Re: Deep __newindex on assignment?

[Alexander Gladysh <agladysh@...>]

> I have a userdata which uses __index to return another userdata and __newindex to set the same data. The first userdata is basically a property bag and the second userdata represents a specific property type. For example, a property like 'Position' might represent a point in 3d space and provide x,y,z members and point math functions.

> I wanted to set just one sub-member of a property in the bag, so I used the following syntax:

>  propBag.Position.z = 10

> I was hoping this would result in a __newindex call for "Position" since this is effectively assigning a value to that index. Instead this performs a __index call for "Position" and then sets the "z" member on the resulting userdata, but the result is then simply lost because all I have modified is the Point on the stack and not the Point in the propBag.

> Now obviously I can work around this problem by changing my script, but this syntax seems logical so I suspect it will be a common error for my end users (who will be authoring scripts that use these userdata objects). My question is can I make the syntax above work as I expected and result in a __newindex call on my propBag?

You have to return a temporary proxy object from the __index handler.
Set __newindex in that proxy object's metatable and handle assignment
there.

See for example how Lua Alchemy AS3 object property access wrapping is done:

http://github.com/lua-alchemy/lua-alchemy/blob/master/alchemy/lua-lib/assets/lua_alchemy/as3/sugar.lua

Alexander.