bevy/crates/bevy_ui
Viktor Gustavsson f86ee32576
Add UI GhostNode (#15341)
# Objective

- Fixes #14826 
- For context, see #15238

## Solution
Add a `GhostNode` component to `bevy_ui` and update all the relevant
systems to use it to traverse for UI children.

- [x] `ghost_hierarchy` module
  - [x] Add `GhostNode`
- [x] Add `UiRootNodes` system param for iterating (ghost-aware) UI root
nodes
- [x] Add `UiChildren` system param for iterating (ghost-aware) UI
children
- [x] Update `layout::ui_layout_system`
  - [x] Use ghost-aware root nodes for camera updates
  - [x] Update and remove children in taffy
    - [x] Initial spawn
    - [x] Detect changes on nested UI children
- [x] Use ghost-aware children traversal in
`update_uinode_geometry_recursive`
- [x] Update the rest of the UI systems to use the ghost hierarchy
  - [x] `stack::ui_stack_system`
  - [x] `update::`
    - [x] `update_clipping_system`
    - [x] `update_target_camera_system`
  - [x] `accessibility::calc_name`

## Testing
- [x] Added a new example `ghost_nodes` that can be used as a testbed.
- [x] Added unit tests for _some_ of the traversal utilities in
`ghost_hierarchy`
- [x] Ensure this fulfills the needs for currently known use cases
  - [x] Reactivity libraries (test with `bevy_reactor`)
- [ ] Text spans (mentioned by koe [on
discord](https://discord.com/channels/691052431525675048/1285371432460881991/1285377442998915246))
  
---
## Performance
[See comment
below](https://github.com/bevyengine/bevy/pull/15341#issuecomment-2385456820)

## Migration guide
Any code that previously relied on `Parent`/`Children` to iterate UI
children may now want to use `bevy_ui::UiChildren` to ensure ghost nodes
are skipped, and their first descendant Nodes included.

UI root nodes may now be children of ghost nodes, which means
`Without<Parent>` might not query all root nodes. Use
`bevy_ui::UiRootNodes` where needed to iterate root nodes instead.

## Potential future work
- Benchmarking/optimizations of hierarchies containing lots of ghost
nodes
- Further exploration of UI hierarchies and markers for root nodes/leaf
nodes to create better ergonomics for things like `UiLayer` (world-space
ui)

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: UkoeHB <37489173+UkoeHB@users.noreply.github.com>
2024-10-02 00:24:28 +00:00
..
src Add UI GhostNode (#15341) 2024-10-02 00:24:28 +00:00
Cargo.toml feature gate picking backends (#15369) 2024-09-22 19:35:15 +00:00
README.md Add README.md to all crates (#13184) 2024-05-02 18:56:00 +00:00

Bevy UI

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