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實例公開方法

class_attribute(*attrs, instance_accessor: true, instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil)

宣告繼承自父類別的類別層級屬性。父類別可以變更自身值，不會影響父類別。

選項

  • :instance_reader - 設定實例讀取方法 (預設為 true)

  • :instance_writer - 設定實例寫入方法 (預設為 true)

  • :instance_accessor - 設定實例方法 (預設為 true)

  • :instance_predicate - 設定述詞方法 (預設為 true)

  • :default - 設定屬性的預設值 (預設為 nil)

範例

class Base
  class_attribute :setting
end

class Subclass < Base
end

Base.setting = true
Subclass.setting            # => true
Subclass.setting = false
Subclass.setting            # => false
Base.setting                # => true

在上方的案例中，只要子類別沒有指派值給設定，並執行 Subclass.setting = something，Subclass.setting 會讀取指派給父類別的值。一旦子類別指派值，子類別指派的值會回傳。

此行為與正常的 Ruby 方法繼承相同：將屬性寫入子類別視為覆寫讀取方法。但是，您在使用可變動結構（例如 Array 或 Hash）時，需要搭配 class_attribute 使用。在這些案例中，您不想就地變更。請改用 setter

Base.setting = []
Base.setting                # => []
Subclass.setting            # => []

# Appending in child changes both parent and child because it is the same object:
Subclass.setting << :foo
Base.setting               # => [:foo]
Subclass.setting           # => [:foo]

# Use setters to not propagate changes:
Base.setting = []
Subclass.setting += [:foo]
Base.setting               # => []
Subclass.setting           # => [:foo]

為了方便，實例述詞方法已定義。要略過它，請傳遞 instance_predicate: false。

Subclass.setting?       # => false

實例可以以相同方式覆寫類別值

Base.setting = true
object = Base.new
object.setting          # => true
object.setting = false
object.setting          # => false
Base.setting            # => true

若要取消實例讀取方法，請傳遞：instance_reader: false。

object.setting          # => NoMethodError
object.setting?         # => NoMethodError

若要取消實例寫入方法，請傳遞：instance_writer: false。

object.setting = false  # => NoMethodError

若要取消兩個實例方法，請傳遞：instance_accessor: false。

若要設定屬性的預設值，請傳遞：default:，如下所示

class_attribute :settings, default: {}

來源：顯示 | 於 GitHub

# File activesupport/lib/active_support/core_ext/class/attribute.rb, line 86
  def class_attribute(*attrs, instance_accessor: true,
    instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil)

    class_methods, methods = [], []
    attrs.each do |name|
      unless name.is_a?(Symbol) || name.is_a?(String)
        raise TypeError, "#{name.inspect} is not a symbol nor a string"
      end

      name = name.to_sym
      ::ActiveSupport::ClassAttribute.redefine(self, name, default)

      unless singleton_class?
        methods << <<~RUBY if instance_reader
          silence_redefinition_of_method def #{name}
            defined?(@#{name}) ? @#{name} : self.class.#{name}
          end
        RUBY
      end

      methods << <<~RUBY if instance_writer
        silence_redefinition_of_method(:#{name}=)
        attr_writer :#{name}
      RUBY

      if instance_predicate
        class_methods << "silence_redefinition_of_method def #{name}?; !!self.#{name}; end"
        if instance_reader
          methods << "silence_redefinition_of_method def #{name}?; !!self.#{name}; end"
        end
      end
    end

    location = caller_locations(1, 1).first
    class_eval(["class << self", *class_methods, "end", *methods].join(";").tr("\n", ";"), location.path, location.lineno)
  end

descendants()

回傳與接收程式相符的所有類別。

class C; end
C.descendants # => []

class B < C; end
C.descendants # => [B]

class A < B; end
C.descendants # => [B, A]

class D < C; end
C.descendants # => [B, A, D]

來源：顯示 | 於 GitHub

# File activesupport/lib/active_support/core_ext/class/subclasses.rb, line 19
def descendants
  subclasses.concat(subclasses.flat_map(&:descendants))
end