ContentSize replacement fix

[ickshonpe]

Objective

If you remove a ContentSize component from a Bevy UI entity and then replace it ui_layout_system will remove the measure func from the internal Taffy layout tree but no new measure func will be generated to replace it since it's the widget systems that are responsible for creating their respective measure funcs not ui_layout_system. The widget systems only perform a measure func update on changes to a widget entity's content. This means that until its content is changed in some way, no content will be displayed by the node.

Example

This example spawns a text node which disappears after a few moments once its ContentSize component is replaced.

use bevy::prelude::*;
use bevy::ui::ContentSize;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, delayed_replacement)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn(Camera2dBundle::default());
    commands.spawn(
        TextBundle::from_section(
            "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.",
            TextStyle::default(),
        )
    );
}

// Waits a few frames to make sure the font is loaded and the text's glyph layout has been generated.
fn delayed_replacement(mut commands: Commands, mut count: Local<usize>, query: Query<Entity, With<Style>>) {
    *count += 1;
    if *count == 10 {
        for item in query.iter() {
            commands
                .entity(item)
                .remove::<ContentSize>()
                .insert(ContentSize::default());
        }
    }
}

Solution

Perform ui_layout_system's ContentSize removal detection and resolution first, before the measure func updates.
Then in the widget systems, generate a new Measure when a ContentSize component is added to a widget entity.

Changelog

• measure_text_system, update_image_content_size_system and update_atlas_content_size_system generate a new Measure when a ContentSize component is added.

[rparrett]

If you remove a ContentSize component from a Bevy UI entity and then replace it

Just for the sake of documentation:

• Is ContentSize ever removed in Bevy / does this fix a bug higher up somewhere?
• What's the use case for a user doing this?

[ickshonpe]

If you remove a ContentSize component from a Bevy UI entity and then replace it

Just for the sake of documentation:

• Is ContentSize ever removed in Bevy / does this fix a bug higher up somewhere?
• What's the use case for a user doing this?

I'm not sure if there are any specific use cases. @viridia seemed to be experimenting with some quite complicated ideas that might require it.

It's about the API obeying the basic principle that an entity is defined by its components.

For example, suppose that you have an entity with the TextBundle components that you are using to display some text. The ideal is that if you remove the TextBundle components and then immediately insert the SpriteBundle components then that entity is now a sprite, the text is gone, you see a sprite instead, there are no panics and nothing weird happens.

[viridia]

I haven't gotten around to messing with ContentSize yet. However...

I am working with adding and removing components. For example, suppose you have a style asset that has a "background-color" attribute. The background color can either be a color (in which case the BackgroundColor component needs to exist) or it can be "transparent", which means "don't render a background color at all" (it does not mean render with alpha 0), which requires that the BackgroundColor component be removed from the entity.

This style property can change due to several causes: a hot reload of the style asset; an override of a context variable farther up the tree; a dynamic style change due to a state change in a widget.

That being said, it's hard to imagine a use case for dynamically changing ContentSize, other than hot reloading of a style asset.

[mockersf]

while a very uncommon case, this should have pretty much no perf impact when not hitting that case

[alice-i-cecile]

@ickshonpe once merge conflicts are fixed I'll merge this for you.