Creating a 3D application with hydra rendering

At a glance

Project structure

Source bundle/
├── source/
│   ├── main.mm
│   ├── AAPLAppDelegate.h
│   ├── AAPLAppDelegate.mm
│   ├── AAPLRenderer.h
│   ├── AAPLRenderer.mm
│   ├── AAPLViewController.h
│   ├── AAPLViewController.mm
│   ├── AAPLCamera.h
│   ├── AAPLCamera.mm
│   └── BlitShaders.metal
└── scripts/
    ├── cleanup_schema.py
    └── relocatable_hydraplayer.py

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++, Python.
• The verified tree contains 4 project/configuration file(s) and 5 source declaration(s).

Overall architecture

flowchart LR
    N1["main"]
    N2["AAPLViewController"]
    N3["AAPLRenderer"]
    N4["MetalKit APIs"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

source/main.mm:10 — architecture anchor

int main(int argc, const char* argv[])
{
    return NSApplicationMain(argc, argv);
}

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 Metal.

Ownership and state

classDiagram
    AAPLAppDelegate o-- NSWindow : window
    AAPLRenderer o-- bool : isZUp
    AAPLRenderer o-- AAPLCamera : viewCamera
    AAPLCamera o-- GfVec3d : position

Ownership evidence

source/AAPLAppDelegate.h:14 — stored dependency or nearest verified ownership anchor

@interface AAPLAppDelegate : NSObject <NSApplicationDelegate>

@property (assign) IBOutlet NSWindow* window;

@end

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.

No source-visible execution, scheduling, or synchronization boundary was found in the indexed source.

@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.

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

source/AAPLAppDelegate.h:12 — representative type boundary

@interface AAPLAppDelegate : NSObject <NSApplicationDelegate>

@property (assign) IBOutlet NSWindow* window;

@end

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

Access control

Reference code

source/AAPLAppDelegate.h:14 — representative boundary

@interface AAPLAppDelegate : NSObject <NSApplicationDelegate>

@property (assign) IBOutlet NSWindow* window;

@end

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 AAPLRenderer
    participant _hgi
    participant hgi
    participant view
    participant hgiTexture
    participant commandBuffer
    AAPLRenderer->>_hgi: get()
    AAPLRenderer->>hgi: StartFrame()
    AAPLRenderer->>view: drawableSize()
    AAPLRenderer->>AAPLRenderer: drawWithHydraAt()
    AAPLRenderer->>hgiTexture: Get()
    AAPLRenderer->>hgi: GetPrimaryCommandBuffer()
    AAPLRenderer->>commandBuffer: addCompletedHandler()
    AAPLRenderer->>hgi: EndFrame()

Reference code

source/AAPLRenderer.mm:379 — drawMainView()

- (bool)drawMainView:(MTKView*)view timeCode:(double)timeCode
{
    if (!_engine)
    {
        return false;
    }

    // Start the next frame.
    dispatch_semaphore_wait(_inFlightSemaphore, DISPATCH_TIME_FOREVER);
    HgiMetal* hgi = static_cast<HgiMetal*>(_hgi.get());
    hgi->StartFrame();

    // Draw the scene using Hydra, and recast the result to a MTLTexture.
    CGSize viewSize = [view drawableSize];
    HgiTextureHandle hgiTexture = [self drawWithHydraAt:timeCode viewSize:viewSize];
    id<MTLTexture> texture = static_cast<HgiMetalTexture*>(hgiTexture.Get())->GetTextureId();

    // Create a command buffer to blit the texture to the view.
    id<MTLCommandBuffer> commandBuffer = hgi->GetPrimaryCommandBuffer();
    __block dispatch_semaphore_t blockSemaphore = _inFlightSemaphore;
    [commandBuffer addCompletedHandler:^(id<MTLCommandBuffer> buffer) {
        dispatch_semaphore_signal(blockSemaphore);
    }];

    // Copy the rendered texture to the view.
    [self blitToView:view commandBuffer:commandBuffer texture:texture];

    // Tell Hydra to commit the command buffer, and complete the work.
    hgi->CommitPrimaryCommandBuffer();
    hgi->EndFrame();

    return true;
}

Naming conventions

• Types: Controller: AAPLViewController; Delegate: AAPLAppDelegate; Renderer: AAPLRenderer.
• Protocols: no local protocol declaration in the scanned source.
• Methods: applicationDidFinishLaunching, applicationWillTerminate, applicationShouldTerminateAfterLastWindowClosed, initWithMetalKitView, setupScene, requestFrame, initializeMaterial, dealloc.
• Files: source/AAPLAppDelegate.h, source/AAPLAppDelegate.mm, source/AAPLRenderer.h, source/AAPLRenderer.mm, source/AAPLViewController.h, source/AAPLViewController.mm.

Architecture takeaways

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