# Element Creation API

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## mount()

Runs a function in a new stable scope and optionally applies its result to a
target instance.

- **Type**
  
    ```lua
    function mount<T>(component: () -> T, target: Instance?): () -> ()
    ```

- **Details**

    The result of the function is applied to a target in the same way
    properties are using `create()`.

    The function is ran in a new stable scope, just like
    [root()](reactivity-core.md#root).

    Returns a function that when called will destroy the stable scope.

- **Example**

    ```lua
    local function App()
        return create "ScreenGui" {
            create "TextLabel" { Text = "Vide" }
        }
    end

    mount(App, game.StarterGui)
    ```

## create()

Creates a new UI element, applying any given properties.

- **Type**

    ```lua
    function create(class: string): (Properties) -> Instance
    function create(instance: Instance): (Properties) -> Instance

    type Properties = Map<string|number, any>
    ```

- **Details**

    The function can take either a `string` or an `Instance` as its first argument.

    - If given a `string`, a new instance with the same class name will be created.
    - If given an `Instance`, a new instance that is a clone of the given instance
    will be created.

    This returns another function that is used to apply any properties to the new
    instance.

- **Property setting rules**

    - **index is string:**
      - **value is function:**
        - **property is event:** connect function as callback
        - **property is not event:** create effect to update property
      - **value is not function:** set property to value
    - **index is number:**
      - **value is action:** run action
      - **value is table:** recurse table
      - **value is function:** create effect to update children
      - **value is instance:** set instance as child

- **Example**

    Basic element creation.

    ```lua
    local frame = create "Frame" {
        Name = "NewFrame",
        Position = UDim2.fromScale(1, 0)
    }
    ```

    A component using property nesting.

    ```lua
    type Layout = {
        Layout = {
            Position: UDim2?,
            Size: UDim2?,
            AnchorPoint: Vector2?
        }
    }

    type Children = {
        Children = Array<Instance>
    }

    function Background(props: Layout & Children & {
        Color: Color3
    })
        return create "Frame" {
            BackgroundColor3 = props.Color,
            props.Layout,
            props.Children
        }
    end
    ```

## action()

Creates a callback that can be passed to `create()` to invoke custom actions on
instances.

- **Type**

    ```lua
    function action((Instance) -> (), priority: number = 1): Action
    ```

- **Details**

    When passed to `create()`, the given callback is called with the instance
    being created as the only argument. Actions take precedence over property
    and child assignments.

    A priority can be optionally specified to ensure certain actions run after
    other actions. Higher priority numbers are ran after lower priority numbers.

- **Example**

    An action to listen to changed properties:

    ```lua
    local function changed(property: string, callback: (new) -> ())
        return action(function(instance)
            local con - instance:GetPropertyChangedSignal(property):Connect(function()
                callback(instance[property])
            end)

            -- disconnect on reactive scope destruction to allow gc of instance
            cleanup(function()
                con:Disconnect()
            end)
        end)
    end

    local output = source ""

    create "TextBox" {
        -- will update the `output` source anytime the text property is changed
        changed("Text", output)
    }
    ```

## changed()

A wrapper for `action()` to listen for property changes.

- **Type**

    ```lua
    function changed(property: string, callback: (...unknown) -> ()): Action
    ```

- **Details**

    Will run the given callback any time the property is changed, as well as
    when the action is initially run.

    The changed connection is disconnected when the scope the action is ran in
    is destroyed.

    Runs with an action priority of 1.