function call on storing a value into the table

[SuperRookie]

Sorry for spamming, but since the question is totally different the previous topic, I had to start a new thread

So, I'm trying to make a table that calls a function when a new value is being stored into the table.

E.g.

table[0] = "value"

or

table.insert(table, "value")

I've done some research and using metatable and its '__settable' method would do a trick, but I could've not found a working example for this.

Does anyone have any idea regarding this?

[SuperRookie]

btw, metamethods doesn't work with table functions

+

Maybe '__index' is the one that I was looking for...

[Ketho]

Originally Posted by SuperRookie So, I'm trying to make a table that calls a function when a new value is being stored into the table. E.g. table[0] = "value"

I could be wrong, but it's something like this with __newindex

• http://lua-users.org/wiki/MetamethodsTutorial
• https://www.lua.org/pil/13.4.2.html

local mt = setmetatable({}, {__newindex = function(t, k, v)
    rawset(t, k, v)
    print(k, v)
end})
 
mt[7] = "Hello World"
-- prints 7, "Hello World"

[SuperRookie]

Hi Ketho,

Awesome!

thx for letting me know !!

* EDIT: One problem with '__newindex' is that if the key already exists, it doesn't call its '__newindex' metamethod as its name indicates.

So, the following would call '__newindex' only once, but for my case, I need a method that runs every single time when there is a change within a table.

local t = {}
 
local mt = {
    __newindex = function(table, key, value)
        print("__newindex called")
 
        rawset(table, key, value)
    end,
};
 
setmetatable(t, mt)
 
t[5] = 10 -- '__newindex' metamethod called & print
 
t[5] = 15 -- '__newindex' metamethod not called

[lightspark]

Originally Posted by SuperRookie Hi Ketho, Awesome! thx for letting me know !! * EDIT: One problem with '__newindex' is that if the key already exists, it doesn't call its '__newindex' metamethod as its name indicates. So, the following would call '__newindex' only once, but for my case, I need a method that runs every single time when there is a change within a table.

You could try using metatable as a proxy then.

local _T, T = {}, {} -- table, proxy
setmetatable(T, {
    __newindex = function(_, k, v)
        print("__newindex")
 
        _T[k] = v
    end,
})
 
T[5] = 10 -- "__newindex"
 
T[5] = 15 -- "__newindex"

Here's a good read: https://www.lua.org/pil/13.4.4.html

[SuperRookie]

Haha! You are genius, lightspark

That's exactly what I was looking for

So, table 'T' is actually a fake while '_T' is the actual table that holds elements, right?

That's just genius!

[lightspark]

Originally Posted by SuperRookie Haha! You are genius, lightspark That's exactly what I was looking for So, table 'T' is actually a fake while '_T' is the actual table that holds elements, right? That's just genius!

Yeah, T is a proxy that's used to track read (__index) and write (__newindex) accesses, _T is our storage

I use something similar in one of my addons, but all credit goes to the author of the article I linked.

[SuperRookie]

Originally Posted by lightspark Yeah, T is a proxy that's used to track read (__index) and write (__newindex) accesses, _T is our storage I use something similar in one of my addons, but all credit goes to the author of the article I linked.

Haha!

All good!

Thanks for the reference (a link) as well