discourse/vendor/assets/javascripts/lock-on.js

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// Dear traveller, you are entering a zone where we are at war with the browser
// the browser is insisting on positioning scrollTop per the location it was in
// the past, we are insisting on it being where we want it to be
2015-03-04 23:09:31 -05:00
// The hack is just to keep trying over and over to position the scrollbar (up to 1 minute)
//
// The root cause is that a "refresh" on a topic page will almost never be at the
// same position it was in the past, the URL points to the post at the top of the
// page, so a refresh will try to bring that post into view causing drift
//
// Additionally if you loaded multiple batches of posts, on refresh they will not
// be loaded.
//
// This hack leads to a slight jerky experience, however other workarounds are more
// complex, the 2 options we have are
//
// 1. onbeforeunload ensure we are scrolled to the right spot
// 2. give up on the scrollbar and implement it ourselves (something that will happen)
(function (exports) {
var scrollEvents = "scroll.lock-on touchmove.lock-on mousedown.lock-on wheel.lock-on DOMMouseScroll.lock-on mousewheel.lock-on keyup.lock-on";
var LockOn = function(selector, options) {
this.selector = selector;
this.options = options || {};
};
LockOn.prototype.elementTop = function() {
var offsetCalculator = this.options.offsetCalculator,
selected = $(this.selector);
if (selected && selected.offset && selected.offset()) {
return selected.offset().top - (offsetCalculator ? offsetCalculator() : 0);
}
};
LockOn.prototype.lock = function() {
var self = this,
previousTop = this.elementTop(),
startedAt = new Date().getTime(),
i = 0;
$(window).scrollTop(previousTop);
var within = function(threshold,x,y) {
return Math.abs(x-y) < threshold;
};
var interval = setInterval(function() {
i = i + 1;
var top = self.elementTop(),
scrollTop = $(window).scrollTop();
if (typeof(top) === "undefined") {
$('body,html').off(scrollEvents);
clearInterval(interval);
return;
}
if (!within(4, top, previousTop) || !within(4, scrollTop, top)) {
$(window).scrollTop(top);
// animating = true;
// $('html,body').animate({scrollTop: parseInt(top,10)+'px'}, 200, 'swing', function(){
// animating = false;
// });
previousTop = top;
}
// We commit suicide after 3s just to clean up
var nowTime = new Date().getTime();
if (nowTime - startedAt > 1000) {
$('body,html').off(scrollEvents);
clearInterval(interval);
}
}, 50);
$('body,html').off(scrollEvents).on(scrollEvents, function(e){
if ( e.which > 0 || e.type === "mousedown" || e.type === "mousewheel" || e.type === "touchmove") {
$('body,html').off(scrollEvents);
clearInterval(interval);
}
});
};
exports.LockOn = LockOn;
})(window);