# Reactivity: Utility

## cleanup()

Queues a callback to run when a scope is reran or destroyed.

- **Type**

    ```luau
    function cleanup(v: Function | Disconnectable | Destroyable | thread)

    type Function = () -> ()
    type Destroyable = { destroy: () -> () }
    type Disconnectable = { disconnect: () -> () }
    ```

- **Example**

    ```luau
    local count = source(0)

    local destroy = root(function()
        effect(function()
            count()

            cleanup(function()
                print "cleaned"
            end)
        end)
    end

    -- nothing printed yet
    count(1) -- prints "cleaned"
    count(2) -- prints "cleaned"
    destroy() -- prints "cleaned"
    ```

## untrack() <Badge type="info" text="STABLE"><a href="/vide/api/reactivity-core#Scopes">STABLE</a></Badge>

Runs a function in a new stable scope.

- **Type**

    ```luau
    function untrack<T>(source: () -> T): T
    ```

- **Details**

    Can be used inside a reactive scope to read from sources you do not want
    tracked by the reactive scope.

- **Example**

    ```luau
    local a = source(0)
    local b = source(0)

    local sum = derive(function()
        return a() + untrack(b)
    end)

    print(sum()) -- 0
    b(1) -- untracked so reactive scope created by derive() does not rerun
    print(sum()) -- 0
    a(1) -- reactive scope created by derive() reruns
    print(sum()) -- 2
    ```

## read()

Utility used to read a value that is either a primitive or a source.

- **Type**

    ```luau
    function read<T>(value: T | () -> T): T
    ```

## batch()

Runs a function where any source updates made within the function do not
trigger effects until after the function ends.

- **Type**

    ```luau
    function batch(fn: () -> ())
    ```

- **Details**

    Improves performance when an effect depends on multiple sources, and those
    sources need to be updated.

- **Example**

    ```luau
    local a = source(0)
    local b = source(0)

    effect(function()
        print(a() + b())
    end)

    -- prints "0"

    batch(function()
        a(1) -- no print
        b(2) -- no print
    end)

    -- prints "3"
    ```

## context() <Badge type="info" text="STABLE"><a href="/vide/api/reactivity-core#Scopes">STABLE</a></Badge>

Creates a new context.

- **Type**

    ```luau
    function context<T>(default: T): Context<T>

    type Context<T> =
        () -> T -- get
        & <U>(T, () -> U) -> U -- set
    ```

- **Details**

    Calling `context()` returns a new context function.
    Call this function with no arguments to get the context value.
    If no context is set, the default value is returned, or an error is thrown
    if no default was given.
    Call this function with a value and a function to create a new context with
    the given value.

    The new context is run under a stable scope.

- **Example**

    ```luau
    local theme = context()

    local function Button()
        print(theme())
    end

    root(function()
        theme("light", function()
             Button() -- prints "light"

            theme("dark", function()
                Button() -- prints "dark"
            end)
        end)
    end)
    ```