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## Introduction This is the first step in my [Next Generation Scene / UI Proposal](https://github.com/bevyengine/bevy/discussions/14437). Fixes https://github.com/bevyengine/bevy/issues/7272 #14800. Bevy's current Bundles as the "unit of construction" hamstring the UI user experience and have been a pain point in the Bevy ecosystem generally when composing scenes: * They are an additional _object defining_ concept, which must be learned separately from components. Notably, Bundles _are not present at runtime_, which is confusing and limiting. * They can completely erase the _defining component_ during Bundle init. For example, `ButtonBundle { style: Style::default(), ..default() }` _makes no mention_ of the `Button` component symbol, which is what makes the Entity a "button"! * They are not capable of representing "dependency inheritance" without completely non-viable / ergonomically crushing nested bundles. This limitation is especially painful in UI scenarios, but it applies to everything across the board. * They introduce a bunch of additional nesting when defining scenes, making them ugly to look at * They introduce component name "stutter": `SomeBundle { component_name: ComponentName::new() }` * They require copious sprinklings of `..default()` when spawning them in Rust code, due to the additional layer of nesting **Required Components** solve this by allowing you to define which components a given component needs, and how to construct those components when they aren't explicitly provided. This is what a `ButtonBundle` looks like with Bundles (the current approach): ```rust #[derive(Component, Default)] struct Button; #[derive(Bundle, Default)] struct ButtonBundle { pub button: Button, pub node: Node, pub style: Style, pub interaction: Interaction, pub focus_policy: FocusPolicy, pub border_color: BorderColor, pub border_radius: BorderRadius, pub image: UiImage, pub transform: Transform, pub global_transform: GlobalTransform, pub visibility: Visibility, pub inherited_visibility: InheritedVisibility, pub view_visibility: ViewVisibility, pub z_index: ZIndex, } commands.spawn(ButtonBundle { style: Style { width: Val::Px(100.0), height: Val::Px(50.0), ..default() }, focus_policy: FocusPolicy::Block, ..default() }) ``` And this is what it looks like with Required Components: ```rust #[derive(Component)] #[require(Node, UiImage)] struct Button; commands.spawn(( Button, Style { width: Val::Px(100.0), height: Val::Px(50.0), ..default() }, FocusPolicy::Block, )); ``` With Required Components, we mention only the most relevant components. Every component required by `Node` (ex: `Style`, `FocusPolicy`, etc) is automatically brought in! ### Efficiency 1. At insertion/spawn time, Required Components (including recursive required components) are initialized and inserted _as if they were manually inserted alongside the given components_. This means that this is maximally efficient: there are no archetype or table moves. 2. Required components are only initialized and inserted if they were not manually provided by the developer. For the code example in the previous section, because `Style` and `FocusPolicy` are inserted manually, they _will not_ be initialized and inserted as part of the required components system. Efficient! 3. The "missing required components _and_ constructors needed for an insertion" are cached in the "archetype graph edge", meaning they aren't computed per-insertion. When a component is inserted, the "missing required components" list is iterated (and that graph edge (AddBundle) is actually already looked up for us during insertion, because we need that for "normal" insert logic too). ### IDE Integration The `#[require(SomeComponent)]` macro has been written in such a way that Rust Analyzer can provide type-inspection-on-hover and `F12` / go-to-definition for required components. ### Custom Constructors The `require` syntax expects a `Default` constructor by default, but it can be overridden with a custom constructor: ```rust #[derive(Component)] #[require( Node, Style(button_style), UiImage )] struct Button; fn button_style() -> Style { Style { width: Val::Px(100.0), ..default() } } ``` ### Multiple Inheritance You may have noticed by now that this behaves a bit like "multiple inheritance". One of the problems that this presents is that it is possible to have duplicate requires for a given type at different levels of the inheritance tree: ```rust #[derive(Component) struct X(usize); #[derive(Component)] #[require(X(x1)) struct Y; fn x1() -> X { X(1) } #[derive(Component)] #[require( Y, X(x2), )] struct Z; fn x2() -> X { X(2) } // What version of X is inserted for Z? commands.spawn(Z); ``` This is allowed (and encouraged), although this doesn't appear to occur much in practice. First: only one version of `X` is initialized and inserted for `Z`. In the case above, I think we can all probably agree that it makes the most sense to use the `x2` constructor for `X`, because `Y`'s `x1` constructor exists "beneath" `Z` in the inheritance hierarchy; `Z`'s constructor is "more specific". The algorithm is simple and predictable: 1. Use all of the constructors (including default constructors) directly defined in the spawned component's require list 2. In the order the requires are defined in `#[require()]`, recursively visit the require list of each of the components in the list (this is a depth Depth First Search). When a constructor is found, it will only be used if one has not already been found. From a user perspective, just think about this as the following: 1. Specifying a required component constructor for `Foo` directly on a spawned component `Bar` will result in that constructor being used (and overriding existing constructors lower in the inheritance tree). This is the classic "inheritance override" behavior people expect. 2. For cases where "multiple inheritance" results in constructor clashes, Components should be listed in "importance order". List a component earlier in the requirement list to initialize its inheritance tree earlier. Required Components _does_ generally result in a model where component values are decoupled from each other at construction time. Notably, some existing Bundle patterns use bundle constructors to initialize multiple components with shared state. I think (in general) moving away from this is necessary: 1. It allows Required Components (and the Scene system more generally) to operate according to simple rules 2. The "do arbitrary init value sharing in Bundle constructors" approach _already_ causes data consistency problems, and those problems would be exacerbated in the context of a Scene/UI system. For cases where shared state is truly necessary, I think we are better served by observers / hooks. 3. If a situation _truly_ needs shared state constructors (which should be rare / generally discouraged), Bundles are still there if they are needed. ## Next Steps * **Require Construct-ed Components**: I have already implemented this (as defined in the [Next Generation Scene / UI Proposal](https://github.com/bevyengine/bevy/discussions/14437). However I've removed `Construct` support from this PR, as that has not landed yet. Adding this back in requires relatively minimal changes to the current impl, and can be done as part of a future Construct pr. * **Port Built-in Bundles to Required Components**: This isn't something we should do right away. It will require rethinking our public interfaces, which IMO should be done holistically after the rest of Next Generation Scene / UI lands. I think we should merge this PR first and let people experiment _inside their own code with their own Components_ while we wait for the rest of the new scene system to land. * **_Consider_ Automatic Required Component Removal**: We should evaluate _if_ automatic Required Component removal should be done. Ex: if all components that explicitly require a component are removed, automatically remove that component. This issue has been explicitly deferred in this PR, as I consider the insertion behavior to be desirable on its own (and viable on its own). I am also doubtful that we can find a design that has behavior we actually want. Aka: can we _really_ distinguish between a component that is "only there because it was automatically inserted" and "a component that was necessary / should be kept". See my [discussion response here](https://github.com/bevyengine/bevy/discussions/14437#discussioncomment-10268668) for more details. --------- Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> Co-authored-by: BD103 <59022059+BD103@users.noreply.github.com> Co-authored-by: Pascal Hertleif <killercup@gmail.com> |
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