local t = {}
local mt = setmetatable({
-- some meta method to know when a key is added or lost and prints a message
}, t)
Is there a way of doing this. I talked about this with someone and they said i couldn't just do it with meta methods but proxies as well. I'm a bit stumped on how to make this work. Can anyone help?
Thanks
To track table keys in lua there are 2 most importaint keys in metatable: __index and __newindex.
__newindex is used to create new key in the table if such key is not found. __index is used to get the value if there is no such key in table.
With __newindex it is possible to track creation, but not assignment, so it is not possible to track key removal:
<script src="https://github.com/fengari-lua/fengari-web/releases/download/v0.1.4/fengari-web.js"></script>
<script type="application/lua">
local t={}
setmetatable(t, {
__newindex = function(self, key, value)
print('Added Key:'..key,'Value:'..value)
rawset(self, key, value)
end
})
t.test = 'test'
t.test = nil -- delete not tracked
t.test = 'test2'
</script>Using proxy table and __newindex with __index we can track every assignment:
<script src="https://github.com/fengari-lua/fengari-web/releases/download/v0.1.4/fengari-web.js"></script>
<script type="application/lua">
local t={}
local proxytable={}
setmetatable(t, {
__newindex = function(self, key, value)
if proxytable[key] then
if value == nil then
print('Deleted Key:'..key)
else
print('Changed Key:'..key,'Value:'..value)
end
else
print('Added Key:'..key,'Value:'..value)
end
rawset(proxytable, key, value)
end,
__index = proxytable
})
t.test = 'test'
t.test = nil
t.test = 'test2'
t.test = 'test3'
t.test = nil
</script>If you want enumerate table keys with pairs(), ipairs(), then you need to use metakeys __pairs and __ipairs as original table is allways empty.