Transition with keepScrollPosition and navigateBack - durandal

Question

We are using Durandal for our SPA application and came to a, in my opinion, common use case. We have two pages: one page is a list of entities (with filters, sorting, virtual scroll) and another is detail preview of an entity. So, user is on list page and set a filter and a list of results comes out. After scrolling a little bit down user notice an entity which he/she would like to see details for. So clicking on a proper link user is navigated to details preview page.
After "work finished" on preview page user click back button (in app itself or browser) and he/she is back on the list page. However, default 'entrance' transition scroll the page to the top and not to the position on list where user pressed preview. So in order to 'read' list further user have to scroll down where he/she was before pressing preview.
So I started to create new transition which will for certain pages (like list-search pages) keep the scroll position and for other pages (like preview or edit pages) scroll to top on transition complete. And this was easy to do however, I was surprised when I noticed that there are strange behavior on preview pages when I hit navigateBack 'button'. My already long story short, after investigation I found out that windows.history.back is completing earlier then the transition is made and this cause that preview pages are scrolled automatically down to position of previous (list) page when back button is hit. This scrolling have a very unpleasant effect on UI not mentioning that it is 'total catastrophe' for my transition.
Any idea or suggestion what could I do in this case?
Here is the code of transition. It is just a working copy not finished yet as far as I have this problem.
define(['../system'], function (system) {
var fadeOutDuration = 100;
var scrollPositions = new Array();
var getScrollObjectFor = function (node) {
var elemObjs = scrollPositions.filter(function (ele) {
return ele.element === node;
});
if (elemObjs.length > 0)
return elemObjs[0];
else
return null;
};
var addScrollPositionFor = function (node) {
var elemObj = getScrollObjectFor(node);
if (elemObj) {
elemObj.scrollPosition = $(document).scrollTop();
}
else {
scrollPositions.push({element: node, scrollPosition: $(document).scrollTop()});
}
};
var scrollTransition = function (parent, newChild, settings) {
return system.defer(function (dfd) {
function endTransition() {
dfd.resolve();
}
function scrollIfNeeded() {
var elemObj = getScrollObjectFor(newChild);
if (elemObj)
{
$(document).scrollTop(elemObj.scrollPosition);
}
else {
$(document).scrollTop(0);
}
}
if (!newChild) {
if (settings.activeView) {
addScrollPositionFor(settings.activeView);
$(settings.activeView).fadeOut(fadeOutDuration, function () {
if (!settings.cacheViews) {
ko.virtualElements.emptyNode(parent);
}
endTransition();
});
} else {
if (!settings.cacheViews) {
ko.virtualElements.emptyNode(parent);
}
endTransition();
}
} else {
var $previousView = $(settings.activeView);
var duration = settings.duration || 500;
var fadeOnly = !!settings.fadeOnly;
function startTransition() {
if (settings.cacheViews) {
if (settings.composingNewView) {
ko.virtualElements.prepend(parent, newChild);
}
} else {
ko.virtualElements.emptyNode(parent);
ko.virtualElements.prepend(parent, newChild);
}
var startValues = {
marginLeft: fadeOnly ? '0' : '20px',
marginRight: fadeOnly ? '0' : '-20px',
opacity: 0,
display: 'block'
};
var endValues = {
marginRight: 0,
marginLeft: 0,
opacity: 1
};
$(newChild).css(startValues);
var animateOptions = {
duration: duration,
easing : 'swing',
complete: endTransition,
done: scrollIfNeeded
};
$(newChild).animate(endValues, animateOptions);
}
if ($previousView.length) {
addScrollPositionFor(settings.activeView);
$previousView.fadeOut(fadeOutDuration, startTransition);
} else {
startTransition();
}
}
}).promise();
};
return scrollTransition;
});

Answer 1

A simpler approach could be to store the scroll position when the module deactivates and restore the scroll on viewAttached.
You could store the positions in some global app variable:
app.scrollPositions = app.scrollPositions || {};
app.scrollPositions[system.getModuleId(this)] = theCurrentScrollPosition;