A slightly Bacon.js inspired Dart package that aims to make functional reactive programming easier in Dart. Frappé extends the behavior of Dart's streams, and introduces new concepts like properties.


An EventStream is just like a Stream in Dart. It inherits the same interface as a Stream, but extends its functionality. This makes it easy to compose both Frappé streams and Dart streams.

For instance, by wrapping window.onMouseMove in an EventStream, we can combine multiple mouse events to perform a mouse drag operation in just a few lines of code:

window.onMouseDown.forEach((mouseDown) {
  var pen = new Pen(mouseDown.client);
  new EventStream(window.onMouseMove).takeUntil(window.onMouseUp.first)
      .forEach((mouseMove) => pen.drawTo(mouseMove.client))
      .then((_) => pen.done())

Or, merging multiple streams together that will signal the close of your application:

var onQuit = new EventStream(quitButton.onClick)
onQuit.listen((_) => closeApp());


Properties are similar to streams, but they remember their last value. This means that if a property has previously emitted the value of x to its subscribers, it will deliver this value to any of its new subscribers.

For instance, a property could be used to unify synchronous and asynchronous calls to get the window's current size:

Size innerSize() => new Size(window.innerWidth, window.innerHeight);

var windowSize = new Property.fromStreamWithInitialValue(innerSize(), => innerSize()));

print(innerSize()); // hypothetical window size {"width": 1024, "height": 768}
windowSize.listen((size) => print(size)); // {"width": 1024, "height": 768}
windowSize.listen((size) => print(size)); // {"width": 1024, "height": 768}

Properties can be created from EventStreams, Streams or Futures.

// Create a property from an EventStream

// Create a property from a Dart Stream

// Create a property from a Future.

Properties can also be combined, and their values will be recomputed whenever a dependency changes. For instance, the property isFormValid will update whenever a character is entered into the username or password fields:

bool isNotEmpty(String value) => value != null && value.isNotEmpty;

var isUsernamePresent = new Property
    .fromStream( => usernameField.value))
var isPasswordPresent = new Property
    .fromStream( => passwordField.value))
var isFormValid = isUsernamePresent.and(isPasswordPresent);