From: Gavin Kistner <gavin@...>
Date: 2005-09-19T21:37:29+09:00
Subject: Re: Metaprogramming problem

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On Sep 19, 2005, at 6:16 AM, Clifford Heath wrote:
> Ok, I can see why "block" isn't defined. How can I pass add_property's
> block in as a block to be used *inside* the evaluated code?

Here's *a* working solution. The big string + eval feels like a  
serious hack, but I'm not cool enough to know how to deal with it  
properly.

class Object
     def add_property(sym, &block)
         n = sym.id2name
         (@_property_callbacks||={})[ n ] = block
         code = %Q{
             def #{n}()
                 @#{n} ||= nil
             end
             def #{n}=(value)
                 if block = @_property_callbacks[ '#{n}' ]
                     @#{n} = block.call(value)
                 else
                     @#{n} = value
                 end
             end
         }
         eval(code)
     end
end

# A test object:
d = 3

# Add a getter and custom setter to 3:
d.add_property(:do_it) {|v|
# Custom setter method
     puts "d.do_it here, v = #{v.inspect}"
     v    # No filtering this time, just return the value.
}

# Assign the property:
d.do_it = "a value"



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