ContentSize

If you remove a `ContentSize` component from a Bevy UI entity and then immediately 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 updates on changes to their content, and don't check `ContentSize` for changes. This means that until the content is changed in some way, no content will be displayed by the node.

This commit fixes this by performing `ui_layout_system`'s `ContentSize` removal detection and resolution first, before measure func updates and in the widget systems generating a new `Measure` when a  `ContentSize` component is added to a widget entity.

crates/bevy_ui/src/layout/mod.rs

@@ -274,6 +274,11 @@ pub fn ui_layout_system(
  }
  }
 
+ // When a `ContentSize` component is removed from an entity, we need to remove the measure from the corresponding taffy node.
+ for entity in removed_content_sizes.iter() {
+ ui_surface.try_remove_measure(entity);
+ }
+
  for (entity, mut content_size) in measure_query.iter_mut() {
  if let Some(measure_func) = content_size.measure_func.take() {
  ui_surface.update_measure(entity, measure_func);
@@ -283,11 +288,6 @@ pub fn ui_layout_system(
  // clean up removed nodes
  ui_surface.remove_entities(removed_nodes.iter());
 
- // When a `ContentSize` component is removed from an entity, we need to remove the measure from the corresponding taffy node.
- for entity in removed_content_sizes.iter() {
- ui_surface.try_remove_measure(entity);
- }
-
  // update window children (for now assuming all Nodes live in the primary window)
  ui_surface.set_window_children(primary_window_entity, root_node_query.iter());
 

crates/bevy_ui/src/widget/image.rs

@@ -3,6 +3,7 @@ use crate::{
 };
 use bevy_asset::{Assets, Handle};
 
+use bevy_ecs::change_detection::DetectChanges;
 use bevy_ecs::query::Without;
 use bevy_ecs::{
  prelude::Component,
@@ -96,7 +97,7 @@ pub fn update_image_content_size_system(
  texture.texture_descriptor.size.height as f32,
  );
  // Update only if size or scale factor has changed to avoid needless layout calculations
- if size != image_size.size || combined_scale_factor != *previous_combined_scale_factor {
+ if size != image_size.size || combined_scale_factor != *previous_combined_scale_factor  || content_size.is_added() {
  image_size.size = size;
  content_size.set(ImageMeasure {
  // multiply the image size by the scale factor to get the physical size
@@ -135,7 +136,7 @@ pub fn update_atlas_content_size_system(
  if let Some(atlas) = atlases.get(atlas) {
  let size = atlas.textures[atlas_image.index].size();
  // Update only if size or scale factor has changed to avoid needless layout calculations
- if size != image_size.size || combined_scale_factor != *previous_combined_scale_factor {
+ if size != image_size.size || combined_scale_factor != *previous_combined_scale_factor || content_size.is_added() {
  image_size.size = size;
  content_size.set(ImageMeasure {
  // multiply the image size by the scale factor to get the physical size

crates/bevy_ui/src/widget/text.rs

@@ -128,7 +128,7 @@ pub fn measure_text_system(
  if *last_scale_factor == scale_factor {
  // scale factor unchanged, only create new measure funcs for modified text
  for (text, content_size, text_flags) in text_query.iter_mut() {
- if text.is_changed() || text_flags.needs_new_measure_func {
+ if text.is_changed() || text_flags.needs_new_measure_func || content_size.is_added() {
  create_text_measure(&fonts, scale_factor, text, content_size, text_flags);
  }
  }