RSpec allow/expect 对比 expect/and_return

Question

在RSpec中，具体版本>=3，有什么区别：

使用 allow 设置消息预期，参数为 return 测试替身，然后使用 expect 对 return 测试替身做出断言
只需使用 expect 设置带参数的期望和 return 测试替身

还是只是语义？我知道 providing/specifying return 值 expect 是 the syntax in RSpec mocks 2.13, but as far as I can see, the syntax changed in RSpec mocks 3 使用 allow。

但是，在下面的（传递的）示例代码中，使用 allow/expect 或仅使用 expect/and_return 似乎会生成相同的结果。如果一种语法比另一种语法更受欢迎，也许我会期望有某种弃用通知，但由于没有，似乎两种语法都被认为是有效的：

class Foo
  def self.bar(baz)
    # not important what happens to baz parameter
    # only important that it is passed in
    new
  end

  def qux
    # perform some action
  end
end

class SomethingThatCallsFoo
  def some_long_process(baz)
    # do some processing
    Foo.bar(baz).qux
    # do other processing
  end
end

describe SomethingThatCallsFoo do
  let(:foo_caller) { SomethingThatCallsFoo.new }

  describe '#some_long_process' do
    let(:foobar_result) { double('foobar_result') }
    let(:baz) { double('baz') }

    context 'using allow/expect' do
      before do
        allow(Foo).to receive(:bar).with(baz).and_return(foobar_result)
      end

      it 'calls qux method on result of Foo.bar(baz)' do
        expect(foobar_result).to receive(:qux)
        foo_caller.some_long_process(baz)
      end
    end

    context 'using expect/and_return' do
      it 'calls qux method on result of Foo.bar(baz)' do
        expect(Foo).to receive(:bar).with(baz).and_return(foobar_result)
        expect(foobar_result).to receive(:qux)
        foo_caller.some_long_process(baz)
      end
    end
  end
end

如果我通过将期望中传入的 baz 参数更改为不同的测试替身来故意使测试失败，则错误几乎相同：

  1) SomethingThatCallsFoo#some_long_process using allow/expect calls quux method on result of Foo.bar(baz)
     Failure/Error: Foo.bar(baz).qux
       <Foo (class)> received :bar with unexpected arguments
         expected: (#<RSpec::Mocks::Double:0x3fe97a0127fc @name="baz">)
              got: (#<RSpec::Mocks::Double:0x3fe97998540c @name=nil>)
        Please stub a default value first if message might be received with other args as well.
     # ./foo_test.rb:16:in `some_long_process'
     # ./foo_test.rb:35:in `block (4 levels) in <top (required)>'

  2) SomethingThatCallsFoo#some_long_process using expect/and_return calls quux method on result of Foo.bar(baz)
     Failure/Error: Foo.bar(baz).qux
       <Foo (class)> received :bar with unexpected arguments
         expected: (#<RSpec::Mocks::Double:0x3fe979935fd8 @name="baz">)
              got: (#<RSpec::Mocks::Double:0x3fe979cc5c0c @name=nil>)
     # ./foo_test.rb:16:in `some_long_process'
     # ./foo_test.rb:43:in `block (4 levels) in <top (required)>'

那么，这两个测试之间是否存在任何真正的差异，无论是结果还是表达的意图，或者只是语义 and/or 个人偏好？ allow/expect 是否应该在 expect/and_return 上使用，因为它看起来像是替换语法，或者它们中的每一个都应该用于特定的测试场景？

更新

看完之后，我注释掉了上面示例代码中的Foo.bar(baz).qux行，得到了以下错误：

  1) SomethingThatCallsFoo#some_long_process using allow/expect calls qux method on result of Foo.bar(baz)
     Failure/Error: expect(foobar_result).to receive(:qux)
       (Double "foobar_result").qux(any args)
           expected: 1 time with any arguments
           received: 0 times with any arguments
     # ./foo_test.rb:34:in `block (4 levels) in <top (required)>'

  2) SomethingThatCallsFoo#some_long_process using expect/and_return calls qux method on result of Foo.bar(baz)
     Failure/Error: expect(Foo).to receive(:bar).with(baz).and_return(foobar_result)
       (<Foo (class)>).bar(#<RSpec::Mocks::Double:0x3fc211944fa4 @name="baz">)
           expected: 1 time with arguments: (#<RSpec::Mocks::Double:0x3fc211944fa4 @name="baz">)
           received: 0 times
     # ./foo_test.rb:41:in `block (4 levels) in <top (required)>'

allow 规范失败，因为 foobar_result double 永远无法代替 Foo.bar(baz) 的结果，因此 #qux 也从未调用它
expect 规范在 Foo 从未收到 .bar(baz) 时失败，因此我们甚至没有达到询问 foobar_result double

有道理：这不仅仅是语法更改，而且 expect/and_return 的目的确实不同于 allow/expect。我真的应该检查最明显的地方：RSpec Mocks README，特别是以下部分：

Answer 1

看经典文章Mocks Aren't Stubs。 allow 制作存根，而 expect 制作模拟。也就是说 allow 允许一个对象 return X 而不是任何它会 return unstubbed 的对象，并且 expect 是一个 allow plus 对某些状态或事件的期望。当你写

allow(Foo).to receive(:bar).with(baz).and_return(foobar_result)

...您告诉规范环境在收到 :bar 和 baz 时将 Foo 修改为 return foobar_result。但是当你写

expect(Foo).to receive(:bar).with(baz).and_return(foobar_result)

...您正在做同样的事情，并告诉规范失败除非 Foo 收到 :bar 和 baz。

要查看差异，请在 Foo 不接收 :bar 和 baz 的示例中尝试两者。

RSpec allow/expect 对比 expect/and_return

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