RSpec 允许/期望与仅期望/and_return

Question

在 RSpec，特别是版本 >= 3 中，有什么区别：

• 使用allow 设置带有返回测试替身的参数的消息预期，然后使用expect 对返回的测试替身进行断言
• 只需使用expect 设置带参数的期望并返回测试替身

还是只是语义？我知道使用expect 提供/指定返回值是the syntax in RSpec mocks 2.13，但据我所知，the syntax changed in RSpec mocks 3 使用allow。

但是，在下面的（通过）示例代码中，使用allow/expect 或仅使用expect/and_return 似乎会产生相同的结果。如果一种语法比另一种更受青睐，也许我会期望会有某种弃用通知，但既然没有，这两种语法似乎都被认为是有效的：

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

如果我故意通过将传入的 baz 参数更改为不同的测试替身来使测试失败，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有任何真正的区别，无论是结果还是表达意图，还是仅仅是语义和/或个人偏好？ allow/expect 应该在一般情况下使用而不是 expect/and_return，因为它似乎是替换语法，还是它们每个都意味着在特定的测试场景中使用？

更新

看了Mori's answer的后，我把上面示例代码中的Foo.bar(baz).qux这一行注释掉了，得到如下错误：

  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 双精度永远不能代替 Foo.bar(baz) 的结果，因此永远不会调用 #qux
• expect 规范在 Foo 从未收到 .bar(baz) 时失败，所以我们甚至没有达到询问 foobar_result 双精度的地步

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

• Mock Objects and Test Stubs
• Test-Specific Extension
• Setting Responses

Answer 1

参见经典文章Mocks Aren't Stubs。 allow 做一个存根，而expect 做一个模拟。那就是allow 允许一个对象返回 X 而不是它会返回未存根的任何东西，而expect 是一个allow 加上 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)

...您告诉规范环境修改Foo 以在收到:bar 和baz 时返回foobar_result。但是当你写

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

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

要查看差异，请尝试在 Foo 没有接收 :bar 和 baz 的示例中。