Ruby: How to add validation for a class field's << method

Question

I understand how to implement a (validating) setter (def item=), but how do I intercept the << operation on a field?

class Bla 
  attr_reader :item

  def initialize
    @item = []
  end

  # only called for =, +=, -= operations (not <<)
  def item=(value)
    puts "Changing value to #{value}"
    # pretend that there is a conditional here
    @item = value
  end

  # This is wrong:
  #def item<<(value)
  #  puts "adding value #{value}"
  #  @item << value
  #end

end

b = Bla.new
b.item = ['one']  # works
b.item += ['one'] # works
b.item << 'two'   # bypasses my setter

I've tried def item<<(value), that doesn't seem to work.

Answer 1

When you call b.item << 'two', you are calling the << method on item directly. So you have a few options here:

1. Implement << directly on your Bla class, then use b << 'two':

   # in class Bla
   def <<(value)
     # do validation here
     @item << value
   end
   

2. Use some other, nicer-named wrapper method name like add_item:

   # in class Bla
   def add_item(value)
     # do validation here
     @item << value
   end
   

3. Use a special array class for @item which has a custom definition for <<:

   class MyArray < Array
     def <<(item)
       # run validation here
       super
     end
   end
   
   # in Bla class
   def initialize
     @item = MyArray.new
   end
   

I would probably go with option 2, it's the most simple and readable.