Flutter Logging

1. Install package logging

Add logging to pubspec.yaml file:

pubspec.yaml

dependencies:
  logging: ^0.11.4

2. Config Logger

Initialize Logger before runApp() in main.dart file:

main.dart

import 'logging.dart';
void main() {
  // config logger before runApp()
  initLogging();
  runApp(MyApp());

  // log a debug message
  log.fine('Running my flutter app.');
}

logging.dart

import 'package:logging/logging.dart';

// create Logger by name 'MyApp'
final log = Logger('MyApp');
void initLogging() {
  // disable hierarchical logger
  hierarchicalLoggingEnabled = false;
  // change to another level as needed.
  Logger.root.level = Level.INFO;
  // skip logging stactrace below the SEVERE level.
  recordStackTraceAtLevel = Level.SEVERE;
  assert(() {
    recordStackTraceAtLevel = Level.WARNING;
    // print all logs on debug build.
    Logger.root.level = Level.ALL;
    return true;
  }());
}

3. Transmit log records from Logger to dart:developer

Logger is a producer, but it will not post any log records if no one is listening.

Print to console:

logging.dart

Logger.root.onRecord.listen((event) {
  print("${event.time}: [${event.level}] [${event.loggerName}] ${event.message}");
});

Save to file:

logging.dart

File logFile = ...
Logger.root.onRecord.listen((event) {
  logFile.writeAsString("${event.time}: [${event.level}] [${event.loggerName}] ${event.message}",
    mode: FileMode.append);
});

It’s strongly recommended to use dart:developer for logging:

logging.dart

import 'dart:developer' as developer;

void initLogging() {
  ...
  Logger.root.onRecord.listen((event) {
    developer.log(
      event.message,
      time: event.time,
      sequenceNumber: event.sequenceNumber,
      level: event.level.value,
      name: event.loggerName,
      zone: event.zone,
      error: event.error,
      stackTrace: event.stackTrace,
    );
  });
}

4. Example

Logging a debug message on track():

import 'package:logging/logging.dart';
class EventReporter {
  // create Logger for this class
  final log = Logger('comic.report');
  void track(String event, [Map<String, dynamic> arguments]) {
    ...
    log.fine('track event $event args: $arguments');
  }
}

If using dart:developer for consuming log records, please check the following.

Flutter console displays logs:

DevTools’ Logging view shows logs:

Complete code

See gist: https://gist.github.com/yrom/1a86a6995720d9a4ff9261617fb13783

Redirect ‘print’

main.dart

void main() {
  runZoned((){
    runApp(MyApp());
  }, zoneSpecification: ZoneSpecification(print: printToDeveloper));
}
/// see [PrintHandler]
void printToDeveloper(Zone self, ZoneDelegate parent, Zone zone, String line) {
  developer.log(
    line,
    time: DateTime.now(),
    level: Level.FINE.value,
    name: 'console',
    zone: zone,
  );
  // if you need print to console
  // Zone.root.print(line); 
}

References

• Using the Logging view: https://flutter.dev/docs/development/tools/devtools/logging
• Debugging Flutter apps programmatically: https://flutter.dev/docs/testing/code-debugging#logging
• logging pacakge: https://pub.dev/packages/logging

Author: Yrom

Link: https://yrom.net/blog/2020/07/08/flutter-logging/

License: 知识共享署名-非商业性使用 4.0 国际许可协议