Debugging in Verse: Print Plus On-Screen HUD Widgets

What you'll learn

You'll learn how to use Print() for log-based tracing with string interpolation, and how to surface those same debug values on-screen by building a canvas with a text_block and adding it to a player's UI via GetPlayerUI[] / AddWidget. Every button press updates both the log and the widget, so you can watch state change without squinting at the console.

How it works

Print() writes a single string to the Output Log (and screen). Its most important rules:

• One string only. Never pass comma-separated arguments. Combine values with string interpolation {...} — the braces evaluate the expression and convert it to text: Print("2 + 2 = {2 + 2}") logs 2 + 2 = 4.
• No + on messages. Build output with interpolation, not concatenation.
• Top-to-bottom execution. The order your prints appear tells you the order your code ran — the cheapest way to see how far execution got.

The log is great for you, but invisible to players. To show debug text on screen we use the player UI:

• Player.GetPlayerUI[] is fallible ([]), so bind it inside an if: if (UI := Player.GetPlayerUI[]):.
• Build a canvas whose slot holds a text_block, then call UI.AddWidget(Canvas).
• Keep a reference to the text_block and call .SetText(...) to update it live. SetText takes a message, so we wrap interpolated text with a <localizes> helper.

Let's build it

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

# A debug device: logs with Print() AND mirrors values on an on-screen widget.
debug_print_device := class<concrete>(creative_device):

    # Drag a Button device into this slot in the Details panel.
    @editable TriggerButton : button_device = button_device{}

    # Mutable counter so we can watch state change across presses.
    var PressCount : int = 0

    # The on-screen text widget we update live. Created in OnBegin.
    var DebugText : text_block = text_block{}

    # <localizes> helper turns interpolated text into a message for SetText.
    MakeMessage<localizes>(Value : string) : message = "{Value}"

    OnBegin<override>()<suspends> : void =
        # 1. Prove OnBegin actually ran.
        Print("[Debug] debug_print_device started!")

        # 2. Interpolate a value and a calculation into the log.
        Counter : int = 42
        Print("[Debug] Counter = {Counter}, Doubled = {Counter * 2}")

        # 3. Build the on-screen widget and add it to every player's UI.
        set DebugText = text_block{DefaultText := MakeMessage("[Debug] Ready. Presses: 0")}
        Canvas := canvas:
            Slots := array:
                canvas_slot:
                    Anchors := anchors{Minimum := vector2{X := 0.05, Y := 0.05}, Maximum := vector2{X := 0.05, Y := 0.05}}
                    Offsets := margin{Top := 0.0, Left := 0.0, Right := 0.0, Bottom := 0.0}
                    Alignment := vector2{X := 0.0, Y := 0.0}
                    SizeToContent := true
                    Widget := DebugText
        for (Player : GetPlayspace().GetPlayers()):
            if (PlayerUI := GetPlayerUI[Player]):
                PlayerUI.AddWidget(Canvas)

        # 4. Subscribe so each press updates log AND widget.
        TriggerButton.InteractedWithEvent.Subscribe(OnButtonPressed)

    # Handler signature must match the button event: (agent):void.
    OnButtonPressed(Agent : agent) : void =
        set PressCount = PressCount + 1
        # Log it...
        Print("[Debug] Button pressed. PressCount = {PressCount}")
        # ...and mirror it on the screen widget.
        DebugText.SetText(MakeMessage("[Debug] Presses: {PressCount}"))```

## Try it yourself

1. Create a new Verse device in UEFN and paste in the code above.
2. Build Verse, then drag `debug_print_device` into your island.
3. Place a **Button** device and assign it to the `TriggerButton` slot.
4. Launch a session. Watch the Output Log for the `[Debug]` startup and counter lines, and confirm the on-screen text appears at the top-left.
5. Press the button repeatedly — the log AND the widget should both show `PressCount` climbing.
6. Change `Counter` or the `* 2` math, rebuild, and confirm the interpolated output updates in the log.

## Recap

- `Print()` takes exactly **one `string`**; use **string interpolation** `{...}` for values and calculations — never `+`, never comma-separated args.
- `GetPlayerUI[]` is fallible — bind it with `if (UI := Player.GetPlayerUI[]):` before calling `AddWidget`.
- Build a `canvas` containing a `text_block`, add it with `AddWidget`, and call `.SetText(...)` (with a `<localizes>` message) to update debug text live on screen.
- Print at the start of `OnBegin` and inside handlers to verify which code ran and in what order.