Simulating physics joints in your RealityKit app

At a glance

Project structure

Source bundle/
├── RealityKit-PhysicsJoint/
│   ├── RealityKit-PhysicsJoint/
│   │   ├── RealityKitPhysicsJointApp.swift
│   │   ├── MainView.swift
│   │   ├── PendulumSettings.swift
│   │   ├── MainView+PendulumCreation.swift
│   │   ├── MainView+PendulumModels.swift
│   │   ├── MainView+PhysicsAndPins.swift
│   │   ├── Info.plist
│   │   └── RealityKit_PhysicsJoint.entitlements
│   └── RealityKit-PhysicsJoint.xcodeproj/
│       ├── .xcodesamplecode.plist
│       └── project.pbxproj
└── Configuration/
    └── SampleCode.xcconfig

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: Swift.
• The verified tree contains 5 project/configuration file(s) and 3 source declaration(s).

Overall architecture

flowchart LR
    N1["RealityKitPhysicsJointApp"]
    N2["MainView"]
    N3["RealityKit APIs"]
    N1 --> N2
    N2 --> N3

Reference code

RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/RealityKitPhysicsJointApp.swift:10 — architecture anchor

@main
struct RealityKitPhysicsJointApp: App {
    // ...
        WindowGroup {
            MainView()
        }
    // ...
}

Interpretation

The arrows summarize the source-visible entry, role-named types or folders, and framework direction; when nodes come from structural folders, the sequence is a high-level interpretation rather than proof that every adjacent node calls the next. Ownership is claimed only where the next section cites a stored property or assignment. The diagram is intentionally limited to the dominant path into RealityKit.

Ownership and state

classDiagram
    MainView *-- PendulumSettings : pendulumSettings
    MainView *-- Array : pendulums
    PendulumSettings *-- Float : ballRadius
    PendulumSettings *-- Float : ballMass

Ownership evidence

RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/MainView.swift:18 — stored dependency or nearest verified ownership anchor

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

Composition arrows indicate a source-visible construction expression or locally owned value state; aggregation means the owner stores or receives a dependency without proving exclusive lifetime ownership.

Concurrency, scheduling, and thread safety

Evidence limit: actor isolation, async/await, or Task creation does not by itself prove background-thread execution; Sendable conformance alone does not prove thread-safe mutation.

@MainActor/MainActor.run, DispatchQueue.main, and RunLoop.main are reported as distinct isolation, queue, and event-loop mechanisms. A plain Task is kept separate from Task.detached; neither is labeled as a background thread.

Reference code

RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/MainView.swift:73 — representative execution boundary

        DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) {
            // Re-enable button.
            pushButtonDisabled = false
        }

State propagation, frameworks, and dependencies

Evidence limit: an import proves a source-level compilation dependency at the cited line; it does not prove runtime use, architectural adoption, or whether a Swift package is a direct application dependency.

receive(on:) describes downstream delivery scheduling, while subscribe(on:) describes upstream subscription/request/cancel scheduling. An import Combine alone establishes neither behavior nor a Store, reducer, Redux, or other application architecture.

Class and protocol design

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

@main
struct RealityKitPhysicsJointApp: App {
    // ...
            MainView()
    // ...
}

No local protocol conformance is claimed as protocol-oriented design; external framework conformances are listed only as dependencies.

Access control

Reference code

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

@main
struct RealityKitPhysicsJointApp: App {
    // ...
            MainView()
    // ...
}

Swift declarations without a modifier are internal; explicit private, fileprivate, private(set), public, or open entries above are interpreted by language semantics. Objective-C/C samples instead rely on header and implementation boundaries, which are not equivalent to Swift lexical privacy.

Logic ownership and placement

Design patterns

Naming conventions

• Types: App: RealityKitPhysicsJointApp; View: MainView.
• Protocols: no local protocol declaration in the scanned source.
• Methods: impulseButtonAction, buildPendulumScene, createPendulum, makeBallEntity, makeStringEntity, makeAttachmentEntity, addPinsTo, addBallPhysics.
• Files: RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/RealityKitPhysicsJointApp.swift, RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/MainView.swift, RealityKit-PhysicsJoint/RealityKit-PhysicsJoint/PendulumSettings.swift.

Architecture takeaways

• RealityKitPhysicsJointApp is the main source-visible entry or composition anchor for this sample.
• Framework work reaches RealityKit, SwiftUI through a deliberately small high-level chain; the detailed API graph remains inside the cited implementation files.
• Stored-property evidence identifies lifecycle collaboration; it does not by itself prove exclusive object ownership.
• Access-control conclusions separate verified language visibility from the likely design rationale.
• The source does not justify labeling the design protocol-oriented.

Source map

• Official Apple documentation