在Rails中编写可继承属性与基本赋值

对于一个简单的类或模块,没有区别,但是对于更复杂的模块,可能会加载多个其他模块,方法如下 write_inheritable_attribute 可以帮助您轻松可靠地修改对象,而不必担心范围、私有/受保护的方法以及来自ruby元编程的各种干扰,比如 method_missing .

简而言之,当你写 foo.sample = "sample" 在设置属性之前、之后或不设置属性的情况下,可能会发生各种情况,特别是当对象使用ActiveModel或ORM时。当你使用 foo.write_inheritable_attribute(:sample, "sample") 你对发生的事情有更大的控制力。

子类不继承实例变量:

>> class B ; @candy = 1 ; end
>> B.instance_variable_get :@candy          # => 1
>> class C < B ; end
>> C.instance_variable_get :@candy          # => nil

在rails中,可继承属性提供了一个解决方案:

>> class B ; end
>> B.write_inheritable_attribute(:candy, 7) # => 7
>> class C < B ; end
>> C.read_inheritable_attribute(:candy)     # => 7

可继承属性的实现主要是为了解决ruby类变量在类继承中共享的问题。考虑这个例子

class Counter
  @@count = 0
  def self.count
    @@count
  end

  def self.increment
    puts "==> #{self} increment"
    @@count += 1
  end
end

class DogCounter < Counter
end

puts "Counter.count:    #{Counter.count}"
puts "DogCounter.count: #{DogCounter.count} -> nice, DogCounter inherits @@count from Counter"

DogCounter.increment
puts "DogCounter.count: #{DogCounter.count} -> as expected"
puts "Counter.count:    #{Counter.count} -> but Counter.count is also changed!"

Counter.increment
puts "Counter.count:    #{Counter.count}"
puts "DogCounter.count: #{DogCounter.count} -> @@count is shared with all the descendants of Counter"

这将产生这个输出

Counter.count:    0
DogCounter.count: 0 -> nice, DogCounter inherits @@count from Counter
==> DogCounter increment
DogCounter.count: 1 -> as expected
Counter.count:    1 -> but Counter.count is also changed!
==> Counter increment
Counter.count:    2
DogCounter.count: 2 -> @@count is shared with all the descendants of Counter

注意,自从Rails 3.2之后,write_inheritable_属性就被删除了。见 http://dev.mensfeld.pl/2012/01/upgrading-to-rails-3-2-0-from-rails-3-1-3/

使用类属性(以前是可继承属性)我们可以实现如下内容:

class Counter
  class_attribute :count
  self.count = 0

  def self.increment
    puts "==> #{self} increment"
    self.count += 1
  end
end

class DogCounter < Counter
end

puts "Counter.count:    #{Counter.count}"
puts "DogCounter.count: #{DogCounter.count} -> nice, DogCounter inherits count from Counter"

DogCounter.increment
puts "DogCounter.count: #{DogCounter.count} -> as expected"
puts "Counter.count:    #{Counter.count} -> nice, it doesn't change count for Counter"

Counter.increment
puts "Counter.count:    #{Counter.count}"
puts "DogCounter.count: #{DogCounter.count} -> now each subclass can have their own class attribute that inherits default value from the superclass"

这将产生这个输出

Counter.count:    0
DogCounter.count: 0 -> nice, DogCounter inherits count from Counter
==> DogCounter increment
DogCounter.count: 1 -> as expected
Counter.count:    0 -> nice, it doesn't change count for Counter
==> Counter increment
Counter.count:    1
DogCounter.count: 1 -> now each subclass can have their own class attribute that inherits default value from the superclass