A Proximity Lantern: Toggle a Light With Player UI

Imagine building a tower out of LEGO blocks. You snap a red block on a base plate, then a blue block on the red one. Move the red block and the blue block travels with it. That parent/child relationship is the heart of the Scene Graph: an entity is the plastic brick, and components are the powers (a mesh for shape, a light for glow) you bolt onto it.

In this beginner guide we build a lantern controller. Rather than leaving code that only prints, we make it do something a player can see: when the game starts we add a text widget to every player's UI, and we drive a placed light device on/off. The UI is the proof the technique works.

What you'll learn

• What an entity is (the empty backpack) and what a component is (the items inside).
• Why Scene Graph parenting means moving the parent moves the child.
• How to reach every player's UI with GetPlayerUI[] and AddWidget.
• How to build and show a text_block inside a canvas.
• How to toggle a light-style state and update the on-screen message.

How it works

An entity on its own is just a container — no color, no light, no sound. You give it powers with components: a mesh paints the brick so it has a shape, and a light puts an LED inside so it glows. When one entity is parented to another, moving the parent drags every child along — exactly like the LEGO tower.

To make the technique observable, we talk to the player UI. For each player we grab their UI with GetPlayerUI[Player] (fallible, so it lives inside an if), build a canvas holding a text_block, and call AddWidget. We drive the actual glow through a placed customizable_light_device reference, flipping a logic flag so we always know its current state. A button device lets the player toggle it.

One entity, two bolted-on powers — and the player UI shows the state.

Let's build it

Place a Light device and a Button device in your level and expose both as @editable. At OnBegin we turn the light off, show a status widget in every player's UI, and subscribe to the button. Each press flips the light and rewrites the on-screen message.

using { /Fortnite.com/Devices }
using { /Fortnite.com/UI }
using { /Verse.org/Simulation }
using { /UnrealEngine.com/Temporary/UI }
using { /UnrealEngine.com/Temporary/SpatialMath }
using { /UnrealEngine.com/Temporary/Diagnostics }

# Player-facing text must be a `message`, produced by a <localizes> function.
LanternOffMsg<localizes>() : message = "Lantern is OFF"
LanternOnMsg<localizes>() : message = "Lantern is ON"

# Drives a lantern light and mirrors its state in player UI.
lantern_controller := class(creative_device):

    # The placed Customizable Light device that acts as the lantern's glow (the LED in the brick).
    @editable
    LanternLight : customizable_light_device = customizable_light_device{}

    # A Button device the player presses to toggle the lantern.
    @editable
    ToggleButton : button_device = button_device{}

    # Tracks whether the lantern is currently glowing.
    var LanternOn : logic = false

    # The on-screen text we update as the state changes (the proof it works).
    var StatusText : text_block = text_block{}

    OnBegin<override>()<suspends> : void =
        set StatusText = text_block{DefaultText := LanternOffMsg()}

        # Start dark so the player can switch it on.
        LanternLight.TurnOff()
        set LanternOn = false

        # Show the status message in every player's UI.
        for (Player : GetPlayspace().GetPlayers()):
            if (UI := GetPlayerUI[Player]):
                # A canvas positions our text_block near the top of the screen.
                Canvas := canvas:
                    Slots := array:
                        canvas_slot:
                            Anchors := anchors{Minimum := vector2{X := 0.5, Y := 0.1}, Maximum := vector2{X := 0.5, Y := 0.1}}
                            Alignment := vector2{X := 0.5, Y := 0.0}
                            SizeToContent := true
                            Widget := StatusText
                UI.AddWidget(Canvas)

        # Each button press toggles the lantern and refreshes the UI text.
        ToggleButton.InteractedWithEvent.Subscribe(OnToggle)

    # Flip the light, update our flag, and rewrite the on-screen message.
    OnToggle(Agent : agent) : void =
        if (LanternOn?):
            LanternLight.TurnOff()
            set LanternOn = false
            StatusText.SetText(LanternOffMsg())
        else:
            LanternLight.TurnOn()
            set LanternOn = true
            StatusText.SetText(LanternOnMsg())```

## Try it yourself

*   Change the anchor `Y` value in the `anchors` to move the status text down the screen.
*   Add a second `text_block` to the canvas showing the player's name via interpolation.
*   Swap the button for a `trigger_device` — subscribe to `TriggeredEvent` instead so stepping on a pad toggles the lantern.
*   In the Scene Graph, parent the light entity under a mesh entity, then move the mesh — watch the glow follow.

## Recap

An **entity** is a container; **components** (mesh, light, custom Verse) give it powers, and parenting makes children follow the parent. We turned that idea into something visible: a placed `customizable_light_device` acts as the lantern glow, `GetPlayerUI[]` + `AddWidget` put a `text_block` on every player's screen, and a button toggles both the light and the message. That's the full loop — an entity with powers, driven by input, reflected in the player UI.