Dynamically Load Code with Dart

Rejoice! Dartium, a build of Chromium with the Dart VM, can now spawn a new isolate from a URI. This means your Dart apps have a new option for more modular code.

Isolates

In Dart, an isolate is an abstraction for an "isolated memory heap". Isolates do not share memory (variables, statics, etc), they are essentially self-contained applications. Isolates communicate by sending and receiving messages over ports.


Why Isolates Matter

Dart programs are static, in that their shape and structure do not change after they are compiled. A static program is great for optimizations like tree shaking (aka dead code elimination), type-inferencing compilers, and more.

However, modular apps often require a way to dynamically load code. Configurable and modular apps need a way to, at run time, pull in new functionality. If Dart apps have a static structure, how can they dynamically alter their behavior?

Isolates to the rescue! The structure inside of an isolate is static, but a Dart program is free to dynamically load other isolates. This means that libraries can be loaded into isolates at run time.

Your First Isolate

This simple reverser isolate listens on its port, receives messages, reverses them, and send the reversed text back to the sender.

// reverser.dart
import 'dart:isolate';

main() {
  port.receive((msg, SendPort replyTo) {
    var reverse = msg as String;
    var buffer = new StringBuffer();
    for (var i = reverse.length - 1; i >= 0; i--) {
      buffer.write(reverse[i]);
    }
    replyTo.send(buffer.toString());
  });
}

The main application (which itself is an isolate), is responsible for instantiating the reverser isolate. Use the spawnUri() function from dart:isolate to spawn a new isolate from some file.

// main.dart

 SendPort reverser;

void main() {
  query("#sample_text_id")
    ..text = "Click me!"
    ..onClick.listen(reverseText);
  
  var serviceFile = 'reverser.dart';
  
  if (identical(1, 1.0)) {  // XXX: horrible hack to detect if we're in JS
    serviceFile = 'reverser.dart.js';
  }
  
  reverser = spawnUri(serviceFile);
}

Notice the horrible hack to detect if this code is running in the Dart VM or JavaScript. I don't know a better way to do it, but I don't recommend this.

When the text from #sample_text_id is clicked, the reverseText function is run:


void reverseText(MouseEvent event) {
  var text = query("#sample_text_id").text;
  reverser.call(text).then((reversed) {
    query("#sample_text_id").text = reversed;
  });
}

Notice how the reverser isolate is sent a message via call(), which returns a Future. When the Future completes, the callback registered with then() is run.

Support

Dartium supports spawnUri since at least version 28.0.1478.0 (194114). Dart2js compiles spawnUri() into a Web worker.

Note that each isolate is its own app, so dart2js must compile the same bootstrap code into each isolate file.

Popular posts from this blog

Lists and arrays in Dart

(This is Part 3 in a series about Dart. Check out Part 2, Function in Dart .) Warning : We expect the Dart libraries to undergo potentially sweeping changes before Dart goes to alpha. This document is relevant as of 2011-12-22. Intro Dart is a "batteries included" effort to help app developers build modern web apps. An important "battery" is the bundled core Dart libraries, providing common and rich functionality. Dart is building a solution for large, complex web apps, and providing well tested, integrated, and common libraries is key to helping a web app developer be more productive out of the box. The Collection libraries are a crucial set of APIs that Dart developers get for free. Much more than simple arrays and maps, the Collection library includes standard ways to filter, iterate, inspect, compose, and sort your data. This post specifically looks at List<E> , Dart's ordered, indexable collection of objects. Aside: The Dart project is lucky
Read more

The 29 Healthiest Foods on the Planet

The 29 Healthiest Foods on the Planet > The following is a "healthy food hot list" consisting of the 29 food that will give you the biggest nutritional bang for you caloric buck, as well as decrease your risk for deadly illnesses like cancer, diabetes and heart disease.
Read more

Converting Array to List in Scala

Now, this has to have a built-in somewhere in Scala , because it just seems too common. So, how to convert an Array to a List in Scala? Why do I need this? I needed to drop to Java for some functionality, which in this case returns an Array. I wanted to get that Array into a List to practice my functional programming skillz. **Update**: I figured out how to convert Arrays to Lists the Scala way. Turns out it's a piece of cake. val myList = List.fromArray(Array("one", "two", "three")) or val myList = Array("one","two","three").elements.toList The call to elements returns an Iterator , and from there you can convert to a List via toList . Nice. Because my first version wasn't actually tail recursive, what follows is a true tail recursive solution, if I were to implement this by hand. The above, built in mechanism is much better, though. object ArrayUtil { def toList[a](array: Array[a]): List[a] = { d
Read more