Derived State

You can create new state from existing states. This is known as deriving state.

A function that wraps a state effectively becomes a state. If a state used inside a function is updated, the whole function can be re-ran to recompute its value.

local count = source(0)

local function text()
    return "count: " .. count()
end

create "TextLabel" {
    Text = text
}

Sometimes when using expensive computations to derive state, you only want to recalculate it once when a source state has changed

If you wrap a source state with a regular function, its value will be recomputed every time you call that function. derive() accepts a functions whose return value will be cached, so that subsequent calls of this derived state will return the same cached value until one of its source states have changed.

local derive = vide.derive

local count = source(0)

local factorial = derive(function()
    local n = 1
    for i = 2, count() do
        n *= i
    end
    return n
end)

This can improve performance for expensive calculations.

create "TextLabel" {
    Text = function()
        return "factorial squared: " .. factorial() * factorial()
    end
}

count(3) -- displays "factorial squared: 36"
count(4) -- displays "factorial squared: 576"

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Derived State

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