vide/docs/tut/crash-course/11-dynamic-scope.md

# Dynamic Scoping

Eventually you may need a way to dynamically create and destroy UI elements
resulting from source updates. Vide provides functions to help you do this,
known as *dynamic scope* functions.

These functions create and destroy components for you in response to source
updates. They return a source containing the created component. This source can
be parented as a child which will update the shown children whenever the source
updates.

The simplest example is using `show()`.

```luau
local source = vide.source
local create = vide.create
local show = vide.show
local root = vide.root

function Button(props: { Text: string, Activated: () -> () })
    return create "TextButton" {
        Text = props.Text,
        Activated = props.Activated
    }
end

function Menu()
    return create "TextLabel" {
        Text = "This is a menu"
    }
end

function App()
    local toggled = source(false)

    return create "ScreenGui" {
        Button {
            Text = "Toggle Menu",
            Activated = function()
                toggled(not toggled())
            end
        },

        show(toggled, Menu) -- [!code highlight]
    }
end

root(function()
    App().Parent = game.StarterGui
end)
```

This is a complete example of rendering UI which has a single button that
toggles the opening of a menu.

--------------------------------------------------------------------------------

Another common function is `indexes()`. This function creates a component for
each index in a table.

Each component created is done so in a new and independent stable scope. The
indexes of the table are checked each source update to prevent redunant
destruction and recreation of UI elements.

```luau
local source = vide.source
local create = vide.create
local indexes = vide.indexes
local root = vide.root

local function Todo(props: {
    Text: () -> string,
    Position: number,
    Activated: () -> ()
})
    return create "TextButton" {
        Text = function() return props.Position .. ": " .. props.Text() end,
        LayoutOrder = props.Position,
        Activated = Activated
    }
end

local function TodoList(props: { List: () -> Array<string> })
    return create "Frame" {
        create "UIListLayout" {},

        indexes(props.List, function(text, i)  -- [!code highlight]
            return Todo {
                Text = text,
                Position = i,
                Activated = function() -- remove the todo when clicked
                    local list = props.List()
                    table.remove(list, i)
                    props.List(list)
                end
            }
        end)
    }
end

function App()
    local list = source {
        "finish the crash course",
        "star Vide's GitHub"
    }

    return create "ScreenGui" {
        TodoList { List = list },
    }
end

root(function()
    App().Parent = game.StarterGui
end)
```

The reactive graph for the above example:

```mermaid
%%{init: {
    "theme": "base",
    "themeVariables": {
        "primaryColor": "#111720",
        "primaryTextColor": "#fff",
        "primaryBorderColor": "#111720",
        "lineColor": "#79B8FF",
        "tertiaryColor": "#0d131b",
        "tertiaryBorderColor": "#0d131b"
    }
}}%%

graph

subgraph root ["root"]
    direction LR
    todoList --> indexes -.- subroot1 & subroot2

    subgraph subroot1 ["indexes scope 1"]
        direction LR
        value1[todo] --> prop1["prop binding"]
    end

    subgraph subroot2 ["indexes scope 2"]
        direction LR
        value2[todo] --> prop2[prop binding]
    end
end
```

When you edit a table in a source, you must set that table again to actually
update the source.

```luau
local src = source { 1, 2 }
local data = src()
table.insert(data, 3) -- no effects will run
src(data) -- effects will run
```