What’s New with Dart’s M2 Release

Written by Seth Ladd
December 2012

Milestone 2 is the second beta release of the Dart SDK. During the Milestone 2 cycle, the team gave a first round of TLC to the Dart libraries. Also included in the M2 cycle are major new features like self-service uploads to pub, SSL in the VM, theme support in Dart Editor, the Web UI package, and much more.

While not an exhaustive list, here are some of the highlights from M2.

Contents

  1. dart2js shrinks generated code size, gets a startup boost
  2. Library refactorings
  3. dart:html simplifications
  4. Dart VM gets SSL and performance improvements
  5. Documentation for new web developers and game developers
  6. Pub lets authors publish their own libraries
  7. Dart Editor helps developers manage more code
  8. Web UI builds on Web Components
  9. New http, oauth2, and serialization packages
  10. Conclusion

dart2js shrinks generated code size, gets a startup boost

The dart2js tool compiles Dart code into ECMAScript 5 JavaScript. During the second milestone, the team worked on reducing the size of the generated JavaScript. They approached this challenge from two angles.

First, a top-to-bottom review of dart:html helped dart2js figure out which DOM classes can’t possibly be in use. If a class isn’t used, it can be “shaken” out and not included in the generated code. The results of this work were impressive, often reducing by half the contribution of dart:html to the application size.

Second, work has started on a more aggressive minifier. Early stats are promising, especially when you factor in gzip. For example, here are some stats from a real-life sample:

There’s still more work to do but we’re encouraged by the early progress in this front. The feedback is loud and clear, dart2js output size is a top priority for the team.

In other dart2js news, the Dart-to-JavaScript compiler removed support for old language features. For example, the following are no longer allowed in code compiled by dart2js:

To greatly speed up the time it takes to run the tool, the Dart SDK now includes a snapshot of dart2js. A snapshot is a binary, serialized file of parsed Dart code, taken before the program starts to execute. The Dart VM can experience up to a 10X startup improvement with snapshots, because the VM avoids loading and parsing textual source code.

Library refactorings

The team made some sweeping changings to the core libraries during M2.

To start, many exceptions are now errors. For example, IndexOutOfRangeException is now RangeError, and NotImplementedException is now UnimplementedError. The library designers feel that many traditionally named exceptions are really unrecoverable errors.

Related, NullPointerException is gone, replaced by NoSuchMethodError. The original NullPointerException had two major issues: there are no pointers in Dart, and it is not an exception. Remember, null is an object in Dart (as is everything else). If code calls a method that does not exist on the null object, noSuchMethod will handle it by simply throwing NoSuchMethodError. You can read more about the justifications for this change.

The dart:coreimpl library is no more. Any functionality worth keeping will find itself moved to other libraries. This reduces confusion about what the Dart SDK’s library surface area is.

The core libraries have embraced getters and setters. For example, the following changes occurred during the M2 cycle:

While the following features did not launch during the M2 cycle, the Dart team did announce a sneak peak at the Streams API and continued clarifying the Mixins proposal. Keep these on your radar for future milestones.

dart:html simplifications

The dart:html library continues to be simplified and refactored to feel more like Dart.

Many names have been changed to conform to Dart style. For example:

In some cases, functionality that is generally considered to be an anti-pattern was removed. For example, Window.prompt is no longer supported. And by “no longer supported” I mean, “removed from dart:html”.

Some functionality that didn’t quite work out was also removed. The asynchronous Element.getBoundingClientRect(), which used to return a Future, is now synchronous. The original thinking was that a Future-based query for DOM element dimensions would mitigate the potential to trigger a reflow (a costly operation). In practice, though, it seemed to cause more harm than good. Based on feedback from the community, Element.getBoundingClientRect() simply returns a ClientRect.

However, reflows and layout measurements can still be costly. The dart:html library now has a requestLayoutFrame function, which will call a callback when the next batch of browser layout measurements has completed.

The dart:html library added new classes for the different types of input fields. For example, you can instantiate a RadioButtonInputElement, instead of setting the right properties on an InputElement. See InputElement API docs for a complete list.

The DOM is a tree, and elements have children, so it only made sense to rename Element.elements to Element.children.

Large sets of functionality have been moved over to their own libraries, cleaning up the old monolithic DOM namespace. For example, SVG, Web Audio, and IndexedDB have been pulled out into their own libraries.

Dart VM gets SSL and performance improvements

Big news for authors of libraries that need to connect to public web services: the Dart VM now supports SSL. You can create a web server that speaks HTTPS, or connect to HTTPS resources.

With the language spec stabilized, the VM team has focused on performance. Initial measurements show a 50% improvement over V8 on some benchmarks. Much more work needs to be done here, but check out dartlang.org/performance/ for charts and details.

Prompted by performance work while integrating Dart VM into Chromium, the VM and the language spec now define strings as UTF-16 code units (no longer treating strings as Unicode code points). This primarily means it is now faster to get strings in and out of the VM when embedded in a browser. While the language spec was changed in M1, the VM implemented this change in M2.

Documentation for new web developers and game developers

If you are comfortable with object-oriented software development, but new to browser programming, the Dart team launched a set of tutorials just for you. These exercises lead you through browser basics (DOM, elements, attributes, and so on) and into the new Web UI package.

There is also a code lab that walks you through building a web app using Web UI. This lab takes you step by step through the client and the server code.

Dart’s performance and structure are especially interesting to game developers. You can find articles and tutorials about building modern browser games with Dart at dartgamedevs.org.

Pub lets authors publish their own libraries

Pub is a package manager for Dart. It helps developers install and update their third-party dependencies, and now it supports self-service uploads of packages to pub.dartlang.org. This means that developers can easily share their libraries for others to use.

Thanks to this new publishing workflow, the SDK packages such as unittest, logging, meta, and more are now found in pub. These packages, which previously lived in the pkg directory of the SDK, now have a permanent home at pub.dartlang.org. You need to update your pubspec.yaml files accordingly.

Learn more at pub.dartlang.org.

Dart Editor helps developers manage more code

The editor added a GUI for editing pubspec.yaml, the file that declares a package’s dependencies. This GUI helps reduce syntax errors.

For developers that like to tweak their editor’s syntax highlighting and colors, Dart Editor now supports themes. The editor comes pre-bundled with many different styles, and you can import your own.

A new WebGL sample was added to the editor’s welcome screen. Try the Solar 3D demo to see how pub, Dart vector math, and WebGL work together.

The editor can now recommend a union of declared and checked types for better code completion. See the following screenshot for an example:

Code completion in Dart Editor

Notice how, inside of the if block, the editor presents options for warrior containing both the methods in Warrior as well as Ninja.

More cleanups were added to help developers stay up to date as the VM and dart2js implement more M1 features and library changes. For example, a cleanup can replace interface declarations with abstract classes and redirecting constructors. See the following screenshot for an example of this in action:

Clean up old code with Dart Editor

Dart Editor had a steady stream of weekly releases; read the full list of editor changes.

Web UI builds on Web Components

The Dart team released a preview of Web UI, a new package for building modern web apps. Built on Web Components, and inspired by model-driven views, Web UI helps you build custom components that encapsulate their style, behavior, and structure. You can also create live templates with one-way and two-way data binding.

Included in Web UI is a compiler that converts the Dart and Web Components code into vanilla JavaScript and HTML. This means you can deploy your Web UI code across modern browsers today.

Learn more about building structured web apps with a tutorial, a code lab, and full documentation.

New http, oauth2, and serialization packages

Sharing is caring, so the Dart team extracted some libraries they use in their projects and published them to pub.dartlang.org.

The serialization package provides a general serialization facility for Dart objects. The serialization library works using either reflection or a more explicit declarative API.

The http package provides a library that has a Future-based composable API for making HTTP requests. This library makes it easier to connect to HTTP resources from command-line Dart apps.

The oauth2 package provides a library for authenticating with a remote service via OAuth2 on behalf of a user, and making authorized HTTP requests with the user’s OAuth2 credentials. This library works only with command-line Dart apps.

Conclusion

We fixed many other bugs and made minor changes and clean-ups, but those are the big ticket items. As always, your feedback, questions, and comments are welcome on the mailing list and our issue tracker.