Running code on variable set in lua

Question

I'm writing a script in lua, what i need to do is run a piece of code every time a variable is set.

The code i have is:

plane = {}
plane.status = "foo"
plane.status = "bar"

I want to run this every time a value is assigned to a variable in plane

print "status=" + <value>

How would i go about this? i've head it has something to do with __newindex.

lhf · Accepted Answer · 2012-07-03 18:12:52Z

3

Yes, you set a proxy table for plane and set a __newindex metamethod for it.

function managed(t,f)
        return setmetatable({},{__index=t, __newindex=f})
end

plane = managed({},print)
plane.status = "foo"
plane.status = "bar"

In practice, in the __newindex metamethod, you'll set the values in the original table using rawset.

edited Jul 3, 2012 at 18:12

answered Jul 3, 2012 at 16:21

lhf

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2 Comments

Michal Kottman Over a year ago

Note that __newindex is called only when the key is not present the table. If you want to be able to store the value and also still intercept the calls, you have to store the values in another table, not in the managed table.

lhf Over a year ago

@MichalKottman, of course. I meant by managed the original table. I've edited my answer.

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