Replace the local text queues in the text systems with flags stored in a component

[ickshonpe]

Objective

text_system and measure_text_system both keep local queues to keep track of text node entities that need recomputations/remeasurement, which scales very badly with large numbers of text entities (O(n^2)) and makes the code quite difficult to understand.

Also text_system filters for Changed<Text>, this isn't something that it should do. When a text node entity fails to be processed by measure_text_system because a font can't be found, the text node will still be added to text_system's local queue for recomputation. Text should only ever be queued by text_system when a text node's geometry is modified or a new measure is added.

Solution

Remove the local text queues and use a component TextFlags to schedule remeasurements and recomputations.

Changelog

• Created a component TextFlags with fields remeasure and recompute, which can be used to schedule a text remeasure or recomputation respectively and added it to TextBundle.
• Removed the local text queues from measure_text_system and text_system and instead use the TextFlags component to schedule remeasurements and recomputations.

Migration Guide

The component TextFlags has been added to TextBundle.

[ickshonpe]

Big gains with lots of text nodes:

cargo run --example many_buttons --profile stress-test --features trace_tracy -- recompute-text

[alice-i-cecile]

This is a much nicer implementation, and the performance benefits are non-trivial.

[ickshonpe]

This is a much nicer implementation, and the performance benefits are non-trivial.

Yep, really pleased with how this PR turned out. I'd rather not have added yet another UI component but there isn't really anywhere to put the flag variables that makes sense and doesn't get filtered for changes.

[nicoburns]

One comment on terminology. Otherwise looks good to me.

[nicoburns]

I feel like remeasure should be called regenerate_measure_function or similar. "measuring" is when you call the measure function not when you generate it. recompute could then be called remeasure, but I think recompute is also fine.

[ickshonpe]

Yeah the terminology everywhere needs to be tightened up. I've been loose with it because I'm not really
clear on what's going to be intuitive for users etc, so just used short easy names. Even regenerate_measure_function is a bit misleading, since the flag is most commonly going to be used when a new text node entity is spawned but its fonts haven't loaded, so it is rescheduling the initial creation of a measure function until the next frame, not regenerating an existing measure function.

[ickshonpe]

Changed it to generate_measure_function, seems okay.

[ickshonpe]

Thought about this again, and I think these renamings are better as they capture the correct semantics:

• generate_measure_func -> needs_new_measure_func
• recompute -> needs_recompute

[ickshonpe]

It's outside of the scope of this PR and I don't want to add any more changes, but it seems like it should be impossible for queue_text to return a NoSuchFont error since the fonts need to be already loaded to create a measure func. It should probably panic on this error, and there might be room for some other simplifications somewhere.