RSpec 匹配器中的“failure_message_when_negated”是什么意思？答案

【问题标题】：What does 'failure_message_when_negated' mean in RSpec Matchers?RSpec 匹配器中的“failure_message_when_negated”是什么意思？
【发布时间】：2017-03-22 07:00:58
【问题描述】：

看了this answer，没看懂failure_message_when_negated部分。

RSpec::Matchers.define :have_attr_accessor do |field|
  match do |object_instance|
    object_instance.respond_to?(field) &&
      object_instance.respond_to?("#{field}=")
  end

  failure_message do |object_instance|
    "expected attr_accessor for #{field} on #{object_instance}"
  end

  failure_message_when_negated do |object_instance|
    "expected attr_accessor for #{field} not to be defined on #{object_instance}"
  end

  description do
    "assert there is an attr_accessor of the given name on the supplied object"
  end
end

用简单的话来说是什么意思？我真的很感谢清晰而不是 foo-bar 的解释。

【问题讨论】：

标签： ruby-on-rails ruby ruby-on-rails-4 rspec rspec-rails

【解决方案1】：

我想我明白了。

# if the Model has `attr_accessor` named `:promotion_type`
# `it { should_not have_attr_accessor :promotion_type }` test
# will return failure (call this `failure_message_when_negated`)
# because the Model is clearly has `:promotion_type`
failure_message_when_negated do |object_instance|
    "expected attr_accessor for #{field} not to be defined on #{object_instance}"
end

示例失败消息：

1) Promotion should not assert there is an attr_accessor of the given name on the supplied object
     Failure/Error: it { should_not have_attr_accessor :promotion_type }
       expected attr_accessor for promotion_type not to be defined on #<Promotion:0x007fd939c840c8>

【讨论】：