rx-fixed-interval.js

var Rx = require('rx')

/**
 * RxFixedInterval
 * 
 * Returns an observable sequence that produces a value after each period.
 * 
 * The difference between FixedInterval and the standard Rx interval is each value is produced when the time
 * is divisable by the interval.
 * 
 * Example: If the interval is 1 hour the next value will be produced at the start of the next hour (00 minutes)
 *          not at 1 hours time from the starting time
 * 
 * To show the difference here is how a Rx.Observable.Interval and a FixedInterval will behave, both with a 10 second interval and starting at 10:07:12.
 * 
 * count  Normal Rx interval  FixedInterval
 * 1      10:07:22            10:07:20
 * 2      10:07:32            10:07:30
 * 3      10:07:42            10:07:40
 * 4      10:07:52            10:07:50
 */
function RxFixedInterval(interval, offset, scheduler) {
	offset = offset || 0

	var now = Date.now()
	var firstTick = (now + interval) - (now - offset) % interval

	return Rx.Observable.timer(new Date(firstTick), interval, scheduler)
		.map(function(count) {
			return {
				count: count,
				time: new Date(firstTick + (count * interval) )
			}
		})
}