
This way might be a little less natural, but it's the recommendedation made by the RSpec documentation. You may also
want to check your code for tight coupling of objects if you're using receive_message_chain
often.
You can find this code on my repo.
You can watch me converting an example on YouTube
Thanks for reading!