Question

所以我有这种情况，我想以编程方式定义一堆类。我在下面使用的方法工作正常，除了我从这里继承的第三方类不喜欢这个类是匿名定义的事实（基本上，它没有它需要的信息，即在我有机会将匿名类设置为常量之前，在'inherited'钩子中的类名。

['one', 'two', 'three'].each do |model|
    cls = Class.new(ThirdPartyClass) do
        define_method :model do
            model
        end
    end
    ThirdPartyClass.const_set(model.capitalize, cls)
end

我可以简单地使用eval并将类定义为：

['one', 'two', 'three'].each do |model|
    eval "class ThirdPartyClass::#{model.capitalize} < ThirdPartyClass; ...; end"
end

但我不喜欢它，因为那时它是凌乱的字符串插值。基于块的方法似乎“更好”，从美学角度讲。

有没有什么方法可以使用基于块的语法非匿名地定义该类（即从头开始使用名称），或者我注定要留在eval杂乱的字符串输入的土地上？

Answer 1

创建类时会调用Class#inherited回调。在实例化匿名类时，这将始终在将类分配给常量之前，为其指定名称。我看不出有什么方法可以解决这个问题。您可以通过使用eval创建一个空类来使>最小化字符串eval中的代码量，使用特殊的类语法使名称在inherited之前可见，然后在块中跟进class_eval用于定义方法的表单。

class Super
  def self.inherited(child)
    puts "#{self.name} inherited by #{child.inspect} named '#{child.name}'"
  end
end

# Your way, inherited can't see name
['one', 'two', 'three'].each do |model|
  klassname = model.capitalize
  klass = Class.new(Super) do
    puts "defining #{model} inheriting from Super"
    define_method :model do
      model
    end
  end
  Super.const_set(klassname, klass)
end

# this way inherited sees name because we use special class definition syntax in minimal string eval
['four', 'five', 'six'].each do |model|
  klassname = model.capitalize

  eval %Q{
    class Super::#{klassname} < Super
      puts %Q[defining Super::#{klassname} inheriting from Super]
    end
  }

  Super.const_get(klassname).class_eval do
    puts "defining methods for Super::#{klassname} inheriting from Super"
    define_method :model do
      model
    end
  end
end

# produces:
Super inherited by #<Class:0x0000010084c988> named ''
defining one inheriting from Super
Super inherited by #<Class:0x0000010084c640> named ''
defining two inheriting from Super
Super inherited by #<Class:0x0000010084c320> named ''
defining three inheriting from Super
Super inherited by Super::Four named 'Super::Four'
defining Super::Four inheriting from Super
defining methods for Super::Four inheriting from Super
Super inherited by Super::Five named 'Super::Five'
defining Super::Five inheriting from Super
defining methods for Super::Five inheriting from Super
Super inherited by Super::Six named 'Super::Six'
defining Super::Six inheriting from Super
defining methods for Super::Six inheriting from Super

带块的动态类定义

1 个答案: