Dart Embedder Hacking

Debugging

Under Mojo, by default the Dart VM is built in Release mode regardless of the mode that Mojo is built in. That is, when Mojo is built in Debug mode, the Dart VM is still built in Release mode. To change this behavior while working on the embedder, pass dart_debug=true to gn to configure a Debug build of the Dart VM. I.e.:

$ gn gen --check out/Debug --args='... dart_debug=true'

Embedder packages

In order to implement the ‘dart:io’ library (and run a service isolate hosting Observatory), the Mojo Dart embedder needs to use some package: imports. Mojo applications should be able to use a different version of these packages than the embedder. In other words, the embedder snapshot cannot include any ‘package:’ imports because they will prohibit an application from using a newer version of the package. In order to allow the embedder to use packages without interfering with an application's intended version, we clone the packages used by the embedder and rewrite the url to start with ‘dart:_’ (not ‘package:’). Each dart:_ import must have a mapping provided to gen_snapshot which maps from the import uri to a real file system path.

The complete list of packages used by the embedder is located at //mojo/dart/embedder/packages.dart.

Adding an embedder package can be done in three steps:

1. Add ‘dart:_’ import to packages.dart, for example:

import ‘dart:_mojo/public/dart/application.dart’;

2. Add dart_embedder_package to //mojo/dart/embedder/BUILD.gn, for example:

dart_embedder_package(“dart_embedder_package_application”) { package = “mojo/public/interfaces/application” }

3. Add the package directory to the list in :generate_snapshot_bin, for example:

rebase_path(“//mojo/public/interfaces/application”),

Dart IO

Under Mojo, the ‘dart:io’ implementation is not complete and likely suffers from subtle differences. Implementation status:

‘dart:io’ feature	Mojo Service	Implemented
Socket	mojo:network_service	Yes
ServerSocket	mojo:network_service	Yes
DNS	mojo:network_srevice	Yes
SecureSocket	N/A	No
SecureServerSocket	N/A	No
Datagram	mojo:network_service	No
File system	mojo:files	No