防止对象被扩展答案

【问题标题】：Prevent object from being extended防止对象被扩展
【发布时间】：2020-11-04 14:56:47
【问题描述】：

我有一个模块，它通过点语法为哈希提供一些惰性查找：

module DotHash
  def method_missing(method_name, *args, &block)
    # look for keys...
  end

  def respond_to_missing?(method_name, _include_all = nil)
    # some logic
  end
end

遇到了不小心扩展nil的问题：

# @hash == nil
@hash.extend(DotHash)

这引起了巨大的问题，因为现在nil 有这个method_missing 逻辑，这把事情搞砸了。

我虽然添加一个钩子可以解决问题：

module DotHash
  def self.extended(base)
    return if base.is_a?(Hash)
    raise "must be a hash"
  end

  def method_missing(method_name, *args, &block)
    # look for keys...
  end

  def respond_to_missing?(method_name, _include_all = nil)
    # some logic
  end
end

确实，它会引发错误：

[1] pry(main)> nil.extend(DotHash)
RuntimeError: must be a hash

但还是添加了逻辑：

[2] pry(main)> nil.foobar
Traceback (most recent call last):
    9707: from bin/rails:6:in `<main>'
    9706: from /usr/local/bundle/gems/activesupport-5.2.4.3/lib/active_support/dependencies.rb:291:in `require'
    9705: from /usr/local/bundle/gems/activesupport-5.2.4.3/lib/active_support/dependencies.rb:257:in `load_dependency'
    9704: from /usr/local/bundle/gems/activesupport-5.2.4.3/lib/active_support/dependencies.rb:291:in `block in require'
    9703: from /usr/local/bundle/gems/bootsnap-1.4.5/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:30:in `require'
    9702: from /usr/local/bundle/gems/bootsnap-1.4.5/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:21:in `require_with_bootsnap_lfi'
    9701: from /usr/local/bundle/gems/bootsnap-1.4.5/lib/bootsnap/load_path_cache/loaded_features_index.rb:92:in `register'
    9700: from /usr/local/bundle/gems/bootsnap-1.4.5/lib/bootsnap/load_path_cache/core_ext/kernel_require.rb:22:in `block in require_with_bootsnap_lfi'
     ... 9695 levels...
       4: from /usr/src/app/app/lib/dot_hash.rb:26:in `respond_to_missing?'
       3: from /usr/src/app/app/lib/dot_hash.rb:14:in `method_missing'
       2: from /usr/src/app/app/lib/dot_hash.rb:26:in `respond_to_missing?'
       1: from /usr/src/app/app/lib/dot_hash.rb:14:in `method_missing'
/usr/src/app/app/lib/mapper/dot_hash.rb:26:in `respond_to_missing?': stack level too deep (SystemStackError)

是否有一个钩子会在对象被扩展之前触发，而不是之后？

【问题讨论】：

标签： ruby module

【解决方案1】：

您可以覆盖extend_object:（文档包含类似示例）

通过添加此模块的常量和方法（作为单例方法添加）来扩展指定对象。这是Object#extend使用的回调方法。

module DotHash
  def self.extend_object(obj)
    raise TypeError, 'must be a hash' unless obj.is_a?(Hash)
    
    super
  end

  def foo
    123
  end
end

h = {}
h.extend(DotHash)
h.foo #=> 123

n = nil
n.extend(DotHash) # TypeError: must be a hash
n.foo             # NoMethodError: undefined method `foo' for nil:NilClass

【讨论】：