Logging in Dart

Dart programs on Fuchsia generally write log messages with the lib.logging package, consuming and initializing it through the fuchsia_logger package.

See the language agnostic logging docs for more information about recording and viewing logs.

Requirements

GN dependency

The necessary packages can be included with an addtion to deps in BUILD.gn:

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

The fuchsia_logger package also provides Dart's lib.logging.

See Dart: Overview for more information about building Dart within Fuchsia.

Component manifest dependency

Ensure that your component has the required capabilities to log by including the following in your component manifest:

• {.cmx}

{
  "include": [
    "sdk/lib/diagnostics/syslog/client.shard.cmx"
  ],
  ...
}

• {.cml}

{
  include: [
    "sdk/lib/diagnostics/syslog/client.shard.cml"
  ],
  ...
}

Note: The above is only available for in-tree development. This is tracked in fxbug.dev/64207. Out of tree developers should copy the snippets shown below instead.

• {.cmx}

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

• {.cml}

{
  use: [
    { protocol: "fuchsia.logger.LogSink" },
  ],
}

The syslog library will fallback to stderr if the LogSink connection fails.

Initialization

In your main function, call the setupLogger() function to initialize logging:

import 'package:fuchsia_logger/logger.dart';

main() {
  // process name will be used if no name is provided here
  setupLogger(name: 'my-component');
}

Configure severity

By default only messages with INFO severity or higher are printed. Severity level can be adjusted by providing the level parameter in the setupLogger() call.

For example, to make all log messages appear in fx log:

setupLogger(name: 'noisy-component', level: Level.ALL);

Recording messages

The log object is a Logger instance.

import 'package:fuchsia_logger/logger.dart';

log.finest('quietest');      // maps to TRACE
log.finer('also quietest');  // maps to TRACE also
log.fine('quiet');           // maps to DEBUG
log.info('hello world!');    // maps to INFO
log.warning('uhhh');         // maps to WARN
log.severe('oh no!');        // maps to ERROR
log.shout('this is fatal.'); // maps to FATAL

Standard streams

print goes to standard out (stdout).

See stdout & stderr in the language-agnostic logging docs for details on the routing of stdio streams in the system.