vide/docs/tut/crash-course/5-effect.md

Effects

Effects are functions that are ran in response to source updates. They are called effects because they cause side-effects when reacting to source updates.

Effects are created using effect().

local source = vide.source
local effect = vide.effect

local count = source(0)

effect(function()
    print("count: " .. count())
end)

-- "count: 0" printed
count(1)
-- "count: 1" printed

The callback given to effect() is initially ran immediately in a reactive scope. Any source read from inside a reactive scope will be tracked, so that if any of those sources update, the effect will be reran too.

Effects also work with derived sources, it doesn't matter how deeply nested inside a function a source is.

local source = vide.source
local effect = vide.effect

local count = source(1)

local doubled = function()
    return count() * 2
end

effect(function()
    print("doubled count: " .. doubled())
end)

-- "doubled count: 2" printed
count(2)
-- "doubled count: 4" printed

If a source is updated with the same value it already had, it will not rerun effects depending on it.