Wednesday, January 16, 2013

First Look at new Dart Libraries

The Dart libraries are implementing a common model for processing a stream of events, and you can try the new libraries now with a preview release of Dart Editor. Developers consistently told us they wanted a more unified method for dealing with asynchronous events. The new Streams API from the dart:async library delivers on the vision of a builtin way to listen for, transform, and emit a sequence of events.

Beautiful streams.

There are numerous changes in the new Dart libraries, above and beyond just Streams. Here is a look at what's new, with some tips for migration. Note: this list is not comprehensive. You can explore the API docs to learn more about these new libraries.

Core library

  • Iterable interface is changed. Now uses "bool moveNext()" and "E get current" to advance and read the current value.
  • Iterable interface is greatly extended.
    • Collection methods moved to Iterable, e.g. any and contains.
    • ‘map’ has been renamed to ‘mappedBy’. The result is lazy.
    • ‘filter’ has been renamed to ‘where’. The result is lazy.
    • Many new operations, and some old operations, now work on all Iterables.
    • Iterable transforming methods (those returning a new Iterable) are lazy - that is, they don't create a new collection immediately.
    • Added Iterable.toList() and Iterable.toSet(). This allows easy porting from old code: foo.map(...) -> foo.mappedBy(...).toList()
  • Added optional function argument to int/double.parse which is called in case of a parsing error. Default behavior is to throw FormatException as before.
  • Added optional arguments to int.parse: “radix”. Example: int.parse(“ff”, radix: 16) -> 255.
  • Changed int.~/ to always return an int value.
  • Added String.replaceAllMapped that takes a function to compute the replacement for each match.
  • Removed Sequence class.
  • The generic List constructor is not fixed length anymore if an argument is given. The list’s size can still be changed afterwards.
  • There are two new named List constructors: List.fixedLength(length, {fill}) and List.filled(len, fill).
  • Strings.join now takes an iterable, but Strings will be removed. Iterable now has a join. Also, Arrays will be removed.
  • map.keys and map.values no longer allocate a copy, so avoid modifying the data structure while iterating. Instead, create the copy by using "for (var key in map.keys.toList())"


Collections


  • Added separate collection library, "dart:collection" for implementations and collections beyond plain Map, Set, and List.


Async


  • Library "dart:async" added.
  • Future and Completer moved to async library.
  • Timer moved to async library.
  • Future interface greatly changed. Most importantly:
    • "chain" and “transform” combined into "then" by checking whether the return value of the handler is a Future.
    • Future is now purely asynchronous - you can't read a value or exception directly from the future, but must register an event handler to receive it at a later time.
    • To fix existing uses of Future: Change "chain" to "then". Always register a "then" or "catchError" handler immediately when you receive a future, or you might not catch an error in time.
  • Added Stream: an asynchronous event source. Single or multiple listeners, produces sequences of both value events and errors.
  • As an asynchronous version of Iterable, Stream has async versions of the transforming methods of Iterable.
  • New AsyncError is now wrapping all errors propagated to async event handlers, both on Future and on Stream.



JSON


  • Library interface changed: No JSON class, methods renamed.
  • Added optional "reviver" argument to parse.
  • Removed length method.



HTML


  • For more information, see our mailing list post.
  • on.click.add is now onClick.listen (streamified)

We invite you try your code with the new libraries, join the discussion on the Dart mailing list, and file issues and bugs at the Dart Issue Tracker.

Photo Credit: ecstaticist (cc)