Simulating physics joints in your RealityKit app

At a glance

Project structure

Source bundle/
└── RealityKit-PhysicsJoint/
    └── RealityKit-PhysicsJoint/
        ├── MainView.swift  # MainView
        ├── RealityKitPhysicsJointApp.swift  # RealityKitPhysicsJointApp
        ├── PendulumSettings.swift  # PendulumSettings
        ├── MainView+PendulumCreation.swift
        ├── MainView+PendulumModels.swift
        └── MainView+PhysicsAndPins.swift

Structure observations

• The runtime boundary is WindowGroup + volumetric window; the tree is pruned to lifecycle, state, framework, rendering, and ECS files.
• The source is Swift; role-named types and feature folders separate the major responsibilities.

Overall architecture

flowchart LR
    RealityKitPhysicsJointApp_1["RealityKitPhysicsJointApp"]
    WindowGroup___volumetric_window_2["WindowGroup + volumetric window"]
    MainView_3["MainView"]
    RealityKit_entity_graph_4["RealityKit entity graph"]
    RealityKit_physics_bodies_joints_5["RealityKit physics bodies/joints"]
    RealityKitPhysicsJointApp_1 --> WindowGroup___volumetric_window_2
    WindowGroup___volumetric_window_2 --> MainView_3
    MainView_3 --> RealityKit_entity_graph_4
    RealityKit_entity_graph_4 --> RealityKit_physics_bodies_joints_5

Reference code

RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/RealityKitPhysicsJointApp.swift:11 — executable or app-lifecycle anchor.

struct RealityKitPhysicsJointApp: App {
    var body: some Scene {
        WindowGroup {
            MainView()
        }
        #if os(visionOS)
        .windowStyle(.volumetric)
        #endif
    }
}

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
    MainView *-- PendulumSettings : pendulumSettings
    MainView *-- Array : pendulums
    PendulumSettings --> Color : stringColor
    PendulumSettings --> Color : attachmentColor

Ownership evidence

RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/MainView.swift:18 — representative stored state or nearest verified lifecycle anchor.

struct MainView: View {
    // ...
    let pendulumSettings = PendulumSettings()
    // ...
}

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), public, open; declarations without a modifier are internal.

Reference code

RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/RealityKitPhysicsJointApp.swift:11 — representative visibility boundary.

struct RealityKitPhysicsJointApp: App {
    // ...
}

Logic ownership and placement

Design patterns

Naming conventions

• Role suffixes are evidence, not decoration: App: RealityKitPhysicsJointApp; View: MainView.
• Protocols: no app-defined protocol in the reviewed source.
• Commands use verb-led methods: impulseButtonAction, buildPendulumScene, createPendulum, makeBallEntity, makeStringEntity, makeAttachmentEntity, addPinsTo, addBallPhysics.
• 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 RealityKitPhysicsJointApp 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.

Source map

• Official Apple documentation