UI Document layout isn't initialized on the first frame

Unity Engine
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1

Apparently UI Documents aren’t initialized on the first frame.

I have this example code that prints the bounds of a Label to the console.

public class UiTest : MonoBehaviour
{
    [SerializeField]
    private UIDocument _uiDocument;

    private Label _label;
    private int frameCount;

    private void Awake()
    {
        _label = _uiDocument.rootVisualElement.Q<Label>("MyLabel");

        //prints (x:0.00, y:0.00, width:NaN, height:NaN)
        PrintWorldBounds("Awake");
    }

    private void Start()
    {
        //prints (x:0.00, y:0.00, width:NaN, height:NaN)
        PrintWorldBounds("Start");
    }

    private void Update()
    {
        //prints (x:0.00, y:0.00, width:NaN, height:NaN) on the first frame, from the second frame on, it prints the right value
        PrintWorldBounds($"Update {frameCount.ToString()}");
        frameCount++;
    }

    private void PrintWorldBounds(string prefix)
    {
        Debug.Log($"{prefix}: {_label.worldBound.ToString()}");
    }
}

The comments explain the issue. On the first frame, the UI Document doesn’t seem to be initialized. Is this by design or a bug? It makes interacting with a UI Document by code tricky because you have to delay everything.
If it’s by design, is there a way to know if the document is initialized other than waiting one frame?

2

Use OnEnable to query your root visual element. Awake is too early.

3

I didn’t test this, but OnEnable happens before the first Update and in the first Update the values are still not initialized.

EDIT: Sorry, you meant that the query happened too early, not the printing of the world bounds. After testing, it seems that it doesn’t make a difference at which point the query happens. Even if the query is made in the first update, uninitialized bound values are still printed.

4

The layout hasn’t run until PreLateUpdate, so until that point the layout of elements is undefined.

5

Is this by design? If yes, I think it’s a pretty bad design decision.

6

Yes, just as adding an element doesn’t mean it’s performed layout.
What reason you have for reading the layout immediately though?

7

I stumbled upon this issue when tinkering around with this problem I have: Interacting with subsections of text. - #29 by Nit_Ram

Interestingly there’s another strange initialization thing going on. Label.uitkTextHandle is only initialized after Alt-Tabbing away from Unity and back again. While this is not an officially accessible property (I access it via reflection), it’s still a weird thing to happen and I wonder why. :smiley:

8

For consistency you would presumably need to update the values when the layout changes. Subscribing to the label’s GeometryChangedEvent would be the way to get a callback whenever the layout is recalculated, and would be the appropriate place for this code imo

9

How can this event be accessed? I can’t find any member like that on a label.

10

Use the RegisterCallback function on the label:

1 Like
11

Ah nice, thank you very much!