From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2011-10-20T23:51:29+09:00
Subject: Re: Call method on attribute update

On Thu, Oct 20, 2011 at 8:01 AM, Gary Wright <gwtmp01@mac.com> wrote:
>
> On Oct 20, 2011, at 1:38 AM, Jason Lillywhite wrote:
>> If @yn is updated, automatically run method "foo.calcArea". The way I
>> have it now requires me to do:
>>
>> foo.yn = 100.0
>> foo.calcArea
>>
>> There must be a way to let ruby call that method for me.
>
>
> Just define the attribute writer method yourself instead of using the helpers:
>
> class Foo
>  attr_reader :yn
>  def yn=(value)
>    @yn = value
>    calcArea
>  end
>
>  def calcArea
>     puts "recomputing area"
>  end
> end
>
>>> f = Foo.new      #=> #<Foo:0x1895458>
>>> f.yn = 100       #=> 100
> recomputing area
>>> f.yn             #=> 100
>
>
> Gary Wright
>
>
>

If you find yourself doing that a lot, you could create a little utility:

class Class
    def attr_writer_with_callbacks attr, *callbacks
        instance_eval do
            define_method("#{attr}=") do |value|
                instance_variable_set "@#{attr}", value
                callbacks.each {|callback| send callback}
            end
        end
    end
end

ruby-1.8.7-p334 :038 > class A
ruby-1.8.7-p334 :039?>   attr_writer_with_callbacks :test, :updated, :modified
ruby-1.8.7-p334 :040?>   def updated
ruby-1.8.7-p334 :041?>     puts "updated"
ruby-1.8.7-p334 :042?>     end
ruby-1.8.7-p334 :043?>   def modified
ruby-1.8.7-p334 :044?>     puts "also modified"
ruby-1.8.7-p334 :045?>     end
ruby-1.8.7-p334 :046?>   end
 => nil
ruby-1.8.7-p334 :047 > A.new.test = "value"
updated
also modified
 => "value"

Jesus.