Migrating OpenGL code to Metal

At a glance

Project structure

Source bundle/
├── Metal/
│   ├── AAPLMetalViewController.m
│   ├── AAPLShaders.metal
│   ├── AAPLMetalRenderer.m
│   └── AAPLShaderTypes.h
├── OpenGL/
│   └── AAPLOpenGLViewController.m
└── Common/
    ├── main.m
    ├── WindowSceneDelegate.m
    ├── AAPLAppDelegate.h
    └── AAPLMeshData.h

Structure observations

• The entry/composition boundary and renderer or operation boundary are separate in the source; AAPLMetalRenderer is the principal feature coordinator.
• GPU-specific logic stays in Metal shader files; shared headers bridge host/shader layouts where present.
• The tree above is intentionally pruned to composition, resource, and shader files.

Overall architecture

flowchart LR
    N1["AAPLMetalViewController"]
    N2["AAPLMetalRenderer"]
    N3["parallel OpenGL and Metal renderers"]
    N4["equivalent backend-specific draw paths"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

Metal/AAPLMetalViewController.m:32 — Metal renderer composition and delegate handoff

    _renderer = [[AAPLMetalRenderer alloc] initWithMetalKitView:_view];

    if(!_renderer)
    {
        assert(!"Renderer failed initialization.");
        return;
    }

    [_renderer mtkView:_view drawableSizeWillChange:_view.drawableSize];
    _view.delegate = _renderer;

Interpretation

This is the dominant control/data path: platform code composes AAPLMetalRenderer; that object controls parallel OpenGL and Metal renderers; GPU-visible work ends in equivalent backend-specific draw paths. The arrows summarize responsibility transfer, not a claim that every node directly calls the next.

Ownership and state

classDiagram
    AAPLOpenGLViewController *-- AAPLOpenGLRenderer : _openGLRenderer

Ownership evidence

OpenGL/AAPLOpenGLViewController.m:56 — AAPLOpenGLViewController creates and stores _openGLRenderer

    _openGLRenderer = [[AAPLOpenGLRenderer alloc] initWithDefaultFBOName:_defaultFBOName];

    if(!_openGLRenderer)
    {
        NSLog(@"OpenGL renderer failed initialization.");
        return;
    }

Class and protocol design

Framework delegate conformance is a callback seam; the source does not justify calling the whole app protocol-oriented.

Access control

This sample does not use Swift private, fileprivate, or public for its native boundary. Objective-C/Objective-C++ use header versus implementation placement, C++ uses access specifiers/linkage, Python uses module conventions, and Metal entry points cross a compiled-library boundary; these are not Swift access levels.

Logic ownership and placement

Shader boundary reference

Metal/AAPLShaders.metal:33 — representative GPU entry/helper

vertex TempleShaderInOut

Design patterns

Naming conventions

• Role suffixes make ownership visible: AAPLMetalViewController, AAPLOpenGLViewController, AAPLMetalRenderer, AAPLOpenGLRenderer, AAPLOpenGLView, AAPLAppDelegate, WindowSceneDelegate.
• Method names describe setup or encoding actions: initWithURL, initWithMetalKitView, buildTempleObjects, buildReflectiveQuadObjects, updateFrameState, mtkView, drawInMTKView.
• Shader entry points use stage/operation names: templeSamplingFragmentShader.
• AAPL is a sample namespace prefix, not a recommendation for production module naming; retain semantic suffixes such as Renderer, Manager, Scene, or Adapter.

Architecture takeaways

• Keep AAPLMetalViewController focused on composition; AAPLMetalRenderer is the owner of parallel OpenGL and Metal renderers.
• Treat equivalent backend-specific draw paths as a separate execution/compilation boundary with explicit resource and data-layout contracts.
• The verified ownership edge is AAPLOpenGLViewController → _openGLRenderer; broader exclusive ownership is not inferred.
• Access-control rationale follows concrete language boundaries rather than translating every header or shader symbol into Swift terms.

Source map

• Official Apple documentation