What to do with `Bus().toProperty()`?

I’ve read (and answered to) numerous complaints that code involving bus.toProperty() “does not work”. And I totally agree that it’s counterintuitive that something like this (from Bacon FAQ) does not work:

var bus = new Bacon.Bus;
var property = bus.toProperty('FIRST');
bus.push('SECOND');
property.onValue(function(val) {
  console.log(val);
});

You’d like the console to log ‘SECOND’ but it logs ‘FIRST’ instead.

What’s happening here is that your Property won’t get updated when there are no listeners. Before you add an actual listener using onValue, the Property is not active; it’s not listening to the underlying Bus. It ignores input until someone’s interested. So it all boils down to the “laziness” of all Bacon streams and properties. This is a key feature too: the streams automatically “plug in” to their sources when a subscriber is added, and “unplug” when there are no more subscribers.

A simple patch for this would be to have Bus::toProperty() always add a dummy subscriber to the resulting Property. Then, again, it would fail in setups like this:

bus.map(x => x*2).toProperty();

… so it’s not such a great patch after all. Or is it? Because if you flipped it like

bus.toProperty().map(x => x*2);

it would be good again!

The underlying problem is that the subscriber-count-based mechanism for change propagation in Bacon.js doesn’t serve you well in cases when you create a stateful Property on top of a stateless EventStream unless you always have at least one subscriber on the Property.

One option I’m half-seriously considering is to have Properties throw an error in cases where you re-subscribe after the last subscriber has been removed. This is probably always a bug in application code, because it can lead to propagating outdated values.

Thoughts?

[raimohanska]

Hi guys! I wrote something on property reactivation on #770. What do you think?

[steve-taylor]

If Bacon.fromClipboard existed, it would probably do the initial readText() for the first subscriber on setup 😛