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DeprecatedConstantAccessor 透過連結 const_missing 將常數轉換為已棄用的常數。

它採用舊（已棄用）常數和新常數（兩者都以字串形式表示）的名稱，以及一個棄用函式。

已棄用的常數現在會傳回與新常數相同的物件，而不是代理物件，因此它可以在 rescue 區塊等中透明地使用。

PLANETS = %w(mercury venus earth mars jupiter saturn uranus neptune pluto)

# (In a later update, the original implementation of `PLANETS` has been removed.)

PLANETS_POST_2006 = %w(mercury venus earth mars jupiter saturn uranus neptune)
include ActiveSupport::Deprecation::DeprecatedConstantAccessor
deprecate_constant 'PLANETS', 'PLANETS_POST_2006', deprecator: ActiveSupport::Deprecation.new

PLANETS.map { |planet| planet.capitalize }
# => DEPRECATION WARNING: PLANETS is deprecated! Use PLANETS_POST_2006 instead.
     (Backtrace information…)
     ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
方法
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類別公開方法

included(base)

來源：顯示 | 在 GitHub 上 

# File activesupport/lib/active_support/deprecation/constant_accessor.rb, line 28
def self.included(base)
  require "active_support/inflector/methods"

  extension = Module.new do
    def const_missing(missing_const_name)
      if class_variable_defined?(:@@_deprecated_constants)
        if (replacement = class_variable_get(:@@_deprecated_constants)[missing_const_name.to_s])
          replacement[:deprecator].warn(replacement[:message] || "#{name}::#{missing_const_name} is deprecated! Use #{replacement[:new]} instead.", caller_locations)
          return ActiveSupport::Inflector.constantize(replacement[:new].to_s)
        end
      end
      super
    end

    def deprecate_constant(const_name, new_constant, message: nil, deprecator: nil)
      ActiveSupport.deprecator.warn("DeprecatedConstantAccessor.deprecate_constant without a deprecator is deprecated") unless deprecator
      deprecator ||= ActiveSupport::Deprecation._instance
      class_variable_set(:@@_deprecated_constants, {}) unless class_variable_defined?(:@@_deprecated_constants)
      class_variable_get(:@@_deprecated_constants)[const_name.to_s] = { new: new_constant, message: message, deprecator: deprecator }
    end
  end
  base.singleton_class.prepend extension
end

執行個體公開方法

const_missing(missing_const_name)

來源：顯示 | 在 GitHub 上 

# File activesupport/lib/active_support/deprecation/constant_accessor.rb, line 32
def const_missing(missing_const_name)
  if class_variable_defined?(:@@_deprecated_constants)
    if (replacement = class_variable_get(:@@_deprecated_constants)[missing_const_name.to_s])
      replacement[:deprecator].warn(replacement[:message] || "#{name}::#{missing_const_name} is deprecated! Use #{replacement[:new]} instead.", caller_locations)
      return ActiveSupport::Inflector.constantize(replacement[:new].to_s)
    end
  end
  super
end

deprecate_constant(const_name, new_constant, message: nil, deprecator: nil)

來源：顯示 | 在 GitHub 上 

# File activesupport/lib/active_support/deprecation/constant_accessor.rb, line 42
def deprecate_constant(const_name, new_constant, message: nil, deprecator: nil)
  ActiveSupport.deprecator.warn("DeprecatedConstantAccessor.deprecate_constant without a deprecator is deprecated") unless deprecator
  deprecator ||= ActiveSupport::Deprecation._instance
  class_variable_set(:@@_deprecated_constants, {}) unless class_variable_defined?(:@@_deprecated_constants)
  class_variable_get(:@@_deprecated_constants)[const_name.to_s] = { new: new_constant, message: message, deprecator: deprecator }
end