Derived State

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

local count = source(0)

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

create "TextLabel" {
    Text = text
}

Assigning a non-event property a function will bind that property to that function, anytime a state being read from inside that function is changed, the function will be re-ran and the property value updated.

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

derive() will cache and return the same value until a source state has changed, where it will recompute and cache a new value.

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

count(3) -- displays "factorial: 6"
count(4) -- displays "factorial: 24"

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Derived State

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