## Instructions - CustomPainter done right: extend `CustomPainter`, override `shouldRepaint(old) => old.data != data`; wrap in `RepaintBoundary`, `size: Size.infinite` handled via `LayoutBuilder`; cache `Path`/`Paint` objects, never allocate in `paint()`. - Canvas perf: batch draws (`drawPoints`, `drawPath`), `canvas.clipRect` before heavy scenes, `isComplex: true, willChange: true` on animating RepaintBoundary; profile with `flutter build --profile` + ShaderWarmUp for jank. - Isolates for CPU work: `await compute(parseJson, rawBody)` or `Isolate.run(() => heavyFilter(bytes))`; keep isolate fns top-level/static, args primitive/serializable; stream progress via `ReceivePort` for >2s jobs. - Platform channels: `const ch = MethodChannel('com.app/sensors')`; Dart `await ch.invokeMethod('stream')`; Kotlin `MethodChannel(flutterEngine.dartExecutor, name).setMethodCallHandler`; Swift `FlutterMethodChannel(name:binaryMessenger:)`; error via `result.error("E1","msg",null)`. - Prefer Pigeon for type-safe channels: define `sensor_api.dart` (`@HostApi getBattery(): double`), run `dart run pigeon --input pigeon/sensor.dart`; commit generated Kotlin/Swift/Dart, never hand-edit. - FFI for hot loops: `dart:ffi` + `flutter_rust_bridge` or C via `NativeLibrary.lookup`; ship `.so`/`.xcframework` per ABI, load with `DynamicLibrary.open`; keep FFI off UI thread (wrap in isolate). - Texture interop: `TextureRegistry` / `ExternalTexture` for camera/ML frames; dispose `textureId` on widget dispose; throttle to 30fps to save battery. - Flavor parity: `android { flavorDimensions "env"; productFlavors { dev { applicationIdSuffix ".dev" }; prod {} } }` + Xcode schemes `dev/prod` with matching bundle IDs; `--flavor dev --dart-define=API_URL=...` everywhere including CI. - Native parity checklist: permissions mirrored (manifest + Info.plist), icons/splash per flavor, ProGuard/R8 keep rules for channels/FFI (`-keep class com.app.**`), Bitcode/dSYM upload for iOS. - Memory: `ImageCache.maximumSizeBytes = 100 << 20`, `dispose()` controllers/animations, `WeakReference` in long-lived listeners; leak-check with DevTools Memory + `leak_tracker`. - Blend with existing multiplatform skill: this file covers render/isolate/channel/FFI depth; defer basic Riverpod/go_router/flavors setup to `flutter-multiplatform`. - Test native bridge: `TestDefaultBinaryMessengerBinding` mock handler for MethodChannel unit tests; `flutter test --platform chrome` for painter goldens; on-device `integration_test` for FFI/channel round-trip. ## Examples - Waveform painter: cached Path + RepaintBoundary, 60fps scrub, compute() decodes peaks off-thread. - Image filter: Rust FFI blur via isolate, texture preview, flavor-aware API upload. - Sensor bridge: Pigeon `getStepCount()` with Kotlin Health Services + Swift CoreMotion parity. ## Workflows 1. Profile first (DevTools timeline) → decide Painter vs FFI vs isolate. 2. Scaffold channel (Pigeon) or FFI binding + flavor defines. 3. Implement painter/isolate with caches + dispose + error states. 4. Parity pass: manifest/plist perms, R8 keeps, icons, bundle IDs per flavor. 5. Test: mock channel unit + golden painter + on-device integration. 6. Release: profile build, shader warmup, mapping/dSYM upload. ## Templates - Painter + isolate: ```dart import 'package:flutter/foundation.dart'; import 'package:flutter/material.dart'; List decodePeaks(List bytes) { // top-level for compute(): heavy DSP here return bytes.map((b) => b / 255.0).toList(); } class WavePainter extends CustomPainter { final List peaks; final Paint _paint = Paint()..color = Colors.blue..strokeWidth = 2; WavePainter(this.peaks); @override void paint(Canvas canvas, Size size) { final path = Path(); for (var i = 0; i < peaks.length; i++) { final x = i / peaks.length * size.width, y = size.height * (1 - peaks[i]); i == 0 ? path.moveTo(x, y) : path.lineTo(x, y); } canvas.drawPath(path, _paint); } @override bool shouldRepaint(covariant WavePainter old) => old.peaks != peaks; } // usage: peaks = await compute(decodePeaks, rawBytes); // RepaintBoundary(child: CustomPaint(painter: WavePainter(peaks), size: Size.infinite)) ``` - Pigeon + FFI: ```dart // pigeon/sensor.dart import 'package:pigeon/pigeon.dart'; @HostApi() abstract class SensorApi { double getBattery(); } // dart run pigeon --input pigeon/sensor.dart --dart_out lib/sensor.g.dart // --kotlin_out android/app/src/main/kotlin/SensorApi.kt --swift_out ios/SensorApi.swift // ffi import 'dart:ffi' as ffi; final lib = ffi.DynamicLibrary.open('libfilter.so'); typedef BlurC = ffi.Void Function(ffi.Pointer, ffi.Int32); Future blurInIsolate(List bytes) => Isolate.run(() { /* call lib.lookup>('blur') */ }); ``` ## Tools - SARAH shell_run for `dart run pigeon`, `flutter build --profile`, DevTools timeline/memory. - SARAH file_write for painter/isolate/pigeon/FFI scaffolds; web_fetch for CustomPainter + Pigeon + FFI docs. ## Advanced - Impeller: test on `--enable-impeller` + Vulkan/Metal devices; precompile shaders (`SkSL warmup` → `flutter build bundle --write-sksl`), watch `RasterCache` misses. - Rust bridge: `flutter_rust_bridge_codegen generate` for zero-copy structs; pin NDK r26 + Xcode 15; strip symbols for size (`split-debug-info`). - Background: `flutter_background_service` + native FGS for audio/tracking; never run isolates forever — checkpoint + WorkManager reschedule. ## Documentation - https://api.flutter.dev/flutter/rendering/CustomPainter-class.html - https://docs.flutter.dev/platform-integration/platform-channels - https://pub.dev/packages/pigeon - https://dart.dev/guides/libraries/c-interop