Ruby自定义错误类:消息属性的继承

Question

我似乎找不到有关自定义异常类的太多信息。

我所知道的

您可以声明您的自定义错误类并让它从Standard ardError继承，因此可以是拯救d：

class MyCustomError < StandardError
end

这允许您使用以下方法提升它：

raise MyCustomError, "A message"

稍后，在救援时获取该消息

rescue MyCustomError => e
  puts e.message # => "A message"

我不知道的是

我想给我的异常提供一些自定义字段，但我想从父类继承＜code＞消息@message不是exception类的实例变量，因此我担心我的继承将不起作用。

有人能告诉我更多细节吗？如何使用＜code＞对象

class MyCustomError < StandardError
  attr_reader :object
  def initialize(message, object)
    super(message)
    @object = object
  end
end

然后：

raise MyCustomError.new(anObject), "A message"

得到：

rescue MyCustomError => e
  puts e.message # => "A message"
  puts e.object # => anObject

它会起作用吗，如果它起作用，这是正确的做事方式吗？

Answer 1

你的想法是对的，但你称呼它的方式是错的。应该是

raise MyCustomError.new(an_object, "A message")

Answer 2

鉴于 Exception 的 ruby 核心文档（所有其他错误都从中继承）声明了#message

返回调用exception.to_ s的结果。通常，这会返回异常的消息或名称。通过提供to_ str方法，异常同意在需要字符串的地方使用。

http://ruby-doc.org/core-1.9.3/Exception.html#method-i-message

我会选择将重新定义为_ s/为_str或初始值设定项。下面是一个示例，我们想以一种基本上人类可读的方式知道外部服务何时失败。

注意：下面的第二种策略使用 rails 漂亮的字符串方法，例如解调，这可能有点复杂，因此在异常情况下这样做可能是不明智的。如果需要，还可以向方法签名添加更多参数。

覆盖#to_s策略而不是#to_str，它的工作方式不同

module ExternalService

  class FailedCRUDError < ::StandardError
    def to_s
      'failed to crud with external service'
    end
  end

  class FailedToCreateError < FailedCRUDError; end
  class FailedToReadError < FailedCRUDError; end
  class FailedToUpdateError < FailedCRUDError; end
  class FailedToDeleteError < FailedCRUDError; end
end

控制台输出

begin; raise ExternalService::FailedToCreateError; rescue => e; e.message; end
# => "failed to crud with external service"

begin; raise ExternalService::FailedToCreateError, 'custom message'; rescue => e; e.message; end
# => "failed to crud with external service"

begin; raise ExternalService::FailedToCreateError.new('custom message'); rescue => e; e.message; end
# => "failed to crud with external service"

raise ExternalService::FailedToCreateError
# ExternalService::FailedToCreateError: failed to crud with external service

重写#初始化策略

这是我在rails中使用的最接近实现的策略。如上所述，它使用< code>demodualize 、< code >下划线和< code >人性化 < code > active support 方法。但这可以很容易地消除，就像在以前的策略中一样。

module ExternalService
  class FailedCRUDError < ::StandardError
    def initialize(service_model=nil)
      super("#{self.class.name.demodulize.underscore.humanize} using #{service_model.class}")
    end
  end

  class FailedToCreateError < FailedCRUDError; end
  class FailedToReadError < FailedCRUDError; end
  class FailedToUpdateError < FailedCRUDError; end
  class FailedToDeleteError < FailedCRUDError; end
end

控制台输出

begin; raise ExternalService::FailedToCreateError; rescue => e; e.message; end
# => "Failed to create error using NilClass"

begin; raise ExternalService::FailedToCreateError, Object.new; rescue => e; e.message; end
# => "Failed to create error using Object"

begin; raise ExternalService::FailedToCreateError.new(Object.new); rescue => e; e.message; end
# => "Failed to create error using Object"

raise ExternalService::FailedCRUDError
# ExternalService::FailedCRUDError: Failed crud error using NilClass

raise ExternalService::FailedCRUDError.new(Object.new)
# RuntimeError: ExternalService::FailedCRUDError using Object

演示工具

这是一个演示程序，展示了上述实现的救援和消息传递。引发异常的类是Cloudinary的一个假API。只需将上述策略之一转储到您的rails控制台中，然后执行以下操作。

require 'rails' # only needed for second strategy 

module ExternalService
  class FailedCRUDError < ::StandardError
    def initialize(service_model=nil)
      @service_model = service_model
      super("#{self.class.name.demodulize.underscore.humanize} using #{@service_model.class}")
    end
  end

  class FailedToCreateError < FailedCRUDError; end
  class FailedToReadError < FailedCRUDError; end
  class FailedToUpdateError < FailedCRUDError; end
  class FailedToDeleteError < FailedCRUDError; end
end

# Stub service representing 3rd party cloud storage
class Cloudinary

  def initialize(*error_args)
    @error_args = error_args.flatten
  end

  def create_read_update_or_delete
    begin
      try_and_fail
    rescue ExternalService::FailedCRUDError => e
      e.message
    end
  end

  private def try_and_fail
    raise *@error_args
  end
end

errors_map = [
  # Without an arg
  ExternalService::FailedCRUDError,
  ExternalService::FailedToCreateError,
  ExternalService::FailedToReadError,
  ExternalService::FailedToUpdateError,
  ExternalService::FailedToDeleteError,
  # Instantiated without an arg
  ExternalService::FailedCRUDError.new,
  ExternalService::FailedToCreateError.new,
  ExternalService::FailedToReadError.new,
  ExternalService::FailedToUpdateError.new,
  ExternalService::FailedToDeleteError.new,
  # With an arg
  [ExternalService::FailedCRUDError, Object.new],
  [ExternalService::FailedToCreateError, Object.new],
  [ExternalService::FailedToReadError, Object.new],
  [ExternalService::FailedToUpdateError, Object.new],
  [ExternalService::FailedToDeleteError, Object.new],
  # Instantiated with an arg
  ExternalService::FailedCRUDError.new(Object.new),
  ExternalService::FailedToCreateError.new(Object.new),
  ExternalService::FailedToReadError.new(Object.new),
  ExternalService::FailedToUpdateError.new(Object.new),
  ExternalService::FailedToDeleteError.new(Object.new),
].inject({}) do |errors, args|
  begin 
    errors.merge!( args => Cloudinary.new(args).create_read_update_or_delete)
  rescue => e
    binding.pry
  end
end

if defined?(pp) || require('pp')
  pp errors_map
else
  errors_map.each{ |set| puts set.inspect }
end

Answer 3

raise已设置消息，因此您不必将其传递给构造函数：

class MyCustomError < StandardError
  attr_reader :object

  def initialize(object)
    @object = object
  end
end

begin
  raise MyCustomError.new("an object"), "a message"
rescue MyCustomError => e
  puts e.message # => "a message"
  puts e.object # => "an object"
end

我已将＜code＞救援异常，请查看为什么“救援异常”是一种糟糕的风格=