Async Loading Runtime UI UXML

Unity Engine
,
1

I need some help understanding the correct way to asynchronously load a VisualElement’s UXML file for my runtime UI.

I’m generally following the Element-first approach from the docs, but instead of Resources.Load, I’m using Addressables.LoadAssetAsync like so:

public class CustomElement : VisualElement
    {
        public CustomElement()
        {
            var handle = Addressables.LoadAssetAsync<VisualTreeAsset>("CustomElement.uxml");

            handle.Completed += _ =>
            {
                var visualTreeAsset = handle.Result;
                visualTreeAsset.CloneTree(this);
            };
        }
    }

This mostly works as expected, except that I get this warning that only occurs in the editor:

SendMessage cannot be called during Awake, CheckConsistency, or OnValidate (New Game Object: OnDidAddComponent)

This seems to happen because the generated code for the CustomElement calls the element’s constructor during OnValidate, and in turn, Addressables.LoadAssetAsync calls SendMessage, which is not allowed during OnValidate.

I can’t use the synchronous version of Addressables.LoadAssetAsync because I’m making a WebGL app which does not support synchronous loading.

Is there some other pattern I should use to async load UXML files for custom VisualElements?

2

EditorApplication.delayCall += () => { … };

Put this in OnValidate and replace … with the code that can‘t execute during OnValidate.

3

OnValidate is defined in the generated code; I can’t modify it.

4

In that case use it inside the element ctor to defer loading. :wink:

5

Ah of course, I understand now! That fixed it. Thank you for your help!