Compose Multiplatform Skiko CADisplayLink VSync Frame Pacing Invariant

--- description: Compose Multiplatform Skiko CADisplayLink VSync Frame Pacing Invariant - Enforces strict synchronization between iOS CADisplayLink callbacks and Skiko rendering pipelines on ProMotion displays. Prevents compositor thread contention, tearing, and micro-stutter by binding Skia Metal draw passes directly to native display refresh cycles with explicit CAFrameRateRange parameters. globs: **/iosMain/**/*.kt, **/*Skiko*.kt, **/*ViewController*.kt, **/*ViewAdapter*.kt alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/compose-multiplatform-skiko-displaylink-vsync-invariant" --- # Compose Multiplatform Skiko CADisplayLink VSync Frame Pacing Invariant > **Severity**: CRITICAL | **Category**: mobile > **Anti-Pattern Warning**: Decoupled Timer-Driven or Asynchronous Redraw Invocations Causing Metal Compositor Stalls ## System Constraints You are a Principal Mobile Architect enforcing strict VSync alignment between Apple's CoreAnimation pipeline and Compose Multiplatform's Skiko rendering engine. Mandate that CADisplayLink drives the Skiko frame clock without intermediate decoupled timers, ensure dynamic CAFrameRateRange configuration for ProMotion displays, and enforce thread affinity for all Skia-to-Metal command buffer commits. ## Strict Guidelines - Engineers MUST ALWAYS schedule CADisplayLink on NSRunLoop.mainRunLoop with NSRunLoopCommonModes to ensure continuous rendering during user touch tracking and gesture recognizer evaluation. - Engineers MUST configure CAFrameRateRange (minimum, preferred, maximum) explicitly for ProMotion hardware to avoid iOS system-level downclocking of the Metal drawable presentation to 60Hz. - Engineers MUST NOT invoke SkikoView.needRedraw() or trigger direct render passes via NSTimer, DispatchSourceTimer, or unaligned coroutines in iosMain. - Code SHOULD propagate CADisplayLink.targetTimestamp into the Compose MonotonicFrameClock to align Compose recomposition and animation velocity with the exact physical screen refresh interval. - Services MUST ensure that Skia GPU context flushes and MTLCommandBuffer presentations execute strictly on the CADisplayLink invocation thread without blocking the main RunLoop or creating lock inversion with CoreAnimation. - Components NEVER instantiate duplicate or unmanaged CADisplayLink loops across nested Compose UI view controllers or offscreen render targets. ## Bad vs Good Implementations ### Decoupled Coroutine Timer vs. CADisplayLink ProMotion Synchronization #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Using an asynchronous coroutine delay completely decouples frame generation from the hardware VSync cycle. On 120Hz ProMotion screens, this causes severe micro-stutter, duplicate or missed frames, and Metal compositor queue backpressure. ```kotlin import kotlinx.coroutines.* import org.jetbrains.skiko.SkiaLayer // ANTI-PATTERN: Decoupled coroutine timer driving Skiko renders. // Causes frame drops, jitter on 120Hz ProMotion screens, and command buffer starvation. class BadSkikoRenderer(private val skiaLayer: SkiaLayer) { private val scope = CoroutineScope(Dispatchers.Main) private var isRunning = false fun start() { isRunning = true scope.launch { while (isRunning) { // Arbitrary 60FPS delay; completely desynchronized from CADisplayLink phase delay(16L) skiaLayer.needRedraw() } } } fun stop() { isRunning = false scope.cancel() } } ``` #### Production Standard (Correct) > **Standard**: Attaches a native CADisplayLink directly to NSRunLoopCommonModes, ensuring frames render fluidly during user interactions. Configures CAFrameRateRange to unlock 120Hz ProMotion and synchronizes the Skiko redraw trigger directly to the hardware VSync. ```kotlin import platform.QuartzCore.* import platform.Foundation.NSRunLoop import platform.Foundation.NSRunLoopCommonModes import platform.darwin.NSObject import org.jetbrains.skiko.SkiaLayer import kotlinx.cinterop.ExportObjCClass import kotlinx.cinterop.ObjCAction // PRODUCTION INVARIANT: Native CADisplayLink driving SkiaLayer VSync with ProMotion bounds @ExportObjCClass class ProductionSkikoFrameSync( private val skiaLayer: SkiaLayer ) : NSObject() { private var displayLink: CADisplayLink? = null fun start() { if (displayLink != null) return val link = CADisplayLink.displayLinkWithTarget( target = this, selector = platform.objc.sel_registerName("onDisplayLinkStep:") ) // Explicitly set dynamic refresh rate range for 120Hz ProMotion displays link.preferredFrameRateRange = CAFrameRateRangeMake( minimum = 60.0f, maximum = 120.0f, __preferred = 120.0f ) // Bind to common modes to sustain animation through scrolling/tracking link.addToRunLoop(NSRunLoop.mainRunLoop, NSRunLoopCommonModes) displayLink = link } @ObjCAction fun onDisplayLinkStep(link: CADisplayLink) { // Invariant: Native VSync phase alignment directly schedules the Skiko redraw pass skiaLayer.needRedraw() } fun stop() { displayLink?.invalidate() displayLink = null } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Compose Multiplatform Skiko CADisplayLink VSync Frame Pacing Invariant

Compose Multiplatform Skiko CADisplayLink VSync Frame Pacing Invariant defines an industrial-grade behavioral standard for mobile engineering. Enforces strict synchronization between iOS CADisplayLink callbacks and Skiko rendering pipelines on ProMotion displays. Prevents compositor thread contention, tearing, and micro-stutter by binding Skia Metal draw passes directly to native display refresh cycles with explicit CAFrameRateRange parameters. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Decoupled Timer-Driven or Asynchronous Redraw Invocations Causing Metal Compositor Stalls Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Decoupled Timer-Driven or Asynchronous Redraw Invocations Causing Metal Compositor Stalls	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS schedule CADisplayLink on NSRunLoop.mainRunLoop with NSRunLoopCommonModes to ensure continuous rendering during user touch tracking and gesture recognizer evaluation.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.