Implementing a multistage image filter using heaps and fences

At a glance

Project structure

Source bundle/
├── Renderer/
│   ├── AAPLRenderer.m
│   ├── AAPLShaders.metal
│   └── AAPLShaderTypes.h
└── Application/
    ├── AAPLViewController.m
    ├── main.m
    ├── WindowSceneDelegate.m
    ├── AAPLAppDelegate.h
    ├── AAPLViewController.h
    └── AAPLAppDelegate.m

Structure observations

• The entry/composition boundary and renderer or operation boundary are separate in the source; AAPLRenderer 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["AAPLViewController"]
    N2["AAPLRenderer"]
    N3["heap-backed intermediate image stages"]
    N4["fence-ordered compute filters"]
    N1 --> N2
    N2 --> N3
    N3 --> N4

Reference code

Renderer/AAPLRenderer.m:402 — explicit render/compute ordering with a fence

        [renderEncoder waitForFence:_fence
                       beforeStages:MTLRenderStageFragment];

        [renderEncoder drawPrimitives:MTLPrimitiveTypeTriangle
                          vertexStart:0
                          vertexCount:6];

        [renderEncoder updateFence:_fence
                       afterStages:MTLRenderStageFragment];

Interpretation

This is the dominant control/data path: platform code composes AAPLRenderer; that object controls heap-backed intermediate image stages; GPU-visible work ends in fence-ordered compute filters. The arrows summarize responsibility transfer, not a claim that every node directly calls the next.

Ownership and state

classDiagram
    AAPLViewController *-- AAPLRenderer : _renderer

Ownership evidence

Application/AAPLViewController.m:29 — AAPLViewController creates and stores _renderer

@implementation AAPLViewController
// ...
    _renderer = [[AAPLRenderer alloc] initWithMetalKitView:_view];
// ...
@end

Class and protocol design

A local protocol/conformance boundary is present, so protocol-oriented collaboration is claimed only for that seam.

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

Renderer/AAPLShaders.metal:20 — representative GPU entry/helper

vertex RasterizerData texturedQuadVertex( uint         vertexID  [[ vertex_id ]],
                             const device AAPLVertex * vertices  [[ buffer(AAPLVertexBufferIndexVertices) ]],
                             constant     float2 &     quadScale [[ buffer(AAPLVertexBufferIndexScale) ]])
{
    // ...
}

Design patterns

Naming conventions

• Role suffixes make ownership visible: AAPLViewController, AAPLRenderer, AAPLAppDelegate, WindowSceneDelegate.
• Method names describe setup or encoding actions: application, viewDidLoad, prefersHomeIndicatorAutoHidden, scene, sceneDidDisconnect, initWithMetalKitView, loadImages.
• Shader entry points use stage/operation names: texturedQuadVertex, texturedQuadFragment, gaussianblurHorizontal, gaussianblurVertical.
• 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 AAPLViewController focused on composition; AAPLRenderer is the owner of heap-backed intermediate image stages.
• Treat fence-ordered compute filters as a separate execution/compilation boundary with explicit resource and data-layout contracts.
• The verified ownership edge is AAPLViewController → _renderer; 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