Specify unique identifier for props

gfx/prop/children.lua

@@ -1,7 +1,7 @@
 local Prop = require("gfx.prop")
 local Element = require("gfx.element")
 local Event = require("gfx.event")
-local Children = Prop:new{ defaultState = {} }
+local Children = Prop:new{ defaultState = {}, uid = "CHILDREN" }
 
 function Children:with(elementType)
   local propSelf = self

gfx/prop/init.lua

@@ -2,11 +2,15 @@ local Prop = {}
 Prop.__index = Prop
 
 function Prop:getState(element)
-  return element[self]
+  return element[self:getId()]
 end
 
 function Prop:setState(element, state)
-  element[self] = state
+  element[self:getId()] = state
+end
+
+function Prop:getId()
+  return self.uid
 end
 
 function Prop.attach(elementType, prop, defaultState)
@@ -15,10 +19,12 @@ function Prop.attach(elementType, prop, defaultState)
 end
 
 function Prop:new(o)
-  local obj = o or {}
+  if o.uid == nil then
-  setmetatable(obj, Prop)
+    error("Property must carry a unique identifier")
-  obj.__index = obj
+  end
-  return obj
+  setmetatable(o, Prop)
+  o.__index = o
+  return o
 end
 
 return Prop

gfx/prop/orientation.lua

@@ -1,5 +1,5 @@
 local Prop = require("gfx.prop")
-local Orientation = Prop:new{ defaultState = false }
+local Orientation = Prop:new{ defaultState = false, uid = "ORIENTATION" }
 
 function Orientation:with(elementType)
   local propSelf = self