📝 Lesson 12: Logging & Debugging

⏱ 25-35 min 📊 Intermediate 📝 Production Ready 🎓 Final Lesson 🔗 Official Source

📝

Why Logging Matters

When your app is running in production, you can't just `print()` everywhere. Logging gives you eyes on your application's behavior, even when things go wrong silently.

🔍

Debug Remotely

See what happened without running the app locally.

📊

Monitor Health

Track performance and error rates over time.

🛡️

Audit Trail

Know exactly what actions occurred and when.

🔧

Filter Noise

Show only critical issues in production.

🎯

Today's Project: "Professional Logger"

We'll build a production-ready logging system:

📊 Configurable log levels
📁 File-based persistent logs
🏗️ Hierarchical loggers for modules
🔌 Dependency injection pattern
📈 Log analysis features

Step 1

Understanding Log Levels

📊 Log Level Hierarchy

Drag the slider to set the minimum log level. Messages below this threshold are filtered out.

ALLFINESTFINERFINECONFIGINFOWARNINGSEVERESHOUT

Current: INFO

🔍 [FINEST] Detailed trace data

🔎 [FINER] Less detailed trace

🐛 [FINE] Debug information

⚙️ [CONFIG] Configuration loaded

ℹ️ [INFO] Application started

⚠️ [WARNING] Low disk space

❌ [SEVERE] Connection failed

🚨 [SHOUT] Critical system failure!

Step 2

Live Logging Simulator

🖥️ Real-Time Log Console

Simulate different application events and watch logs appear:

Logs will appear here...

📊 INFO: 0 ⚠️ WARN: 0 ❌ ERR: 0 🐛 DEBUG: 0

Step 3

Writing Logs to Files

📁 File Logger Configuration

Configure where and how logs are persisted:

Log Directory:

File Name Pattern:

Console Threshold:

📄 Generated Logger Setup:

final logFile = File('./logs/app_2026_05_25.txt');
logger.onRecord.listen((record) {
  final msg = '[${record.time}] ${record.level.name}: ${record.message}';
  logFile.writeAsStringSync('$msg\n', mode: FileMode.append);
  if (record.level >= Level.WARNING) print(msg);
});

Step 4

Dependency Injection with Loggers

🔌 Pass Loggers via Constructor

See how to inject loggers into your service classes:

🏠 main() Creates Logger

→ Logger →

🔍 SearchCommand Logger logger

→ Logger →

📄 GetArticleCommand Logger logger

💡 Implementation Pattern:

class SearchCommand {
  final Logger logger;
  SearchCommand({required this.logger});
  
  Future<void> execute(String query) async {
    logger.info('Searching for: $query');
    // ... implementation
  }
}

🏆

Build: Production Logger System

📝 Logger Configuration

🔧 Service with Injected Logger

🎮 Main Application

void main(List<String> arguments) async {
  // Setup loggers with different levels
  final appLogger = setupLogger('app', level: Level.INFO);
  final searchLogger = setupLogger('search', level: Level.ALL);
  final errorLogger = setupLogger('errors', level: Level.WARNING);
  
  appLogger.info('Application starting...');
  appLogger.config('Arguments: $arguments');
  
  // Create service with injected logger
  final searchService = WikipediaSearchService(logger: searchLogger);
  
  try {
    final query = arguments.isNotEmpty ? arguments[0] : 'Dart';
    final results = await searchService.search(query);
    
    if (results.isNotEmpty) {
      appLogger.info('Displaying ${results.length} results');
      for (var result in results) {
        print('  📄 $result');
      }
    } else {
      appLogger.warning('No results found');
    }
    
    appLogger.info('Application finished successfully');
    
  } catch (e) {
    errorLogger.severe('Application crashed: $e');
    print('An error occurred. Check logs for details.');
  }
}

📤 Simulated Log Output

Click "Run Simulation" to see log output

🎮 Final Challenge: Logging Master Quiz

📝 Score: 0/5

⚡ Streak: 0

⏱ Time: 0s

📊 Logs: 100%