Lua: Calling c method that returns userdata to Lua

Question

Im using LuaObjcBridge and I'm calling a method from Lua like this:

local position = owner:position()

In objective-c this returns a CGPoint struct with X and Y which the bridge converts into Userdata. So CGPoint struct -> Lua userdata. In Lua how would I access the X and Y values from the Userdata?

I have not used LuaObjcBridge but I have a fair amount of experience with Lua and a custom bridge. I looked at the code and I do not see any support in the metatable to allow access to struct members. It appears that the metatable only tries to bridge objective C methods for full objects.You could try position.x or position.y but it does not look like those would work. In this case you could write your own bridged C functions that would extract the values from the userdata. — Jon Steinmetz
– Jon Steinmetz, Commented Jan 14, 2011 at 3:09
I wrote a lengthy answer for doing this in c++ by hand. I know its not exactly what you are asking for but it may help. stackoverflow.com/questions/3481856/… The only difference would be the way that classes are composed is c++ vs. obj-c — Nick Van Brunt
– Nick Van Brunt, Commented Jan 14, 2011 at 20:37

Łukasz Gruner · Accepted Answer · 2011-01-24 07:42:17Z

1

you need to:

define metatable, that contains methods that operate on userdata
set metatable on userdata
set the same metatable as __index field on userdata

or:

set some methods that operate on userdata
expose them to Lua

look at http://www.lua.org/pil/28.1.html

userdata can't do anything by itself, you need to add methods that do something with it.

answered Jan 24, 2011 at 7:42

Łukasz Gruner

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