Question

在RSpec中，特别是版本＆gt; = 3，是否有任何区别：

使用allow设置带有返回测试双精度的参数的消息期望，然后使用expect对返回的测试进行双重断言
只需使用expect设置参数的期望值并返回测试结果

还是只是语义？我知道提供/指定expect的返回值为the syntax in RSpec mocks 2.13，但据我所见，the syntax changed in RSpec mocks 3使用allow。

但是，在下面的（传递）示例代码中，使用allow / expect或expect / and_return似乎会生成相同的结果。如果一种语法比另一种语言更受青睐，也许我会期望有某种弃用通知，但由于没有，所以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

如果我故意通过将期望中的传入baz参数更改为不同的测试双精度来使测试失败，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差异，无论是结果还是表达意图，还是只是语义和/或个人偏好？ allow / expect一般应该使用expect / and_return，因为它似乎是替换语法，或者它们中的每一个都是要使用的在特定的测试场景中？

更新

在阅读Mori's answer后，我从上面的示例代码中注释掉了Foo.bar(baz).qux行，并收到以下错误：

  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永远不会代表Foo.bar(baz)的结果，因此永远不会#qux调用它
expect规范在Foo从未接收.bar(baz)时失败，因此我们甚至无法审问foobar_result双重

有道理：它不仅仅是语法更改，expect / and_return的目的与allow / expect不同。我真的应该检查一下最明显的地方：RSpec Mocks README，特别是以下几节：

Answer 1

请参阅经典文章Mocks Aren't Stubs。 allow制作存根，expect进行模拟。即allow允许对象返回X而不是返回未被取消的任何对象，而expect是allow 加上对某个州或事件的期望。当你写

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

...当您通过Foo收到foobar_result时，您告诉规范环境修改:bar以返回baz。但是当你写作

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 baz。

要查看差异，请在Foo <{1}} :bar的示例中尝试这两种情况。

RSpec允许/期望vs期望/ and_return

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