bacon.js - Ignoring certain values in a stream

Question

Assuming the following code (current state can be viewed here):

function scrollTopFromEvent(evt) {
    return $(evt.target).scrollTop();
}

function scrollDirection(evt) {
    return -evt.originalEvent.wheelDelta / 120 || evt.originalEvent.detail / 3;
}

function pageNumber(previous, next) {
    return previous + next;
}

function scrollToPage(pageNumber) {
    var container = $('.parallax');
    TweenLite.to(container, 1, {
        scrollTop: container.height() * (pageNumber)
    });
}

function inRange(pageNumber) {
    var totalPages = $('.parallax').find('div[id^="group"]').length;
    return pageNumber >= 0 || pageNumber <= totalPages;
}
var scroll = $('.parallax')
    .asEventStream('mousewheel DOMMouseScroll')
    .doAction('.preventDefault').flatMap(scrollDirection);

var page = scroll
    .scan(0, pageNumber)
    .filter(inRange)
    .log()
    .onValue(scrollToPage);

---

I want to be able to limit the values emitted by the page stream from 0 to n, n being the totalPages (defined in the inRange method). In that particular case, there are 7 groups, so the range would be from 0 to 7

If a user scroll down to '7', and continues scrolling, no values would be emitted from the scroll stream.

Ideas?

Thanks

Answer 1

I think what you actually want is to limit the value of scroll.scan(0, pageNumber) to a certain range, not to maintain a "virtual" scroll position and display that only when it currently is in the range.

Try

var totalPages = $('.parallax').find('div[id^=group]').length; // static, right?

function pageNumber(previous, next) {
    return Math.max(0, Math.min(totalPages, previous + next));
}

scroll.scan(0, pageNumber).onValue(scrollToPage);

Answer 2

These are the changes I made:

In your pageNumber function:

return previous + next >= $('.parallax__group').length ? previous : previous + next;

And in your inRange function:

return pageNumber >= 0 && pageNumber < totalPages;

Here is a fiddle containing your code with my changes.