Ruby中的访客模式,还是只使用一个块？

Question

嘿那里,我已经阅读了关于何时/如何使用访问者模式的一些帖子,以及它上面的一些文章/章节,如果你正在遍历AST并且它是高度结构化的,并且你想要封装逻辑分成一个单独的"访问者"对象等.但是使用Ruby,它似乎有点过分,因为你可以使用块来做几乎相同的事情.

我想使用Nokogiri的pretty_print xml.作者建议我使用访问者模式,这需要我创建一个FormatVisitor或类似的东西,所以我可以说"node.accept(FormatVisitor.new)".

问题是,如果我想开始自定义FormatVisitor中的所有内容(例如,它允许您指定节点的选项卡方式,属性如何排序,属性如何间隔等等).

• 有一次,我希望节点为每个嵌套级别都有1个选项卡,并且属性可以按任何顺序排列
• 下一次,我希望节点有2个空格,属性按字母顺序排列
• 下一次,我希望它们有3个空格,每行有两个属性.

我有几个选择:

• 在构造函数中创建选项哈希(FormatVisitor.new({:tabs => 2})
• 在构建访问者之后设置值
• 为每个新实现子类化FormatVisitor
• 或者只使用块,而不是访问者

而不是必须构造一个FormatVisitor,设置值,并将其传递给node.accept方法,为什么不这样做:

node.pretty_print do |format|
  format.tabs = 2
  format.sort_attributes_by {...}
end

这与我觉得访客模式看起来形成鲜明对比:

visitor = Class.new(FormatVisitor) do
  attr_accessor :format
  def pretty_print(node)
    # do something with the text
    @format.tabs = 2 # two tabs per nest level
    @format.sort_attributes_by {...}
  end
end.new
doc.children.each do |child|
  child.accept(visitor)
end

也许我的访客模式都是错误的,但是从我在红宝石中读到的那些,看起来有点矫枉过正.你怎么看？无论哪种方式都适合我,只是想知道你们对它的看法.

非常感谢Lance

Answer 1

实质上,Ruby块是没有额外样板的访问者模式.对于琐碎的情况,块就足够了.

例如,如果要对Array对象执行简单操作,则只需#each使用块调用该方法,而不是实现单独的Visitor类.

但是,在某些情况下实施具体访客模式有一些优势:

• 对于多个类似但复杂的操作,访问者模式提供继承而块不提供.
• Cleaner为Visitor类编写单独的测试套件.
• 将较小的哑类合并为更大的智能类比将复杂的智能类分成更小的哑类更容易.

你的实现似乎有点复杂,Nokogiri希望一个Visitor实例是impelment #visit方法,所以Visitor模式实际上非常适合你的特定用例.以下是访问者模式的基于类的实现:

FormatVisitor实现该#visit方法并使用Formatter子类根据节点类型和其他条件格式化每个节点.

# FormatVisitor implments the #visit method and uses formatter to format
# each node recursively.
class FormatVistor

  attr_reader :io

  # Set some initial conditions here.
  # Notice that you can specify a class to format attributes here.
  def initialize(io, tab: "  ", depth: 0, attributes_formatter_class: AttributesFormatter)
    @io = io
    @tab = tab
    @depth = depth
    @attributes_formatter_class = attributes_formatter_class
  end

  # Visitor interface. This is called by Nokogiri node when Node#accept
  # is invoked.
  def visit(node)
    NodeFormatter.format(node, @attributes_formatter_class, self)
  end

  # helper method to return a string with tabs calculated according to depth
  def tabs
    @tab * @depth
  end

  # creates and returns another visitor when going deeper in the AST
  def descend
    self.class.new(@io, {
      tab: @tab,
      depth: @depth + 1,
      attributes_formatter_class: @attributes_formatter_class
    })
  end
end

这里执行AttributesFormatter上面使用的.

# This is a very simple attribute formatter that writes all attributes
# in one line in alphabetical order. It's easy to create another formatter
# with the same #initialize and #format interface, and you can then
# change the logic however you want.
class AttributesFormatter
  attr_reader :attributes, :io

  def initialize(attributes, io)
    @attributes, @io = attributes, io
  end

  def format
    return if attributes.empty?

    sorted_attribute_keys.each do |key|
      io << ' ' << key << '="' << attributes[key] << '"'
    end
  end

  private

  def sorted_attribute_keys
    attributes.keys.sort
  end
end

NodeFormatters使用Factory模式为特定节点实例化正确的格式化程序.在这种情况下,我将文本节点,叶元素节点,元素节点与文本和常规元素节点区分开来.每种类型都有不同的格式要求.另请注意,这不完整,例如注释节点不被考虑在内.

class NodeFormatter
  # convience method to create a formatter using #formatter_for
  # factory method, and calls #format to do the formatting.
  def self.format(node, attributes_formatter_class, visitor)
    formatter_for(node, attributes_formatter_class, visitor).format
  end

  # This is the factory that creates different formatters
  # and use it to format the node
  def self.formatter_for(node, attributes_formatter_class, visitor)
    formatter_class_for(node).new(node, attributes_formatter_class, visitor)
  end

  def self.formatter_class_for(node)
    case
    when text?(node)
      Text
    when leaf_element?(node)
      LeafElement
    when element_with_text?(node)
      ElementWithText
    else
      Element
    end
  end

  # Is the node a text node? In Nokogiri a text node contains plain text
  def self.text?(node)
    node.class == Nokogiri::XML::Text
  end

  # Is this node an Element node? In Nokogiri an element node is a node
  # with a tag, e.g. <img src="foo.png" /> It can also contain a number
  # of child nodes
  def self.element?(node)
    node.class == Nokogiri::XML::Element
  end

  # Is this node a leaf element node? e.g. <img src="foo.png" />
  # Leaf element nodes should be formatted in one line.
  def self.leaf_element?(node)
    element?(node) && node.children.size == 0
  end

  # Is this node an element node with a single child as a text node.
  # e.g. <p>foobar</p>. We will format this in one line.
  def self.element_with_text?(node)
    element?(node) && node.children.size == 1 && text?(node.children.first)
  end

  attr_reader :node, :attributes_formatter_class, :visitor

  def initialize(node, attributes_formatter_class, visitor)
    @node = node
    @visitor = visitor
    @attributes_formatter_class = attributes_formatter_class
  end

  protected

  def attribute_formatter
    @attribute_formatter ||= @attributes_formatter_class.new(node.attributes, io)
  end

  def tabs
    visitor.tabs
  end

  def io
    visitor.io
  end

  def leaf?
    node.children.empty?
  end

  def write_tabs
    io << tabs
  end

  def write_children
    v = visitor.descend
    node.children.each { |child| child.accept(v) }
  end

  def write_attributes
    attribute_formatter.format
  end

  def write_open_tag
    io << '<' << node.name
    write_attributes
    if leaf?
      io << '/>'
    else
      io << '>'
    end
  end

  def write_close_tag
    return if leaf?
    io << '</' << node.name << '>'
  end

  def write_eol
    io << "\n"
  end

  class Element < self
    def format
      write_tabs
      write_open_tag
      write_eol
      write_children
      write_tabs
      write_close_tag
      write_eol
    end
  end

  class LeafElement < self
    def format
      write_tabs
      write_open_tag
      write_eol
    end
  end

  class ElementWithText < self
    def format
      write_tabs
      write_open_tag
      io << text
      write_close_tag
      write_eol
    end

    private

    def text
      node.children.first.text
    end
  end

  class Text < self
    def format
      write_tabs
      io << node.text
      write_eol
    end
  end
end

要使用此类:

xml = "<root><aliens><alien><name foo=\"bar\">Alf<asdf/></name></alien></aliens></root>"
doc = Nokogiri::XML(xml)

# the FormatVisitor accepts an IO object and writes to it 
# as it visits each node, in this case, I pick STDOUT.
# You can also use File IO, Network IO, StringIO, etc.
# As long as it support the #puts method, it will work.
# I'm using the defaults here. ( two spaces, with starting depth at 0 )
visitor = FormatVisitor.new(STDOUT)

# this will allow doc ( the root node ) to call visitor.visit with
# itself. This triggers the visiting of each children recursively
# and contents written to the IO object. ( In this case, it will
# print to STDOUT.
doc.accept(visitor)

# Prints:
# <root>
#   <aliens>
#     <alien>
#       <name foo="bar">
#         Alf
#         <asdf/>
#       </name>
#     </alien>
#   </aliens>
# </root>

使用上面的代码,您可以通过构造NodeFromatters的额外子类来更改节点格式化行为,并将它们插入到工厂方法中.您可以通过各种实现来控制属性的格式AttributesFromatter.只要您遵守其界面,就可以将其插入attributes_formatter_class参数而无需修改任何其他内容.

使用的设计模式列表:

• 访问者模式:处理节点遍历逻辑.(也是Nokogiri的接口要求.)
• 工厂模式,用于根据节点类型和其他格式条件确定格式化程序.注意,如果您不喜欢上面的类方法NodeFormatter,可以将它们提取到NodeFormatterFactory更合适的位置.
• 依赖注入(DI/IoC),用于控制属性的格式.

这演示了如何将几种模式组合在一起以实现所需的灵活性.虽然,如果你需要这些灵活性,你必须要做出决定.

Answer 2

我会选择简单而有效的方法.我不知道细节,但你写的与访客模式相比,看起来更简单.如果它也适合你,我会用它.就个人而言,我厌倦了所有这些技术,要求你创建一个巨大的相互关联的"网络",只是为了解决一个小问题.

有些人会说,是的,但是如果你使用模式来做,那么你可以涵盖许多未来的需求,等等等等.我说,现在做什么有效,如果需要,你可以在将来进行重构.在我的项目中,几乎从未出现过,但这是一个不同的故事.