trigger Text & Text2d re-render when Children is removed

[Austreelis]

Objective

As of at least bevy 0.16.0, Removing Children from a Text Node would not be detected by bevy_text::text::detect_text_needs_rerender, leading to rendering glitches (#19657). After pinning down the bug, the same happens with Text2d, assuming its docs are correct about its use of TextSpan (I didn't actually went down the rabbit hole a second time to make sure).

Fixes #19657

Solution

• Observe On<Remove, Children>, and query then set ComputedTextBlock.needs_rerendering if present.

Testing

• The reproducer in In TextLayout, when deleting all TextSpans, TextLayout still retains the last text #19657 works as intended, I did some extra logging and monitoring to be extra sure too.

[rendaoer]

Haha, I still want to come back to repair him today.

[ickshonpe]

As well the problems with using an observer, this won't work because the bug is recursive and it affects the Children of TextSpan entities too.

Maybe we should just give up on fine grained change detection for now, walk every TextSpan entity building an ordered list of spans for each ComputedTextBlock entity, and relayout if the list of spans or a TextSpan's contents changes. I'm not sure.

[ickshonpe]

This won't work I think, it's going to trigger and do a ComputedTextBlock look up everytime any Children component is despawned, not just those belonging to text entities.

[Austreelis]

Should we explicitely observe entities that have Text/Text2d ? I don't know if that dupplicates observer entities for every observed text root, I'm not sure it would be an issue either.

[ickshonpe]

The same observer could update both root types. The discriminator also means that the observer is going to run twice every time any Children component is removed.

[Austreelis]

I'll at least dedupplicate that first

[Austreelis]

Maybe we should just give up on fine grained change detection for now, walk every TextSpan entity building an ordered list of spans for each ComputedTextBlock entity

I originally wished I could use a custom relation (e.g. ChildTextOf) that would do exactly what ChildOf is, with the added hooks needed to take care of text-specific stuff. But I didn't want to overthrow ChildOf, or effectively dupplicate it. Though I didn't consider the recursive TextSpan children.

[Austreelis]

I've come back to this, to try and handle any of Text, Text2d and TextSpan: we now observe when any of those component is added, then add an observer to the target - that will track when its children are removed - to traverse up the hierarchy, find the root computed text block and set needs_rerender. This avoids the bug, including with recursive TextSpans, and does not trigger observers needlessly. It does create an observer per text node (+ 1 global) however :/