本质上，Ruby 块是没有额外样板的访问者模式。对于琐碎的情况，一个块就足够了。
例如，如果您想对 Array 对象执行简单的操作，您只需#each使用块调用该方法，而不是实现单独的 Visitor 类。
然而，在某些情况下实现一个具体的访问者模式是有优势的：
- 对于多个相似但复杂的操作，访问者模式提供继承而块不提供。
- 为访客类编写单独的测试套件的清洁工。
- 将更小的哑类合并到更大的智能类中总是比将复杂的智能类分成更小的哑类更容易。
您的实现似乎有点复杂，并且 Nokogiri 需要一个执行#visit方法的访问者实例，因此访问者模式实际上非常适合您的特定用例。这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来格式化每个节点。
# 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AttributesFormatter使用上面的实现。
# 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 使用工厂模式为特定节点实例化正确的格式化程序。在这种情况下，我区分了文本节点、叶元素节点、带有文本的元素节点和常规元素节点。每种类型都有不同的格式要求。另请注意，这并不完整，例如未考虑评论节点。
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
要使用这个类：
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>
NodeFromatter使用上面的代码，您可以通过构造额外的s 子类并将它们插入工厂方法来更改节点格式化行为。您可以使用AttributesFromatter. 只要您遵守它的接口，您就可以将其插入attributes_formatter_class参数中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化条件确定格式化程序。请注意，如果您不喜欢 上的类方法
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化。
这演示了如何将几个模式组合在一起以实现所需的灵活性。虽然，如果你需要那些灵活性是你必须决定的。