middleclass: add getter-setter support for properties - oop

Question

I'm trying to add automatic support for property declarations, so that a class gets getters and setters generated automatically for them. I use the middleclass library as a base for classes. I've defined a root class that handles property creation. However, in tests, only an immediate subclass of the root class works properly. Others give me stack overflow error deep inside the middleclass code ([string "local middleclass = {..."]:82: stack overflow).
My code is:
local CBaseObject=class('CObjectBase');
function CBaseObject:initialize()
self._init=true;
end;
function CBaseObject:finalize()
self._init=false;
end;
function CBaseObject:_getter_setter(v)
return v;
end;
function CBaseObject:_gen_prop_cache()
rawset(self,'_properties',rawget(self,'_properties') or {});
end;
function CBaseObject:__index(k)
print('GET',k);
self:_gen_prop_cache();
local prop=self._properties[k];
if prop~=nil
then
local getter=self[prop[2] or '_getter_setter'];
return getter(self,prop[1]);
else return nil;end;
end;
function CBaseObject:__newindex(k,v)
print('ME',self.class.name);
print('SET',k,v);
self:_gen_prop_cache();
local prop=self._properties[k];
if prop==nil and self._init or prop
then
if prop==nil then prop={};self._properties[k]=prop;end;
local vv=prop[1];
if type(v)=='table' and #v<4
then
for i=1,3 do prop[i]=v[i];end;
else
prop[1]=v;
end;
local setter=self[prop[3] or '_getter_setter'];
prop[1]=setter(self,prop[1],vv);
else
rawset(self,k,v);
end;
end;
Test classes:
local cls=CBaseObject:subclass('test');
function cls:initialize()
self.class.super.initialize(self);
self.health={1000,'_gethealth','_sethealth'};
self.ammo=100;
self:finalize();
end;
function cls:_sethealth(value,old)
print('WRITE:',value,old);
if value<0 then return old;else return value;end;
end;
function cls:_gethealth(value)
print('READ:',value);
return value/1000;
end;
local cls2=cls:subclass('test2');
function cls2:initialize()
self.class.super.initialize(self);
self.ammo=200;
self:finalize();
end;
function cls2:_sethealth(value,old)
print('WRITE_OVERRIDEN:',value,old);
return value;
end;
local obj=cls2(); --change this to cls() for working example.
obj.health=100;
obj.health=-100;
print(obj.health,obj._properties.health[1]);
print(obj.ammo,obj._properties.ammo[1]);
I used https://repl.it/languages/lua to run my code. So, the questions is, is what I do even the right approach? Is it possible to add property support in an easier way compatible with the used library? Or should I use another, and what then?
EDIT: after experimenting I found out that the construct self.class.parent.<method>(<...>) is to blame for the error. I replaced all such occurrences by actual parent classes. That was the only problem, it seems, after that the code started to work error-free so far.

Answer 1

Enrique García Cota (middleclass creator) has enlightened me with what I think is a nice way to implement getters/setters on middleclass.
He suggested to create and use a mixin.
I've done some minor changes while testing/using this mixin recipe. Currently the one I'm using looks like this:
-- properties.lua
local Properties = {}
function Properties:__index(k)
local getter = self.class.__instanceDict["get_" .. k]
if getter ~= nil then
return getter(self)
end
end
function Properties:__newindex(k, v)
local setter = self["set_" .. k]
if setter ~= nil then
setter(self, v)
else
rawset(self, k, v)
end
end
return Properties
You would have to create function get_* and function set_* for your properties (or modify the string pattern above as you wish).
Example:
local class = require('middleclass')
local Properties = require('properties')
local Rect = class('Rect'):include(Properties)
function Rect:initialize(x, y, w, h)
self.x = x
self.y = y
self.w = w
self.h = h
end
function Rect:get_right()
return self.x + self.w
end
function Rect:set_right(right)
self.x = self.x + right - self.right
end
r = Rect:new(10,10, 100, 100)
print(r.right) -- 110
print(r:get_right()) -- 110
r.right = 200
print(r.right) -- 200
print(r.x) -- 100
This way you can use this mixin in whatever classes you want to have properties and simply create the get_* and set_* functions on it.
He also said, however:
I am not a huge fan of getters/setters in Lua. In other languages, I can accept them; for example in ruby, they are integrated in the language's message-passing mechanism.
But in Lua they are extra syntactic sugar, and risk making things more "magical" (unexpected to someone unfamiliar with the code).
Performance note: It's worth mentioning, however, that using __index functions, like the example does, should impact heavily on your code's performance (if compared to __index tables).
Personally, after some time using getters and setters (due to my Python bias), I've decided to write things explicitly and no longer rely on them.
It depends, of course, if performance is critical or not for your code.