dhruvanbhalara/flutter-debugging

Logging

• Use a centralized AppLogger class for all logging — NEVER use print() or raw debugPrint()
• Define log levels: verbose, debug, info, warning, error, fatal
• In dev flavor: log everything (verbose and above)
• In staging: log info and above
• In production: log warning and above only, route to Crashlytics
• Include context in logs: AppLogger.error('Failed to fetch user', error: e, stackTrace: st)
• NEVER log sensitive data (passwords, tokens, PII) at any level

Flutter DevTools

• Use Widget Inspector to debug layout issues and identify unnecessary rebuilds
• Use Performance Overlay (showPerformanceOverlay: true) to monitor frame rates
• Use Timeline View to identify jank — target 16ms per frame (60fps)
• Use Memory View to detect memory leaks and monitor allocation patterns
• Use Network Profiler to inspect Dio requests/responses during development

Debugging Strategies

• Layout Issues: Use debugPaintSizeEnabled = true to visualize widget boundaries
• Overflow Errors: Check RenderFlex overflowed — use Expanded, Flexible, or constrain dimensions
• Unbounded Height: Wrap ListView in SizedBox or use shrinkWrap: true with NeverScrollableScrollPhysics
• Rebuild Tracking: Add debugPrint('$runtimeType rebuild') temporarily to identify excessive rebuilds — remove before commit
• Async Errors: Always catch and log errors in try-catch blocks with stack traces
• Use assert() for development-time invariant checks that are stripped in release builds

Memory Management

• Dispose ALL controllers, subscriptions, Timer, and AnimationController in dispose()
• Use late initialization in initState() — never inline-initialize disposable objects
• Use WeakReference for caches that should not prevent garbage collection
• Profile memory with DevTools Memory tab — watch for monotonically increasing allocations
• Watch for common leaks: undisposed listeners, closures capturing BuildContext, global streams without cancellation

Performance Profiling

• Always profile with --profile mode (not debug): flutter run --profile --flavor dev -t lib/main_dev.dart
• Use Timeline.startSync / Timeline.finishSync for custom performance tracing of critical paths
• Monitor shader compilation jank on first run — use --cache-sksl for warmup:

  flutter run --profile --cache-sksl --purge-persistent-cache
  

• Target metrics: < 16ms frame build time, < 100ms screen transition, < 2s cold start

Error Boundaries

• Route errors to Crashlytics in staging/prod (FlutterError.onError = FirebaseCrashlytics.instance.recordFlutterFatalError)
• Set FlutterError.onError and PlatformDispatcher.instance.onError to catch framework and async errors
• Wrap critical widget subtrees in custom error boundary widgets that show fallback UI instead of red screens
• In release mode: NEVER show stack traces to users — show user-friendly error messages only

Runtime Error Taxonomy

Categorize and fix Dart runtime errors systematically using static analysis and type system knowledge.

Type System Soundness

• Method Overrides: Maintain sound return types (covariant) and parameter types (contravariant). Use covariant keyword when intentionally tightening parameter types.
• Generics: Add explicit type annotations to generic classes (List<int> not List<dynamic>). Never assign List<dynamic> to a typed list.
• Downcasting: Avoid implicit downcasts from dynamic. Use explicit casts (as Type) only when runtime type is guaranteed.
• Enable Strict Casts: Add to analysis_options.yaml:

  analyzer:
    language:
      strict-casts: true
  

Null Safety Error Patterns

| Error | Cause | Fix | |---|---|---| | Property cannot be accessed on nullable receiver | Accessing member on Type? | Use ?. or null check | | Non-nullable instance field must be initialized | Uninitialized non-null field | Use late or make nullable | | The argument type can't be assigned | Type? passed where Type expected | Add null check or ! (with caution) |

Rules:

• Avoid ! operator — prefer pattern matching or early returns.
• Use late only when initialization is guaranteed before first access.
• Use _ wildcard (Dart 3.7+) for unused variables.

Automated Resolution Workflow

# 1. Identify all errors
dart analyze . --fatal-infos

# 2. Preview automated fixes
dart fix --dry-run

# 3. Apply automated fixes
dart fix --apply

# 4. Verify resolution
dart analyze .
dart test

Feedback Loop: If dart test fails with TypeError after fixing → you introduced an invalid cast (as T) or accessed an uninitialized late variable. Locate and correct.