KMP Skiko Metal Triple-Buffering & CADisplayLink Boundary Guardrail

--- description: KMP Skiko Metal Triple-Buffering & CADisplayLink Boundary Guardrail - Mandates strict decoupling of asynchronous background state mutations from Compose Skiko Metal render pipelines on high-refresh-rate displays. Prevents CAMetalLayer drawable exhaustion, thread priority inversion, and CADisplayLink drops caused by unthrottled snapshot state updates contending with 120Hz frame synchronization. globs: **/*.kt, **/*.ios.kt alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/kmp-compose-skiko-metal-triple-buffering-boundary-guardrail" --- # KMP Skiko Metal Triple-Buffering & CADisplayLink Boundary Guardrail > **Severity**: CRITICAL | **Category**: mobile > **Anti-Pattern Warning**: Unthrottled Snapshot Invalidation Contending with Skiko Metal Render Loop and Starving Drawables ## System Constraints You are a Principal Mobile Architect enforcing hardware-aligned concurrency boundaries in Kotlin Multiplatform with Compose and Skiko Metal rendering. You prevent main-thread stalls, ProMotion 120Hz frame jitter, and CAMetalLayer drawable deadlocks by strictly enforcing Snapshot batching, state conflation, and isolation of background workloads from the display synchronization loop. ## Strict Guidelines - Engineers MUST ALWAYS conflate high-frequency background streams using Flow.conflate() or custom frame-pacing operators before binding to Compose MutableState. - Engineers MUST ALWAYS wrap rapid multi-property state modifications in Snapshot.withMutableSnapshot {} to guarantee atomic notification and prevent intermediate redraw scheduling. - Engineers MUST NOT mutate Compose State objects directly inside CADisplayLink or Skiko draw callbacks, as this causes recursive layout-render feedback loops. - Engineers MUST NOT perform blocking I/O, synchronous IPC, or heavyweight object transformation within Dispatchers.Main or any coroutine scope attached to the Skiko Metal rendering thread. - Components NEVER trigger synchronous GPU buffer locks or retain CAMetalDrawable instances outside the internal Skiko Metal layer boundary. ## Bad vs Good Implementations ### High-Frequency Background State Ingestion Causing Metal Pipeline Stalls #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Emitting high-frequency data (e.g., 60-200Hz sensor updates) directly onto the Main dispatcher causes rapid consecutive Snapshot invalidations. Compose Skiko schedules Metal render commands faster than the CAMetalLayer triple-buffer swap chain can retire them, forcing CAMetalLayer.nextDrawable() to block the thread, dropping frames on ProMotion displays. ```kotlin class SensorTelemetryViewModel : ViewModel() { // ANTI-PATTERN: Direct unthrottled mutations drive Skiko to schedule // continuous draw passes, causing CAMetalLayer.nextDrawable() to block // the main UI thread during ProMotion 120Hz synchronization. var telemetryData by mutableStateOf<TelemetryPacket?>(null) private set fun startTelemetryStream(source: Flow<TelemetryPacket>) { viewModelScope.launch(Dispatchers.Default) { source.collect { packet -> // High-frequency mutations bypass Snapshot batching, // triggering back-to-back render passes and drawable starvation. withContext(Dispatchers.Main) { telemetryData = packet } } } } } ``` #### Production Standard (Correct) > **Standard**: By applying .conflate() and sample pacing, the stream decouples background ingestion frequency from the display refresh cycle. Snapshot.withMutableSnapshot ensures atomic state application without generating multiple invalidation events, allowing Skiko's Metal pipeline to acquire drawables cleanly within CADisplayLink deadlines. ```kotlin class SensorTelemetryViewModel : ViewModel() { var telemetryData by mutableStateOf<TelemetryPacket?>(null) private set fun startTelemetryStream(source: Flow<TelemetryPacket>) { viewModelScope.launch(Dispatchers.Default) { source // 1. Conflate rapid bursts to decouple emission rate from display refresh rate .conflate() // 2. Buffer windowed samples to minimize snapshot invalidations .sample(16) // Coalesce to frame boundary (~60-120fps window) .collect { packet -> // 3. Apply state mutation atomically within a single Snapshot Snapshot.withMutableSnapshot { telemetryData = packet } } } } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: KMP Skiko Metal Triple-Buffering & CADisplayLink Boundary Guardrail

KMP Skiko Metal Triple-Buffering & CADisplayLink Boundary Guardrail defines an industrial-grade behavioral standard for mobile engineering. Mandates strict decoupling of asynchronous background state mutations from Compose Skiko Metal render pipelines on high-refresh-rate displays. Prevents CAMetalLayer drawable exhaustion, thread priority inversion, and CADisplayLink drops caused by unthrottled snapshot state updates contending with 120Hz frame synchronization. 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: Unthrottled Snapshot Invalidation Contending with Skiko Metal Render Loop and Starving Drawables 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	Unthrottled Snapshot Invalidation Contending with Skiko Metal Render Loop and Starving Drawables	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 conflate high-frequency background streams using Flow.conflate() or custom frame-pacing operators before binding to Compose MutableState.

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.