SwiftUI Metal Shaders (iOS 17+)

The [[stitchable]] attribute + ShaderLibrary API lets you write Metal fragment functions that SwiftUI applies directly to views — no MTKView, no pipeline setup, no command buffers. This skill covers the exact contract, argument mappings, known bugs, and patterns.

File Setup

Add a .metal file to your Xcode target. Xcode auto-compiles all .metal files into the default metallib and registers [[stitchable]] functions with SwiftUI's runtime.

#include <metal_stdlib>
#include <SwiftUI/SwiftUI_Metal.h>   // required ONLY when using SwiftUI::Layer
using namespace metal;

The [[stitchable]] attribute is mandatory — without it the function exists in the metallib but ShaderLibrary.yourFunction() silently fails to render.

The Three Modifier Types

Each modifier has a rigid function signature. The first parameters are implicitly provided by SwiftUI — you never pass them from Swift. Your custom arguments come after.

colorEffect — per-pixel color transform

[[stitchable]] half4 name(float2 position, half4 color, args...)

• Receives the destination pixel position and the view's current color at that pixel.
• Use for: tinting, palette mapping, generative patterns, dithering, color grading.
• Cannot read neighboring pixels.

distortionEffect — geometric remap

[[stitchable]] float2 name(float2 position, args...)

• Receives the destination position; returns the source position SwiftUI should sample instead.
• Use for: waves, ripples, lens distortion, page curl, glitch shifts.
• If you return a position outside the padded region, the pixel is transparent.

layerEffect — full filter with arbitrary sampling

[[stitchable]] half4 name(float2 position, SwiftUI::Layer layer, args...)

• Receives a SwiftUI::Layer you can .sample(float2) at any position.
• Use for: blur, emboss, pixelate, RGB-split, any multi-sample effect.
• Most flexible, most expensive — each layerEffect is a separate render pass.

SwiftUI::Layer details

layer.sample(float2 position) returns a premultiplied, linearly-filtered half4 in the destination color space. Two consequences:

• Alpha is already multiplied into RGB — blending with mix() works, but multiplying by an additional alpha double-darkens.
• Sampling outside the layer's bounds returns (0,0,0,0), not an error.

Argument Type Mapping

This mapping is a hard contract — a type mismatch compiles cleanly but silently produces garbage at runtime. There is no runtime type-checking.

Swift Shader.Argument	MSL parameter type
.float(T)	float
.float2(CGPoint) / .float2(T, T)	float2
.float3(T, T, T)	float3
.float4(T, T, T, T)	float4
.color(Color)	half4 (premultiplied, target color space)
.colorArray([Color])	device const half4 *ptr, int count
.image(Image)	texture2d<half>
.floatArray([Float])	device const float *ptr, int count
.data(Data)	device const void *ptr, int size_in_bytes
.boundingRect	float4 as (x, y, width, height)

The one-image limit

Only ONE .image() argument is supported per Shader instance. The SwiftUI::Layer in layerEffect does not count against this limit. If you need multiple textures, drop to MTKView.

Array arguments expand to TWO MSL parameters

.colorArray([Color]) becomes device const half4 *colors, int colorCount in MSL — two parameters from one Swift argument. Same for .floatArray and .data. Account for this in your parameter ordering.

Reading View Size

Use visualEffect { content, proxy in } to read the view's size without affecting layout:

myView
    .visualEffect { content, proxy in
        content.colorEffect(
            ShaderLibrary.myShader(
                .float2(proxy.size)   // pass size as float2
            )
        )
    }

Animation with TimelineView

TimelineView(.animation) { context in
    let elapsed = startDate.distance(to: context.date)
    myView.colorEffect(
        ShaderLibrary.myShader(.float(Float(elapsed)))
    )
}

On ProMotion displays this drives the shader at up to 120 Hz. Cost is zero when the TimelineView is off-screen.

Shader Compilation Warming (iOS 18+)

Pre-compile shaders to avoid first-frame stutter:

.task {
    let shader = ShaderLibrary.myEffect(.float2(.zero), .float(0))
    try? await shader.compile(as: .colorEffect)  // or .distortionEffect, .layerEffect
}

Pass the same argument types you will use at draw time. The cache key is function name + argument type signature (not values). Changing argument values every frame is cheap; changing argument types triggers a recompile.

Render Pass Optimization

• Sequential .colorEffects on the same view fold into a single render pass.
• A .colorEffect before a .distortionEffect also folds into one pass.
• Every .layerEffect and every .distortionEffect after the first one introduces a new render pass.
• Reorder shaders to put .colorEffects first when possible.

maxSampleOffset

maxSampleOffset on .distortionEffect and .layerEffect is not optional advisory metadata — it expands the rasterized region SwiftUI prepares. If your shader samples beyond maxSampleOffset from the destination pixel, the result is clipped to transparent (the classic "shader cut off at edges" bug).

The common fix pattern:

myView
    .padding(40)                  // expand layout to make room
    .drawingGroup()               // rasterize before the effect
    .distortionEffect(
        ShaderLibrary.ripple(...),
        maxSampleOffset: CGSize(width: 40, height: 40)
    )

Known Bugs and Silent Failures

1. Argument type mismatch — compiles fine, renders garbage. Always verify argument order and types match the MSL signature exactly.

2. Too many arguments — no published hard limit, but shaders may fail silently when many parameters are passed. Keep argument lists short (≤ 8 scalars). Pack vectors when possible.

3. UIKit-backed views don't render into layerEffect — Apple's docs state: "Views backed by AppKit or UIKit views may not render into the filtered layer." Workaround: apply .drawingGroup() before the effect, or apply the effect to a pure SwiftUI view layered above.

4. Second .image() breaks layer sampling — passing a second .image() to a layerEffect either fails silently or corrupts SwiftUI::Layer.sample() (the Layer appears to occupy the single texture slot internally).

5. Core Image + SwiftUI shader conflict — if the same target has both [[stitchable]] SwiftUI shaders and extern "C" Core Image custom kernels, Xcode's -fcikernel build rule applies to all .metal files. Solution: put CI kernels in a separate target.

6. No runtime MTLLibrary → ShaderLibrary bridge — ShaderLibrary(data:) accepts metallib Data from disk but is undocumented for runtime-compiled shaders. For dynamic shader generation, use MTKView.

7. No backdrop sampling — there is no API to sample what's behind a view. For Material-like backdrop blur, apply the effect to a common ancestor containing both background and foreground.

When to Drop to MTKView

Use SwiftUI shaders for ~95% of UI effects. Drop to MTKView (via UIViewRepresentable) when you need:

• ≥ 2 texture inputs
• Compute kernels
• Offscreen render targets / bake-to-texture
• MTLFunctionConstantValues for shader specialization
• Arbitrary numbers of inputs or full pipeline control

See the metal-motion-effects skill for the MTKView wrapper pattern.