cc69fdd0c6
16 Commits
| Author | SHA1 | Message | Date | |
|---|---|---|---|---|
Carter Anderson
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21f1e3045c
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Relationships (non-fragmenting, one-to-many) (#17398)
This adds support for one-to-many non-fragmenting relationships (with planned paths for fragmenting and non-fragmenting many-to-many relationships). "Non-fragmenting" means that entities with the same relationship type, but different relationship targets, are not forced into separate tables (which would cause "table fragmentation"). Functionally, this fills a similar niche as the current Parent/Children system. The biggest differences are: 1. Relationships have simpler internals and significantly improved performance and UX. Commands and specialized APIs are no longer necessary to keep everything in sync. Just spawn entities with the relationship components you want and everything "just works". 2. Relationships are generalized. Bevy can provide additional built in relationships, and users can define their own. **REQUEST TO REVIEWERS**: _please don't leave top level comments and instead comment on specific lines of code. That way we can take advantage of threaded discussions. Also dont leave comments simply pointing out CI failures as I can read those just fine._ ## Built on top of what we have Relationships are implemented on top of the Bevy ECS features we already have: components, immutability, and hooks. This makes them immediately compatible with all of our existing (and future) APIs for querying, spawning, removing, scenes, reflection, etc. The fewer specialized APIs we need to build, maintain, and teach, the better. ## Why focus on one-to-many non-fragmenting first? 1. This allows us to improve Parent/Children relationships immediately, in a way that is reasonably uncontroversial. Switching our hierarchy to fragmenting relationships would have significant performance implications. ~~Flecs is heavily considering a switch to non-fragmenting relations after careful considerations of the performance tradeoffs.~~ _(Correction from @SanderMertens: Flecs is implementing non-fragmenting storage specialized for asset hierarchies, where asset hierarchies are many instances of small trees that have a well defined structure)_ 2. Adding generalized one-to-many relationships is currently a priority for the [Next Generation Scene / UI effort](https://github.com/bevyengine/bevy/discussions/14437). Specifically, we're interested in building reactions and observers on top. ## The changes This PR does the following: 1. Adds a generic one-to-many Relationship system 3. Ports the existing Parent/Children system to Relationships, which now lives in `bevy_ecs::hierarchy`. The old `bevy_hierarchy` crate has been removed. 4. Adds on_despawn component hooks 5. Relationships can opt-in to "despawn descendants" behavior, meaning that the entire relationship hierarchy is despawned when `entity.despawn()` is called. The built in Parent/Children hierarchies enable this behavior, and `entity.despawn_recursive()` has been removed. 6. `world.spawn` now applies commands after spawning. This ensures that relationship bookkeeping happens immediately and removes the need to manually flush. This is in line with the equivalent behaviors recently added to the other APIs (ex: insert). 7. Removes the ValidParentCheckPlugin (system-driven / poll based) in favor of a `validate_parent_has_component` hook. ## Using Relationships The `Relationship` trait looks like this: ```rust pub trait Relationship: Component + Sized { type RelationshipSources: RelationshipSources<Relationship = Self>; fn get(&self) -> Entity; fn from(entity: Entity) -> Self; } ``` A relationship is a component that: 1. Is a simple wrapper over a "target" Entity. 2. Has a corresponding `RelationshipSources` component, which is a simple wrapper over a collection of entities. Every "target entity" targeted by a "source entity" with a `Relationship` has a `RelationshipSources` component, which contains every "source entity" that targets it. For example, the `Parent` component (as it currently exists in Bevy) is the `Relationship` component and the entity containing the Parent is the "source entity". The entity _inside_ the `Parent(Entity)` component is the "target entity". And that target entity has a `Children` component (which implements `RelationshipSources`). In practice, the Parent/Children relationship looks like this: ```rust #[derive(Relationship)] #[relationship(relationship_sources = Children)] pub struct Parent(pub Entity); #[derive(RelationshipSources)] #[relationship_sources(relationship = Parent)] pub struct Children(Vec<Entity>); ``` The Relationship and RelationshipSources derives automatically implement Component with the relevant configuration (namely, the hooks necessary to keep everything in sync). The most direct way to add relationships is to spawn entities with relationship components: ```rust let a = world.spawn_empty().id(); let b = world.spawn(Parent(a)).id(); assert_eq!(world.entity(a).get::<Children>().unwrap(), &[b]); ``` There are also convenience APIs for spawning more than one entity with the same relationship: ```rust world.spawn_empty().with_related::<Children>(|s| { s.spawn_empty(); s.spawn_empty(); }) ``` The existing `with_children` API is now a simpler wrapper over `with_related`. This makes this change largely non-breaking for existing spawn patterns. ```rust world.spawn_empty().with_children(|s| { s.spawn_empty(); s.spawn_empty(); }) ``` There are also other relationship APIs, such as `add_related` and `despawn_related`. ## Automatic recursive despawn via the new on_despawn hook `RelationshipSources` can opt-in to "despawn descendants" behavior, which will despawn all related entities in the relationship hierarchy: ```rust #[derive(RelationshipSources)] #[relationship_sources(relationship = Parent, despawn_descendants)] pub struct Children(Vec<Entity>); ``` This means that `entity.despawn_recursive()` is no longer required. Instead, just use `entity.despawn()` and the relevant related entities will also be despawned. To despawn an entity _without_ despawning its parent/child descendants, you should remove the `Children` component first, which will also remove the related `Parent` components: ```rust entity .remove::<Children>() .despawn() ``` This builds on the on_despawn hook introduced in this PR, which is fired when an entity is despawned (before other hooks). ## Relationships are the source of truth `Relationship` is the _single_ source of truth component. `RelationshipSources` is merely a reflection of what all the `Relationship` components say. By embracing this, we are able to significantly improve the performance of the system as a whole. We can rely on component lifecycles to protect us against duplicates, rather than needing to scan at runtime to ensure entities don't already exist (which results in quadratic runtime). A single source of truth gives us constant-time inserts. This does mean that we cannot directly spawn populated `Children` components (or directly add or remove entities from those components). I personally think this is a worthwhile tradeoff, both because it makes the performance much better _and_ because it means theres exactly one way to do things (which is a philosophy we try to employ for Bevy APIs). As an aside: treating both sides of the relationship as "equivalent source of truth relations" does enable building simple and flexible many-to-many relationships. But this introduces an _inherent_ need to scan (or hash) to protect against duplicates. [`evergreen_relations`](https://github.com/EvergreenNest/evergreen_relations) has a very nice implementation of the "symmetrical many-to-many" approach. Unfortunately I think the performance issues inherent to that approach make it a poor choice for Bevy's default relationship system. ## Followup Work * Discuss renaming `Parent` to `ChildOf`. I refrained from doing that in this PR to keep the diff reasonable, but I'm personally biased toward this change (and using that naming pattern generally for relationships). * [Improved spawning ergonomics](https://github.com/bevyengine/bevy/discussions/16920) * Consider adding relationship observers/triggers for "relationship targets" whenever a source is added or removed. This would replace the current "hierarchy events" system, which is unused upstream but may have existing users downstream. I think triggers are the better fit for this than a buffered event queue, and would prefer not to add that back. * Fragmenting relations: My current idea hinges on the introduction of "value components" (aka: components whose type _and_ value determines their ComponentId, via something like Hashing / PartialEq). By labeling a Relationship component such as `ChildOf(Entity)` as a "value component", `ChildOf(e1)` and `ChildOf(e2)` would be considered "different components". This makes the transition between fragmenting and non-fragmenting a single flag, and everything else continues to work as expected. * Many-to-many support * Non-fragmenting: We can expand Relationship to be a list of entities instead of a single entity. I have largely already written the code for this. * Fragmenting: With the "value component" impl mentioned above, we get many-to-many support "for free", as it would allow inserting multiple copies of a Relationship component with different target entities. Fixes #3742 (If this PR is merged, I think we should open more targeted followup issues for the work above, with a fresh tracking issue free of the large amount of less-directed historical context) Fixes #17301 Fixes #12235 Fixes #15299 Fixes #15308 ## Migration Guide * Replace `ChildBuilder` with `ChildSpawnerCommands`. * Replace calls to `.set_parent(parent_id)` with `.insert(Parent(parent_id))`. * Replace calls to `.replace_children()` with `.remove::<Children>()` followed by `.add_children()`. Note that you'll need to manually despawn any children that are not carried over. * Replace calls to `.despawn_recursive()` with `.despawn()`. * Replace calls to `.despawn_descendants()` with `.despawn_related::<Children>()`. * If you have any calls to `.despawn()` which depend on the children being preserved, you'll need to remove the `Children` component first. --------- Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> |
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Antony
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02bb151889
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Rename PickingBehavior to Pickable (#17266)
# Objective PR #17225 allowed for sprite picking to be opt-in. After some discussion, it was agreed that `PickingBehavior` should be used to opt-in to sprite picking behavior for entities. This leads to `PickingBehavior` having two purposes: mark an entity for use in a backend, and describe how it should be picked. Discussion led to the name `Pickable`making more sense (also: this is what the component was named before upstreaming). A follow-up pass will be made after this PR to unify backends. ## Solution Replace all instances of `PickingBehavior` and `picking_behavior` with `Pickable` and `pickable`, respectively. ## Testing CI ## Migration Guide Change all instances of `PickingBehavior` to `Pickable`. |
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Alice Cecile
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48fe2a6e21
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Rename "focus" in bevy_picking to "hover" (#16872)
# Objective With the introduction of bevy_input_focus, the uses of "focus" in bevy_picking are quite confusing and make searching hard. Users will intuitively think these concepts are related, but they actually aren't. ## Solution Rename / rephrase all uses of "focus" in bevy_picking to refer to "hover", since this is ultimately related to creating the `HoverMap`. ## Migration Guide Various terms related to "focus" in `bevy_picking` have been renamed to refer to "hover" to avoid confusion with `bevy_input_focus`. In particular: - The `update_focus` system has been renamed to `generate_hovermap` - `PickSet::Focus` and `PostFocus` have been renamed to `Hover` and `PostHover` - The `bevy_picking::focus` module has been renamed to `bevy_picking::hover` - The `is_focus_enabled` field on `PickingPlugin` has been renamed to `is_hover_enabled` - The `focus_should_run` run condition has been renamed to `hover_should_run` |
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Harun Ibram
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ad4144ad7a
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Rename Pointer<Down/Up> -> Pointer<Pressed/Released> in bevy_picking. (#16331)
# Objective Fixes #16192 ## Solution I renamed the Pointer<Down/Up> to <Pressed/Released> and then I resolved all the errors. Renamed variables like "is_down" to "is_pressed" to maintain consistency. Modified the docs in places where 'down/up' were used to maintain consistency. ## Testing I haven't tested this in any way beside the checks from rust analyzer and the examples in the examples/ directory. --- ## Migration Guide ### `bevy_picking/src/pointer.rs`: #### `enum PressDirection`: - `PressDirection::Down` changes to `PressDirection::Pressed`. - `PressDirection::Up` changes to `PressDirection::Released`. These changes are also relevant when working with `enum PointerAction` ### `bevy_picking/src/events.rs`: Clicking and pressing Events in events.rs categories change from [Down], [Up], [Click] to [Pressed], [Released], [Click]. - `struct Down` changes to `struct Pressed` - fires when a pointer button is pressed over the 'target' entity. - `struct Up` changes to `struct Released` - fires when a pointer button is released over the 'target' entity. - `struct Click` now fires when a pointer sends a Pressed event followed by a Released event on the same 'target'. - `struct DragStart` now fires when the 'target' entity receives a pointer Pressed event followed by a pointer Move event. - `struct DragEnd` now fires when the 'target' entity is being dragged and receives a pointer Released event. - `PickingEventWriters<'w>::down_events: EventWriter<'w, Pointer<Down>>` changes to `PickingEventWriters<'w>::pressed_events: EventWriter<'w, Pointer<Pressed>>`. - `PickingEventWriters<'w>::up_events changes to PickingEventWriters<'w>::released_events`. --------- Co-authored-by: Harun Ibram <harun.ibram@outlook.com> Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> |
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Aevyrie
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61b98ec80f
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Rename trigger.entity() to trigger.target() (#16716)
# Objective - A `Trigger` has multiple associated `Entity`s - the entity observing the event, and the entity that was targeted by the event. - The field `entity: Entity` encodes no semantic information about what the entity is used for, you can already tell that it's an `Entity` by the type signature! ## Solution - Rename `trigger.entity()` to `trigger.target()` --- ## Changelog - `Trigger`s are associated with multiple entities. `Trigger::entity()` has been renamed to `Trigger::target()` to reflect the semantics of the entity being returned. ## Migration Guide - Rename `Trigger::entity()` to `Trigger::target()`. - Rename `ObserverTrigger::entity` to `ObserverTrigger::target` |
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Derick M
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0ac495f7f4
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Remove accesskit re-export from bevy_a11y (#16257)
# Objective - Fixes #16235 ## Solution - Both Bevy and AccessKit export a `Node` struct, to reduce confusion Bevy will no longer re-export `AccessKit` from `bevy_a11y` ## Testing - Tested locally ## Migration Guide ```diff # main.rs -- use bevy_a11y::{ -- accesskit::{Node, Rect, Role}, -- AccessibilityNode, -- }; ++ use bevy_a11y::AccessibilityNode; ++ use accesskit::{Node, Rect, Role}; # Cargo.toml ++ accesskit = "0.17" ``` - Users will need to add `accesskit = "0.17"` to the dependencies section of their `Cargo.toml` file and update their `accesskit` use statements to come directly from the external crate instead of `bevy_a11y`. - Make sure to keep the versions of `accesskit` aligned with the versions Bevy uses. |
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Nolan Darilek
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817f160d35
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Bump accesskit and accesskit_winit. (#16234)
# Objective - Bumps accesskit and accesskit_winit dependencies ## Solution - Fixes several breaking API changes introduced in accesskit 0.23. ## Testing - Tested with the ui example and seems to work comparably |
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Carter Anderson
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015f2c69ca
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Merge Style properties into Node. Use ComputedNode for computed properties. (#15975)
# Objective Continue improving the user experience of our UI Node API in the direction specified by [Bevy's Next Generation Scene / UI System](https://github.com/bevyengine/bevy/discussions/14437) ## Solution As specified in the document above, merge `Style` fields into `Node`, and move "computed Node fields" into `ComputedNode` (I chose this name over something like `ComputedNodeLayout` because it currently contains more than just layout info. If we want to break this up / rename these concepts, lets do that in a separate PR). `Style` has been removed. This accomplishes a number of goals: ## Ergonomics wins Specifying both `Node` and `Style` is now no longer required for non-default styles Before: ```rust commands.spawn(( Node::default(), Style { width: Val::Px(100.), ..default() }, )); ``` After: ```rust commands.spawn(Node { width: Val::Px(100.), ..default() }); ``` ## Conceptual clarity `Style` was never a comprehensive "style sheet". It only defined "core" style properties that all `Nodes` shared. Any "styled property" that couldn't fit that mold had to be in a separate component. A "real" style system would style properties _across_ components (`Node`, `Button`, etc). We have plans to build a true style system (see the doc linked above). By moving the `Style` fields to `Node`, we fully embrace `Node` as the driving concept and remove the "style system" confusion. ## Next Steps * Consider identifying and splitting out "style properties that aren't core to Node". This should not happen for Bevy 0.15. --- ## Migration Guide Move any fields set on `Style` into `Node` and replace all `Style` component usage with `Node`. Before: ```rust commands.spawn(( Node::default(), Style { width: Val::Px(100.), ..default() }, )); ``` After: ```rust commands.spawn(Node { width: Val::Px(100.), ..default() }); ``` For any usage of the "computed node properties" that used to live on `Node`, use `ComputedNode` instead: Before: ```rust fn system(nodes: Query<&Node>) { for node in &nodes { let computed_size = node.size(); } } ``` After: ```rust fn system(computed_nodes: Query<&ComputedNode>) { for computed_node in &computed_nodes { let computed_size = computed_node.size(); } } ``` |
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VitalyR
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eb19a9ea0b
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Migrate UI bundles to required components (#15898)
# Objective - Migrate UI bundles to required components, fixes #15889 ## Solution - deprecate `NodeBundle` in favor of `Node` - deprecate `ImageBundle` in favor of `UiImage` - deprecate `ButtonBundle` in favor of `Button` ## Testing CI. ## Migration Guide - Replace all uses of `NodeBundle` with `Node`. e.g. ```diff commands - .spawn(NodeBundle { - style: Style { + .spawn(( + Node::default(), + Style { width: Val::Percent(100.), align_items: AlignItems::Center, justify_content: JustifyContent::Center, ..default() }, - ..default() - }) + )) ``` - Replace all uses of `ButtonBundle` with `Button`. e.g. ```diff .spawn(( - ButtonBundle { - style: Style { - width: Val::Px(w), - height: Val::Px(h), - // horizontally center child text - justify_content: JustifyContent::Center, - // vertically center child text - align_items: AlignItems::Center, - margin: UiRect::all(Val::Px(20.0)), - ..default() - }, - image: image.clone().into(), + Button, + Style { + width: Val::Px(w), + height: Val::Px(h), + // horizontally center child text + justify_content: JustifyContent::Center, + // vertically center child text + align_items: AlignItems::Center, + margin: UiRect::all(Val::Px(20.0)), ..default() }, + UiImage::from(image.clone()), ImageScaleMode::Sliced(slicer.clone()), )) ``` - Replace all uses of `ImageBundle` with `UiImage`. e.g. ```diff - commands.spawn(ImageBundle { - image: UiImage { + commands.spawn(( + UiImage { texture: metering_mask, ..default() }, - style: Style { + Style { width: Val::Percent(100.0), height: Val::Percent(100.0), ..default() }, - ..default() - }); + )); ``` --------- Co-authored-by: Carter Anderson <mcanders1@gmail.com> |
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ickshonpe
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6f7d0e5725
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split up TextStyle (#15857)
# Objective
Currently text is recomputed unnecessarily on any changes to its color,
which is extremely expensive.
## Solution
Split up `TextStyle` into two separate components `TextFont` and
`TextColor`.
## Testing
I added this system to `many_buttons`:
```rust
fn set_text_colors_changed(mut colors: Query<&mut TextColor>) {
for mut text_color in colors.iter_mut() {
text_color.set_changed();
}
}
```
reports ~4fps on main, ~50fps with this PR.
## Migration Guide
`TextStyle` has been renamed to `TextFont` and its `color` field has
been moved to a separate component named `TextColor` which newtypes
`Color`.
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UkoeHB
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c2c19e5ae4
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Text rework (#15591)
**Ready for review. Examples migration progress: 100%.** # Objective - Implement https://github.com/bevyengine/bevy/discussions/15014 ## Solution This implements [cart's proposal](https://github.com/bevyengine/bevy/discussions/15014#discussioncomment-10574459) faithfully except for one change. I separated `TextSpan` from `TextSpan2d` because `TextSpan` needs to require the `GhostNode` component, which is a `bevy_ui` component only usable by UI. Extra changes: - Added `EntityCommands::commands_mut` that returns a mutable reference. This is a blocker for extension methods that return something other than `self`. Note that `sickle_ui`'s `UiBuilder::commands` returns a mutable reference for this reason. ## Testing - [x] Text examples all work. --- ## Showcase TODO: showcase-worthy ## Migration Guide TODO: very breaking ### Accessing text spans by index Text sections are now text sections on different entities in a hierarchy, Use the new `TextReader` and `TextWriter` system parameters to access spans by index. Before: ```rust fn refresh_text(mut query: Query<&mut Text, With<TimeText>>, time: Res<Time>) { let text = query.single_mut(); text.sections[1].value = format_time(time.elapsed()); } ``` After: ```rust fn refresh_text( query: Query<Entity, With<TimeText>>, mut writer: UiTextWriter, time: Res<Time> ) { let entity = query.single(); *writer.text(entity, 1) = format_time(time.elapsed()); } ``` ### Iterating text spans Text spans are now entities in a hierarchy, so the new `UiTextReader` and `UiTextWriter` system parameters provide ways to iterate that hierarchy. The `UiTextReader::iter` method will give you a normal iterator over spans, and `UiTextWriter::for_each` lets you visit each of the spans. --------- Co-authored-by: ickshonpe <david.curthoys@googlemail.com> Co-authored-by: Carter Anderson <mcanders1@gmail.com> |
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Tim
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d454db8e58
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Rename the Pickable component and fix incorrect documentation (#15707)
# Objective
- Rename `Pickable` to `PickingBehavior` to counter the easily-made
assumption that the component is required. It is optional
- Fix and clarify documentation
- The docs in `crates/bevy_ui/src/picking_backend.rs` were incorrect
about the necessity of `Pickable`
- Plus two minor code quality changes in this commit
(
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Joona Aalto
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25bfa80e60
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Migrate cameras to required components (#15641)
# Objective Yet another PR for migrating stuff to required components. This time, cameras! ## Solution As per the [selected proposal](https://hackmd.io/tsYID4CGRiWxzsgawzxG_g#Combined-Proposal-1-Selected), deprecate `Camera2dBundle` and `Camera3dBundle` in favor of `Camera2d` and `Camera3d`. Adding a `Camera` without `Camera2d` or `Camera3d` now logs a warning, as suggested by Cart [on Discord](https://discord.com/channels/691052431525675048/1264881140007702558/1291506402832945273). I would personally like cameras to work a bit differently and be split into a few more components, to avoid some footguns and confusing semantics, but that is more controversial, and shouldn't block this core migration. ## Testing I ran a few 2D and 3D examples, and tried cameras with and without render graphs. --- ## Migration Guide `Camera2dBundle` and `Camera3dBundle` have been deprecated in favor of `Camera2d` and `Camera3d`. Inserting them will now also insert the other components required by them automatically. |
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Clar Fon
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efda7f3f9c
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Simpler lint fixes: makes ci lints work but disables a lint for now (#15376)
Takes the first two commits from #15375 and adds suggestions from this comment: https://github.com/bevyengine/bevy/pull/15375#issuecomment-2366968300 See #15375 for more reasoning/motivation. ## Rebasing (rerunning) ```rust git switch simpler-lint-fixes git reset --hard main cargo fmt --all -- --unstable-features --config normalize_comments=true,imports_granularity=Crate cargo fmt --all git add --update git commit --message "rustfmt" cargo clippy --workspace --all-targets --all-features --fix cargo fmt --all -- --unstable-features --config normalize_comments=true,imports_granularity=Crate cargo fmt --all git add --update git commit --message "clippy" git cherry-pick e6c0b94f6795222310fb812fa5c4512661fc7887 ``` |
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Piefayth
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1d9ee56457
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Fix horizontal scrolling in scroll example for macOS (#15407)
# Objective Fixes #15401 ## Solution Changes the scroll inversion hotkey in the example from Shift to Control. Shift is idiomatic for this. Since we cannot use Shift per #15401, I picked another modifier arbitrarily. A production app would handle this in a platform specific way until the platform behaviors are unified upstream, but no point here. ## Testing I don't have a mac readily available for testing, if someone wouldn't mind testing. I would also appreciate confirmation that trackpad is working nicely. |
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Piefayth
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55dddaf72e
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UI Scrolling (#15291)
# Objective - Fixes #8074 - Adopts / Supersedes #8104 ## Solution Adapted from #8104 and affords the same benefits. **Additions** - [x] Update scrolling on relayout (height of node or contents may have changed) - [x] Make ScrollPosition component optional for ui nodes to avoid checking every node on scroll - [x] Nested scrollviews **Omissions** - Removed input handling for scrolling from `bevy_ui`. Users should update `ScrollPosition` directly. ### Implementation Adds a new `ScrollPosition` component. Updating this component on a `Node` with an overflow axis set to `OverflowAxis::Scroll` will reposition its children by that amount when calculating node transforms. As before, no impact on the underlying Taffy layout. Calculating this correctly is trickier than it was in #8104 due to `"Update scrolling on relayout"`. **Background** When `ScrollPosition` is updated directly by the user, it can be trivially handled in-engine by adding the parent's scroll position to the final location of each child node. However, _other layout actions_ may result in a situation where `ScrollPosition` needs to be updated. Consider a 1000 pixel tall vertically scrolling list of 100 elements, each 100 pixels tall. Scrolled to the bottom, the `ScrollPosition.offset_y` is 9000, just enough to display the last element in the list. When removing an element from that list, the new desired `ScrollPosition.offset_y` is 8900, but, critically, that is not known until after the sizes and positions of the children of the scrollable node are resolved. All user scrolling code today handles this by delaying the resolution by one frame. One notable disadvantage of this is the inability to support `WinitSettings::desktop_app()`, since there would need to be an input AFTER the layout change that caused the scroll position to update for the results of the scroll position update to render visually. I propose the alternative in this PR, which allows for same-frame resolution of scrolling layout. **Resolution** _Edit: Below resolution is outdated, and replaced with the simpler usage of taffy's `Layout::content_size`._ When recursively iterating the children of a node, each child now returns a `Vec2` representing the location of their own bottom right corner. Then, `[[0,0, [x,y]]` represents a bounding box containing the scrollable area filled by that child. Scrollable parents aggregate those areas into the bounding box of _all_ children, then consider that result against `ScrollPosition` to ensure its validity. In the event that resolution of the layout of the children invalidates the `ScrollPosition` (e.g. scrolled further than there were children to scroll to), _all_ children of that node must be recursively repositioned. The position of each child must change as a result of the change in scroll position. Therefore, this implementation takes care to only spend the cost of the "second layout pass" when a specific node actually had a `ScrollPosition` forcibly updated by the layout of its children. ## Testing Examples in `ui/scroll.rs`. There may be more complex node/style interactions that were unconsidered. --- ## Showcase  ## Alternatives - `bevy_ui` doesn't support scrolling. - `bevy_ui` implements scrolling with a one-frame delay on reactions to layout changes. |