Bacon.waitUntil

waitUntil.js

/**
 * Waits until the supplied stream emits a value, and then
 * continues by emitting the last value emitted on the target.
 * Useful when you want to gate on an event emitter.
 */
Bacon.Observable.prototype.waitUntil = function(item) {
    // Cache the last value to come out of the source.
    let last;
    const unsub = this.onValue(x => last = x);

    return Bacon.fromBinder(sink => {
        // And sink it once the target emits a value.
        // Note that the target's emitted value is ignored.
        const outerUnsub = item.onValue(_ => sink(last));

        return () => {
            unsub();
            outerUnsub();
        };
    });
};

// Example: Same functionality as Bacon's Property.sample, gets the latest emitted value of the first stream every 300ms.
Bacon.repeatedly(100, [1,2,3,4,5,6])
  .waitUntil(Bacon.interval(300))
  .onValue(console.log.bind(console))
  
// Example: Wait for clicks and emit whatever the last value the first stream has emitted.
Bacon.repeatedly(100, [1,2,3,4,5,6])
  .waitUntil($(document).asEventStream('click'))
  .onValue(console.log.bind(console))