Animating entity rotation with a system

At a glance

Project structure

Source bundle/
├── Packages/SceneAssets/Sources/SceneAssets/RotationSystem.swift  # RotationSystem
├── Packages/SceneAssets/Sources/SceneAssets/RotationComponent.swift  # RotationComponent
├── Rotation/RotationApp.swift  # RotationApp
├── Rotation/ContentView.swift  # ContentView
├── Packages/SceneAssets/Sources/SceneAssets/RotationAxis.swift  # RotationAxis
├── Packages/SceneAssets/Sources/SceneAssets/SceneAssets.swift
└── Packages/SceneAssets/Package.realitycomposerpro/ProjectData/main.json  # authored RealityKit content

Structure observations

• The runtime boundary is WindowGroup; the tree is pruned to lifecycle, state, framework, rendering, and ECS files.
• The source is Swift; its compact size does not erase any explicit owner shown below.
• Authored .reality, .rkassets, or Reality Composer Pro content is an implementation boundary; Swift loads or drives it rather than reproducing its entity graph.

Overall architecture

flowchart LR
    RotationApp_1["RotationApp"]
    WindowGroup_2["WindowGroup"]
    ContentView_3["ContentView"]
    RotationSystem_4["RotationSystem"]
    RealityKit_entity_graph_5["RealityKit entity graph"]
    RealityKit_components_and_systems_6["RealityKit components and systems"]
    RotationApp_1 --> WindowGroup_2
    WindowGroup_2 --> ContentView_3
    ContentView_3 --> RotationSystem_4
    RotationSystem_4 --> RealityKit_entity_graph_5
    RealityKit_entity_graph_5 --> RealityKit_components_and_systems_6

Reference code

Rotation/RotationApp.swift:12 — executable or app-lifecycle anchor.

struct RotationApp: App {
    // ...
}

The diagram is a responsibility flow, not a claim that every adjacent node directly calls the next. It separates lifecycle, presentation, stable state, framework/session work, and the RealityKit entity graph.

Ownership and state

classDiagram
    ContentView *-- Entity : telescope
    ContentView *-- RotationAxis : axis
    RotationSystem *-- EntityQuery : rotationQuery
    ContentView *-- Bool : isRotating

Ownership evidence

Rotation/ContentView.swift:12 — representative stored state or nearest verified lifecycle anchor.

struct ContentView: View {
    @State var telescope: Entity? = nil
    // ...
}

Ownership is intentionally narrow: environment, bindings, observed objects, weak links, and delegate callbacks do not prove lifetime ownership. Initialized stored services and wrapper-managed state do; entities added inside a RealityKit content closure belong to that entity graph.

Class and protocol design

The reviewed source defines no local substitution protocol. Its App, View, delegate, renderer, ARKit, and RealityKit conformances are framework-facing contracts, so this document does not label the whole app protocol-oriented.

Access control

No reviewed Swift access example uses fileprivate, private(set), open; declarations without a modifier are internal.

Reference code

Packages/SceneAssets/Sources/SceneAssets/RotationAxis.swift:8 — representative visibility boundary.

    public var id: String {
        return self.rawValue
    }

Logic ownership and placement

Design patterns

Naming conventions

• Role suffixes are evidence, not decoration: App: RotationApp; System: RotationSystem; Component: RotationComponent; View: ContentView.
• ECS names pair data and behavior: components RotationComponent; systems RotationSystem.
• Protocols: no app-defined protocol in the reviewed source.
• Commands use verb-led methods: update.
• Files and folders use primary-type or responsibility names such as Views, Models, Rendering, Components, Systems, Packages, or feature names where those boundaries exist.

Architecture takeaways

• Treat RotationApp as the owner of executable or scene lifecycle, not automatically as the owner of every entity created later.
• Keep view/controller input near presentation, but move sessions, reconstruction, capture, rendering, shared game state, or documents into stable owners when their lifetime exceeds one callback or render pass.
• Use components for per-entity data and systems for repeated simulation instead of centralizing every update in a view model or controller.
• Count authored Reality Composer Pro assets as implementation; the Swift shell is not the complete architecture.
• The compact source does not justify repository, coordinator, or protocol layers beyond boundaries the sample actually declares.

Source map

• Official Apple documentation