Help with metatables(?)

kaels · 02-09-12, 05:41 AM

2-part question.

(1) Let's say I have an object created as follows:

lua Code:

local object = CreateFrame("Button", name, UIParent, "SecureUnitButtonTemplate")

and a library of functions:

lua Code:

local lib = CreateFrame("Frame")
lib.someFunction = function(stuff)
    DoStuff(stuff)
end

and I want the object to inherit the methods I defined in the library, so that I can call object:someFunction. How would I do that? I know I need to use a metatable, but the metatable examples I've been able to dig up don't seem to make sense.

(2) Let's say my object already has a metatable defined, because the object was constructed in an embedded addon. Is there a way I can let the object inherit my library functions without losing its existing inherited methods?

Torhal · 02-09-12, 08:48 AM

You don't need to use a metatable for this.

Code:

object.some_func = lib.some_func

kaels · 02-09-12, 09:11 AM

If I do that, does that make object.some_func refer to lib.some_func, or does it create a duplicate of some_func for each object?

I know I don't want to do the latter, but I never quite seem to know in Lua when I'm creating something new and when I'm just assigning a pointer.

haste · 02-09-12, 10:47 AM

Originally Posted by kaels

If I do that, does that make object.some_func refer to lib.some_func, or does it create a duplicate of some_func for each object?

I know I don't want to do the latter, but I never quite seem to know in Lua when I'm creating something new and when I'm just assigning a pointer.

You are creating a reference to the function:

Code:

Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> lib = { some_func = function(self) print(self) end }
> obj = { some_func = lib.some_func }
> = lib.some_func, obj.some_func -- same function
function: 0x214ddd0	function: 0x214ddd0
> lib:some_func() obj:some_func() -- different self
table: 0x214e770
table: 0x214ea70

Frames (and all other UI widgets) already have metatables. It's possible to add methods to it, while still supporting its frame functionality, but you should consider if you really need to do this.

Lua Code:

local frame_metatable = {
   __index = CreateFrame('Button')
}
 
function frame_metatable.__index:tostring()
   return tostring(self)
end
 
local lib = CreateFrame('Button')
lib = setmetatable(lib, frame_metatable)
print(lib:tostring()) -- works

This is pretty much how oUF is implemented.

kaels · 02-09-12, 12:18 PM

Thanks!

SDPhantom · 02-09-12, 02:28 PM

Originally Posted by haste

Frames (and all other UI widgets) already have metatables. It's possible to add methods to it, while still supporting its frame functionality, but you should consider if you really need to do this.

Lua Code:

local frame_metatable = {
   __index = CreateFrame('Button')
}
 
function frame_metatable.__index:tostring()
   return tostring(self)
end
 
local lib = CreateFrame('Button')
lib = setmetatable(lib, frame_metatable)
print(lib:tostring()) -- works

This is pretty much how oUF is implemented.

I had developed a way of metatable injection a long time ago that would be more efficient than this. As such, it would create no extra tables/frames in the use of the code itself. The source and destination objects would already exist. The code quoted creates an extra frame and has the CPU transverse an extra table when looking up a value.

This is the code I had in mind for metatable injection.

lua Code:

function InjectMetatable(tbl,meta)
    return setmetatable(tbl,setmetatable(meta,getmetatable(tbl)));
end

Example of usage:

lua Code:

local meta={};
meta.tostring=tostring;
 
local frame=InjectMetatable(CreateFrame("Button"),meta);
print(frame:tostring());