From: Dave Burt <dave@...>
Date: 2006-05-04T14:44:55+09:00
Subject: Re: Ruby idiom for attributes / properties

John Lam wrote:
> On 5/4/06, James Britt <james_b@neurogami.com> wrote:
>>
>> What would you consider to be an attribute?
> 
> 
> I'm just borrowing the ActiveRecord terminology for attributes. In my mind,
> an attribute == a property. In .NET, a property is a bit of syntactical
> sugar (but also a distinct metadata entity) for a getter and setter method.
> ...
> I would consider it equivalent to this Ruby code:
> 
> class Foo
>  def string_property
>     'string'
>  end
>  def string_property=(value)
>    puts "setting a string to #{value}"
>  end
> end
> 
> The problem is that I can't distinguish a string_property method from any
> other method. In .NET, I can walk the property metadata to discover all of
> those properties (and those are things that can be bound to data-aware
> controls in the framework).

RDoc considers anything declared with the attr_* class methods an
"attribute", so that your string_property above isn't, but this is:

class Foo
  attr_accessor :string_property
end

One approach might be to hack attr_* to generate your metadata (as I see
Logan Capaldo has now suggested).

Another approach might be to simply narrow down what valid "attribute"
methods look like. Does it have to have a foo=() and foo() pair? Does
foo() have to be parameterless, or is it accept optional parameters
(like Ara's traits)? Then you can just pull those methods out of the class:

class Module
  def attributes
    instance_methods.select do |name|
      next if name =~ /!$/ || name =~ /=$/
      setter = name.sub(/\?$/, "") + "="
      instance_methods.include?(setter) &&
        [0, -1].include?(instance_method(name).arity) &&
        [1, -2].include?(instance_method(setter).arity)
    end
  end
end

struct = Struct.new(:foo, :bar)
[Dir, File, IO, Hash, Thread, Struct::Tms, struct].each do |c|
  p [c, c.attributes]
end

=> [Dir, ["pos"]]
[File, ["sync", "lineno", "pos"]]
[IO, ["sync", "lineno", "pos"]]
[Hash, ["default"]]
[Thread, ["abort_on_exception", "priority"]]
[Struct::Tms, ["cutime", "cstime", "stime", "utime"]]
[#<Class:0x2c32660>, ["bar", "foo"]]

Cheers,
Dave