# 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.
```rust
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.
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