Supporting Simulator in a Metal app

At a glance

Project structure

Source bundle/
├── Objective-C/
│   ├── Application/
│   │   ├── main.m
│   │   ├── AppDelegate.h
│   │   ├── AppDelegate.m
│   │   ├── ViewController.h
│   │   ├── ViewController.m
│   │   ├── WindowSceneDelegate.h
│   │   └── WindowSceneDelegate.m
│   └── Renderer/
│       ├── Renderer.h
│       └── Renderer.m
└── Swift/
    ├── Application/
    │   ├── AppDelegate.swift
    │   └── ViewController.swift
    └── Renderer/
        └── Renderer.swift

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: C/Objective-C header, Metal, Objective-C, Swift.
• The verified tree contains 19 project/configuration file(s) and 16 source declaration(s).

Overall architecture

flowchart LR
    Bundle["Sample bundle"]
    V1["Objective-C"]
    V2["Swift"]
    Boundary["MetalKit APIs"]
    Bundle --> V1
    V1 --> Boundary
    Bundle --> V2
    V2 --> Boundary

Reference code

Objective-C/Application/main.m:19 — architecture anchor

#if defined(TARGET_IOS) || defined(TARGET_TVOS)
int main(int argc, char * argv[]) {

    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}
#endif

Interpretation

The branches represent separate entry-bearing project variants in the downloaded bundle, not runtime calls between those variants. Each branch is intentionally collapsed at the documented framework boundary; the detailed target-local flow remains in the cited files. Ownership is claimed only where the next section cites a stored property or assignment.

Ownership and state

classDiagram
    Renderer o-- MTLDevice : device
    Renderer o-- MTLCommandQueue : commandQueue
    Renderer o-- MTLBuffer : dynamicUniformBuffer
    Renderer *-- Array : constantData

Ownership evidence

Swift/Renderer/Renderer.swift:31 — stored dependency or nearest verified ownership anchor

class Renderer: NSObject, MTKViewDelegate {
    // ...
    public let device: MTLDevice
    // ...
}

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

Swift/Renderer/Renderer.swift:46 — representative execution boundary

class Renderer: NSObject, MTKViewDelegate {
    // ...
    let inFlightSemaphore = DispatchSemaphore(value: maxBuffersInFlight)
    // ...
}

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

Swift/Application/AppDelegate.swift:12 — representative type boundary

#if os(macOS)
@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {
    // ...
}
#endif

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

Access control

Reference code

Objective-C/Application/ViewController.h:22 — representative boundary

#ifdef TARGET_IOS
@property (weak, nonatomic) IBOutlet UISlider *transparencySlider;
#endif

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

Main application flow

sequenceDiagram
    participant Renderer
    participant _commandQueue
    participant commandBuffer
    participant renderEncoder
    Renderer->>_commandQueue: commandBuffer()
    Renderer->>commandBuffer: addCompletedHandler()
    Renderer->>Renderer: updateState()
    Renderer->>commandBuffer: renderCommandEncoderWithDescriptor()
    Renderer->>renderEncoder: setFrontFacingWinding()
    Renderer->>renderEncoder: setCullMode()
    Renderer->>commandBuffer: presentDrawable()
    Renderer->>commandBuffer: commit()

Reference code

Objective-C/Renderer/Renderer.m:539 — drawInMTKView()

@implementation Renderer
// ...
    [commandBuffer commit];
// ...
@end

Naming conventions

• Types: Controller: ViewController; Delegate: AppDelegate, WindowSceneDelegate; Renderer: Renderer.
• Protocols: no local protocol declaration in the scanned source.
• Methods: applicationShouldTerminateAfterLastWindowClosed, application, buildMetalVertexDescriptor, loadTexture, updateDynamicBufferState, uniformsForObject, updateGameState, bindVertexDescriptorsForMesh.
• Files: Swift/Application/AppDelegate.swift, Swift/Renderer/Renderer.swift, Objective-C/Application/AppDelegate.h, Objective-C/Application/AppDelegate.m, Objective-C/Application/ViewController.h, Objective-C/Application/ViewController.m.

Architecture takeaways

• main is the main source-visible entry or composition anchor for this sample.
• Framework work reaches simd, UIKit, MetalKit, Cocoa 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