ruby - 如何使用acts_like_string？在 ruby

Question

http://api.rubyonrails.org/classes/String.html#method-i-acts_like_string-3F

我正在研究 ruby​​ 中的 acts_like_string?，但找不到任何有关如何使用的示例。我试过了，但没有成功。

puts str.acts_like_string?

Answer 1

这些是在我的 gems 文件夹中搜索 acts_like_string 的结果，grep --recursive --context 2 acts_like_string $(gem environment gemdir):

activesupport-4.1.0.rc1/lib/active_support/core_ext/string/behavior.rb-class String
activesupport-4.1.0.rc1/lib/active_support/core_ext/string/behavior.rb-  # Enable more predictable duck-typing on String-like classes. See <tt>Object#acts_like?</tt>.
activesupport-4.1.0.rc1/lib/active_support/core_ext/string/behavior.rb:  def acts_like_string?
activesupport-4.1.0.rc1/lib/active_support/core_ext/string/behavior.rb-    true
activesupport-4.1.0.rc1/lib/active_support/core_ext/string/behavior.rb-  end
--
activesupport-4.1.0.rc1/lib/active_support/multibyte/chars.rb-      alias to_str wrapped_string
activesupport-4.1.0.rc1/lib/active_support/multibyte/chars.rb-
activesupport-4.1.0.rc1/lib/active_support/multibyte/chars.rb:      delegate :<=>, :=~, :acts_like_string?, :to => :wrapped_string
activesupport-4.1.0.rc1/lib/active_support/multibyte/chars.rb-
activesupport-4.1.0.rc1/lib/active_support/multibyte/chars.rb-      # Creates a new Chars instance by wrapping _string_.
--
mail-2.5.4/lib/mail/multibyte/chars.rb-
mail-2.5.4/lib/mail/multibyte/chars.rb-      # Enable more predictable duck-typing on String-like classes. See Object#acts_like?.
mail-2.5.4/lib/mail/multibyte/chars.rb:      def acts_like_string?
mail-2.5.4/lib/mail/multibyte/chars.rb-        true
mail-2.5.4/lib/mail/multibyte/chars.rb-      end

如您所见，acts_like_string? 为 ActiveSupport::Multibyte::Chars 返回 true .它解决了哪个问题？

假设您必须检查某个对象是否是 String:您会编写类似 object.is_a 的代码吗？字符串;但通过这种方式，您可以排除那些不由 String 继承但可以作为字符串计算的类，例如 f.e. ActiveSupport::Multibyte::Chars，它增强了一些字符串方法并将每个缺失的方法委托(delegate)给它的 @wrapped_string 实例变量。

而不是 object.is_a？ String 您可以使用 object.acts_like?(:string):这可以正常工作，因为 ActiveSupport 定义了(我的评论):

# Provides acts_like? for every Ruby object, so you can call it
# on everything without worrying about its presence
class Object
  def acts_like?(duck)
    respond_to? :"acts_like_#{duck}?"
  end
end

# Provides acts_like_string? for String and every class which inherits by String
class String
  def acts_like_string?
    # this actually could be false or nil or whatever,
    # since acts_like? checks only the method presence
    true
  end
end

# Provides acts_like_string? ActiveSupport::Multibyte::Chars, delegating it
# to @wrapped_string instance variable (which in turn defines it if it is
# a String)
class ActiveSupport::Multibyte::Chars
  delegate :<=>, :=~, :acts_like_string?, :to => :wrapped_string
end

这允许您编写例如:

gem 'activesupport'
require 'active_support/all'

def method_which_works_only_with_a_string_argument(argument)
  unless argument.acts_like? :string
    raise ArgumentError, 'argument must act like a string'
  end
  argument.capitalize
end

argument = ActiveSupport::Multibyte::Chars.new 'über'
method_which_works_only_with_a_string_argument argument
# => "Über"
method_which_works_only_with_a_string_argument 123
# => ArgumentError: argument must act like a string

例如，ActiveRecord 在内部使用它 values sanitization :

[...]
def quote_bound_value(value, c = connection, column = nil) #:nodoc:
  if column
    c.quote(value, column)
  elsif value.respond_to?(:map) && !value.acts_like?(:string)
    if value.respond_to?(:empty?) && value.empty?
      c.quote(nil)
    else
      value.map { |v| c.quote(v) }.join(',')
    end
  else
    c.quote(value)
  end
end
[...]