pkgs/logging/README.md

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## Initializing

By default, the logging package does not do anything useful with the log
messages. You must configure the logging level and add a handler for the log
messages.

Here is a simple logging configuration that logs all messages via `print`.

```dart
Logger.root.level = Level.ALL; // defaults to Level.INFO
Logger.root.onRecord.listen((record) {
  print('${record.level.name}: ${record.time}: ${record.message}');
});
```

First, set the root `Level`. All messages at or above the current level are sent to the
`onRecord` stream. Available levels are:

+ `Level.OFF`
+ `Level.SHOUT`
+ `Level.SEVERE`
+ `Level.WARNING`
+ `Level.INFO`
+ `Level.CONFIG`
+ `Level.FINE`
+ `Level.FINER`
+ `Level.FINEST`

Then, listen on the `onRecord` stream for `LogRecord` events. The `LogRecord`
class has various properties for the message, error, logger name, and more.

To listen for changed level notifications use:

```dart
Logger.root.onLevelChanged.listen((level) {
  print('The new log level is $level');
});
```

## Logging messages

Create a `Logger` with a unique name to easily identify the source of the log
messages.

```dart
final log = Logger('MyClassName');
```

Here is an example of logging a debug message and an error:

```dart
var future = doSomethingAsync().then((result) {
  log.fine('Got the result: $result');
  processResult(result);
}).catchError((e, stackTrace) => log.severe('Oh noes!', e, stackTrace));
```

When logging more complex messages, you can pass a closure instead that will be
evaluated only if the message is actually logged:

```dart
log.fine(() => [1, 2, 3, 4, 5].map((e) => e * 4).join("-"));
```

Available logging methods are:

+ `log.shout(logged_content);`
+ `log.severe(logged_content);`
+ `log.warning(logged_content);`
+ `log.info(logged_content);`
+ `log.config(logged_content);`
+ `log.fine(logged_content);`
+ `log.finer(logged_content);`
+ `log.finest(logged_content);`

## Configuration

Loggers can be individually configured and listened to. When an individual logger has no
specific configuration, it uses the configuration and any listeners found at `Logger.root`.

To begin, set the global boolean `hierarchicalLoggingEnabled` to `true`.

Then, create unique loggers and configure their `level` attributes and assign any listeners to
their `onRecord` streams.


```dart
  hierarchicalLoggingEnabled = true;
  Logger.root.level = Level.WARNING;
  Logger.root.onRecord.listen((record) {
    print('[ROOT][WARNING+] ${record.message}');
  });

  final log1 = Logger('FINE+');
  log1.level = Level.FINE;
  log1.onRecord.listen((record) {
    print('[LOG1][FINE+] ${record.message}');
  });

  // log2 inherits LEVEL value of WARNING from `Logger.root`
  final log2 = Logger('WARNING+');
  log2.onRecord.listen((record) {
    print('[LOG2][WARNING+] ${record.message}');
  });


  // Will NOT print because FINER is too low level for `Logger.root`.
  log1.finer('LOG_01 FINER (X)');

  // Will print twice ([LOG1] & [ROOT])
  log1.fine('LOG_01 FINE (√√)');

  // Will print ONCE because `log1` only uses root listener.
  log1.warning('LOG_01 WARNING (√)');

  // Will never print because FINE is too low level.
  log2.fine('LOG_02 FINE (X)');

  // Will print twice ([LOG2] & [ROOT]) because warning is sufficient for all
  // loggers' levels.
  log2.warning('LOG_02 WARNING (√√)');

  // Will never print because `info` is filtered by `Logger.root.level` of
  // `Level.WARNING`.
  log2.info('INFO (X)');
```

Results in:

```
[LOG1][FINE+] LOG_01 FINE (√√)
[ROOT][WARNING+] LOG_01 FINE (√√)
[LOG1][FINE+] LOG_01 WARNING (√)
[ROOT][WARNING+] LOG_01 WARNING (√)
[LOG2][WARNING+] LOG_02 WARNING (√√)
[ROOT][WARNING+] LOG_02 WARNING (√√)
```