Rendering Pipeline & Jank Hunting

Copy codeMain Thread                    Render Thread               GPU
─────────────────────────────────────────────────────────────
Input handling
Animations
Layout (measure/layout)
Drawing (Canvas operations) → DisplayList → GPU rasterization → Screen

Copy codeProfile → CPU → Select "System Trace"
→ Record while scrolling
→ Look for frames that exceed 16ms in the "Frames" row
→ Click a long frame → see which method consumed the time

Copy codeadb shell dumpsys gfxinfo com.example.myapp framestats
# Outputs per-frame timing for the last 120 frames
# Look for "Janky frames: N (X.XX%)"

Copy code// In debug builds — catch IO on main thread
if (BuildConfig.DEBUG) {
    StrictMode.setThreadPolicy(
        StrictMode.ThreadPolicy.Builder()
            .detectDiskReads()
            .detectDiskWrites()
            .detectNetwork()
            .penaltyLog()  // logcat warning
            // .penaltyDeath()  // crash — useful in CI
            .build()
    )
}

Copy code// ❌ Disk read on main thread — StrictMode will catch this
class MainActivity : AppCompatActivity() {
    override fun onResume() {
        val prefs = getSharedPreferences("prefs", MODE_PRIVATE)  // synchronous disk read
        val token = prefs.getString("token", null)
    }
}

// ✅ Move to background thread; use DataStore
lifecycleScope.launch {
    val token = dataStore.data.first()[TOKEN_KEY]
}

Copy code// ❌ Reading layout properties forces a layout pass mid-frame
view.width  // forces measure if dirty
view.translationX = 50f  // marks layout dirty
view.width  // forces measure AGAIN — thrashing

// ✅ Cache layout properties; batch writes
val currentWidth = view.width  // one read
view.translationX = 50f       // write
// Don't read width again in the same frame

Copy code// ❌ Path creation in onDraw — called every frame
override fun onDraw(canvas: Canvas) {
    val path = Path()  // allocation every frame
    path.moveTo(0f, 0f)
    canvas.drawPath(path, paint)
}

// ✅ Create path once; update only when data changes
private val path = Path()

fun updatePath(points: List<PointF>) {
    path.reset()
    points.firstOrNull()?.let { path.moveTo(it.x, it.y) }
    points.drop(1).forEach { path.lineTo(it.x, it.y) }
    invalidate()  // trigger redraw with the pre-built path
}

override fun onDraw(canvas: Canvas) {
    canvas.drawPath(path, paint)  // no allocation
}

Copy codewindow.addOnFrameMetricsAvailableListener({ _, frameMetrics, _ ->
    val totalDuration = frameMetrics.getMetric(FrameMetrics.TOTAL_DURATION) / 1_000_000
    if (totalDuration > 16) {
        Log.w("Jank", "Slow frame: ${totalDuration}ms")
    }
}, Handler(Looper.getMainLooper()))

Copy code// Animate a view frequently (alpha, translation, scale)
view.setLayerType(View.LAYER_TYPE_HARDWARE, null)  // cached on GPU
startAnimation()
// After animation completes:
view.setLayerType(View.LAYER_TYPE_NONE, null)  // release GPU layer