docs/development/languages/dart/logging.md

# Logging


It is highly recommended that you use `lib.logging` package when you want to add
logging statements to your Dart package.

Include the `fuchsia_logger`, which wraps `lib.logging`, package in your
BUILD.gn target as a dependency:

```
deps = [
  ...
   "//topaz/public/dart/fuchsia_logger",
  ...
]
```

In the .cmx file the `fuchsia.logger.LogSink` needs to be added to the sandbox:

```
{
    "sandbox": {
        "services": [
            "fuchsia.logger.LogSink",
            ...
        ]
    }
}
```


In the main function of your Dart / Flutter app, call the `setupLogger()`
function to make sure logs appear in the Fuchsia console in the desired format.

```dart
import 'package:fuchsia_logger/logger.dart';

main() {
  setupLogger();
}
```

After setting this up, you can call one of the following log methods to add log
statements to your code:

```dart
import 'package:fuchsia_logger/logger.dart';


// add logging statements somewhere in your code as follows:
log.info('hello world!');
```

The `log` object is a [Logger][logger-doc] instance.


## Log Levels

The log methods are named after the supported log levels. To list the log
methods in descending order of severity:

```dart
    log.shout()    // maps to LOG_FATAL in FXL.
    log.severe()   // maps to LOG_ERROR in FXL.
    log.warning()  // maps to LOG_WARNING in FXL.
    log.info()     // maps to LOG_INFO in FXL.
    log.fine()     // maps to VLOG(1) in FXL.
    log.finer()    // maps to VLOG(2) in FXL.
    log.finest()   // maps to VLOG(3) in FXL.
```

By default, all the logs of which level is INFO or higher will be shown in the
console. Because of this, Dart / Flutter app developers are highly encouraged to
use `log.fine()` for their typical logging statements for development purposes.

Currently, the log level should be adjusted in individual Dart apps by providing
the `level` parameter in the `setupLogger()` call. For example:

```dart
setupLogger(level: Level.ALL);
```

will make all log statements appear in the console.  The console is visible by
running [`fx syslog`][getting_logs].


[logger-doc]: https://pub.dev/documentation/logging/latest/logging/Logger-class.html
[getting_logs]: /docs/development/build/fx.md#getting_logs