使用instance_double测试#game_over?时调用#check_row失败

问题背景 #

尝试用instance_double测试Game类的#game_over?方法，该方法会调用Board类的#check_row，但测试失败。

测试代码 #

describe '#game_over' do
    subject(:game_over) { described_class.new }
    let(:board) { instance_double(Board) }
    context 'when #game_over is called' do
      it 'calls #check_row in Board' do
        game_over.game_over?
        expect(board).to receive(:check_row)
        #game_over.game_over?
      end
    end
  end

被测试的#game_over?方法 #

def game_over?
    return true if @board.check_row || @board.check_column || @board.check_diagonal || @board.check_antidiagonal

    false
  end

Game类初始化方法 #

def initialize
    @board = Board.new
  end

失败信息 #

1) Game#game_over when #game_over is called calls #check_row in Board
     Failure/Error: expect(board).to receive(:check_row)
     
       (InstanceDouble(Board) (anonymous)).check_row(*(any args))
           expected: 1 time with any arguments
           received: 0 times with any arguments

问题原因 #

1. 测试用的double对象未关联到Game实例：Game初始化时会自行创建一个新的Board实例，测试里定义的board这个instance_double对象并没有被Game使用，两者完全独立。
2. 方法调用与期望设置顺序错误：先调用了game_over.game_over?，之后才设置expect(board).to receive(:check_row)，此时方法已经执行完毕，不会触发后续的期望检测。

解决方案 #

步骤1：修改Game初始化方法，支持依赖注入 #

让Game的初始化可以接受一个可选的Board参数，这样测试时可以传入我们的double对象：

def initialize(board = Board.new)
  @board = board
end

步骤2：调整测试代码，关联double对象并修正执行顺序 #

describe '#game_over?' do
  subject(:game) { described_class.new(board) }
  let(:board) { instance_double(Board) }

  context 'when called' do
    it 'calls #check_row on the board' do
      # 先设置期望
      expect(board).to receive(:check_row)
      # 再调用被测试方法
      game.game_over?
    end
  end
end

额外说明 #

如果需要测试#game_over?的返回值逻辑，还可以给double对象的方法设置返回值，比如：

it 'returns true when check_row returns true' do
  allow(board).to receive(:check_row).and_return(true)
  allow(board).to receive(:check_column).and_return(false)
  allow(board).to receive(:check_diagonal).and_return(false)
  allow(board).to receive(:check_antidiagonal).and_return(false)

  expect(game.game_over?).to be(true)
end

内容的提问来源于stack exchange，提问作者Ruiyot Abby