Compose Multiplatform Skiko VSync Frame Pacing Invariant

--- description: Compose Multiplatform Skiko VSync Frame Pacing Invariant - Enforces strict synchronization between Skiko rendering dispatchers and native display refresh subsystems (iOS CADisplayLink/Metal and Android Choreographer/Vulkan). Prevents visual tearing, jitter, and UI thread starvation caused by unthrottled recomposition loops or off-beat Skia draw invalidations. globs: **/iosMain/**/*.kt, **/androidMain/**/*.kt, **/commonMain/**/ui/**/*.kt, **/graphics/**/*.kt alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/compose-multiplatform-skiko-vsync-pacing-guardrail" --- # Compose Multiplatform Skiko VSync Frame Pacing Invariant > **Severity**: CRITICAL | **Category**: mobile > **Anti-Pattern Warning**: Unbounded Redraw Dispatching and Asynchronous Canvas Invalidation Outside Native VSync Ticks ## System Constraints You are a Principal Mobile Architect enforcing Compose Multiplatform and Skiko frame pacing invariants. You must mandate that all custom graphics dispatching, canvas invalidations, and high-frequency animations align strictly with the target platform's hardware refresh cycle (CADisplayLink on iOS Metal, Choreographer on Android Vulkan). Reject unthrottled CoroutineScope.launch rendering loops, direct recursive redraw requests, and canvas mutability spanning across rendering boundaries. ## Strict Guidelines - Engineers MUST ALWAYS bind Skiko render loop ticks directly to the host platform display synchronization primitives (CADisplayLink on iOS, Choreographer.FrameCallback on Android). - Engineers MUST NOT invoke SkiaLayer.needRedraw() or SkikoView.requestRedraw() from asynchronous I/O coroutines or background worker threads without dispatching to the designated UI frame clock. - Code SHOULD leverage withFrameNanos or withFrameMillis within the MonotonicFrameClock rather than delay() or Unconfined/Default coroutine dispatchers for frame transitions. - Services MUST throttle frame updates to the native hardware refresh rate (e.g., 60Hz, 120Hz ProMotion) and avoid queueing redundant draw passes while an existing draw pass is awaiting hardware presentation. - Components NEVER execute raw Canvas rendering passes directly inside Composable evaluation bodies; custom rendering must be encapsulated in DrawModifier nodes or Canvas composables utilizing bounded invalidation flags. ## Bad vs Good Implementations ### Asynchronous High-Frequency Redraw vs VSync-Synchronized Frame Clock #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Launching an unconstrained draw loop on Dispatchers.Default with an arbitrary delay causes Skiko to flood the Metal/Vulkan command queues out of sync with VSync, leading to thread starvation, missed display deadlines, and visible screen tearing. ```kotlin // Anti-Pattern: Unbounded coroutine loop triggering Skia redraws off-clock @Composable fun ShimmeringCanvas(skikoView: SkikoView) { val scope = rememberCoroutineScope() LaunchedEffect(Unit) { scope.launch(Dispatchers.Default) { while (isActive) { // Bypasses CADisplayLink/Choreographer, driving excessive CPU/GPU contention skikoView.needRedraw() delay(8) // Arbitrary delay creates frame pacing mismatch and visual tearing } } } } ``` #### Production Standard (Correct) > **Standard**: Using withFrameNanos subscribes directly to Compose Multiplatform's platform-bound MonotonicFrameClock (bridged to CADisplayLink on iOS and Choreographer on Android), guaranteeing zero tearing and bounded draw invalidation. ```kotlin // Production Guardrail: Synchronized with Skiko MonotonicFrameClock via withFrameNanos @Composable fun ShimmeringCanvas(modifier: Modifier = Modifier) { var frameTimeNanos by remember { mutableLongStateOf(0L) } LaunchedEffect(Unit) { while (isActive) { // Perfectly aligns calculation and invalidation with platform VSync pulse withFrameNanos { frameTime -> frameTimeNanos = frameTime } } } Canvas(modifier = modifier) { // Skia rendering executed within standard Compose DrawScope tied to native display cadence drawCircle(color = Color.Cyan, radius = (frameTimeNanos % 100).toFloat()) } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Compose Multiplatform Skiko VSync Frame Pacing Invariant

Compose Multiplatform Skiko VSync Frame Pacing Invariant defines an industrial-grade behavioral standard for mobile engineering. Enforces strict synchronization between Skiko rendering dispatchers and native display refresh subsystems (iOS CADisplayLink/Metal and Android Choreographer/Vulkan). Prevents visual tearing, jitter, and UI thread starvation caused by unthrottled recomposition loops or off-beat Skia draw invalidations. 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: Unbounded Redraw Dispatching and Asynchronous Canvas Invalidation Outside Native VSync Ticks 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	Unbounded Redraw Dispatching and Asynchronous Canvas Invalidation Outside Native VSync Ticks	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 bind Skiko render loop ticks directly to the host platform display synchronization primitives (CADisplayLink on iOS, Choreographer.FrameCallback on Android).

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.