events - What is *like* a promise but can resolve mutliple times?

Question

I am looking for a pub/sub mechanism that behaves like a promise but can resolve multiple times, and behaves like an event except if you subscribe after a notification has happened it triggers with the most recent value.

I am aware of notify, but deferred.notify is order-sensitive, so in that way it behaves just like an event. eg:

d.notify('notify before'); // Not observed :(
d.promise.progress(function(data){ console.log(data) });
d.notify('notify after');  // Observed
setTimeout(function(){ d.notify('notify much later') }, 100); // Also Observed

fiddle: http://jsfiddle.net/foLhag3b/

The notification system I'd like is a good fit for a UI component that should update to reflect the state of the data behind it. In these cases, you don't want to care about whether the data has arrived yet or not, and you want updates when they come in.

Maybe this is similar to Immediate mode UIs, but is distinct because it is message based.

The state of the art for message based UI updating, as far as I'm aware, is something which uses a promise or callback to initialize, then also binds an update event:

myUIComponent.gotData(model.data);
model.onUpdate(myUIComponent.gotData); // doing two things is 2x teh workz :(

I don't want to have to do both. I don't think anyone should have to, the use case is common enough to abstract.

model.whenever(myUIComponent.gotData); // yay one intention === one line of code

I could build a mechanism to do what I want, but I wanted to see if a pub/sub mechanism like this already exists. A lot of smart people have done a lot in CS and I figure this concept must exist, I just am looking for the name of it.

To be clear, I'm not looking to change an entire framework, say to Angular or React. I'm looking only for a pub/sub mechanism. Preferably an implementation of a known concept with a snazzy name like notifr or lemme-kno or touch-base.

Answer 1

You'll want to have a look at (functional) reactive programming. The concept you are looking for is known as a Behavior or Signal in FRP. It models the change of a value over time, and can be inspected at any time (continuously holds a value, in contrast to a stream that discretely fires events).

var ui = state.map(render); // whenever state updates, so does ui with render() result

Some popular libraries in JS are Bacon and Rx, which use their own terminology however: you'll find properties and observables.