Re: OOP sortof

[steve donovan <steve.j.donovan@...>]

On Sat, Jan 1, 2011 at 3:33 PM, Philippe Lhoste <PhiLho@gmx.net> wrote:
> Scala (to remain in the JVM realm) resolved that quite elegantly: just leave
> the variables in the open, and should you add code around access (get or
> set), you can do it without disturbing calling code.
> That's indeed a point that a modern language should address elegantly.

Yes, there are cases where the property pattern is the most elegant
solution - VB, borrowed by Delphi, brought back into MS by Delphi's
creator when he did C#.

With Lua one can write general setters and getters. For instance, a
common pattern is some GUI element where setting a property must cause
the element to update itself. Classic OOP would look like this,
repeated for each property:

function Box.set_width(x)
   if x ~= self.width then
      self.width = x
      self:update()
   end
end

A more dynamic solution could look like this:

local property = {width=true,height=true,text=true}

function Box.__newindex(self,prop,value)
   if property[prop] then
     local name = '_'..prop  -- i.e. actual field is _width, etc
     if self[name] ~= value then
        self[name] = value
        self:update()
    end
  else
    error("not a property or method",2)
  end
end

steve d.