How to handle scroll position on hashchange in Backbone.js application?

Question

I\'ve read through many related threads but none of them seem to provide a solution.

What I\'m trying to do is handle the scrollbar intelligently in my Backbone.js app.

Answer 1

My solution to this ended up being something less automatic than I wanted, but at least it's consistent.

This was my code for saving and restoring. This code was pretty much carried from my attempt over to my actual solution, just called it on different events. "soft" is a flag that this came from a browser action (back, forward, or hash click) as opposed to a "hard" call to Router.navigate(). During a navigate() call I wanted to just scroll to the top.

restoreScrollPosition: function(route, soft) {
    var pos = 0;
    if (soft) {
        if (this.routesToScrollPositions[route]) {
            pos = this.routesToScrollPositions[route];
        }
    }
    else {
        delete this.routesToScrollPositions[route];
    }
    $(window).scrollTop(pos);
},

saveScrollPosition: function(route) {
    var pos = $(window).scrollTop();
    this.routesToScrollPositions[route] = pos;
}

I also modified Backbone.History so that we can tell the difference between reacting to a "soft" history change (which calls checkUrl) versus programmatically triggering a "hard" history change. It passes this flag to the Router callback.

_.extend(Backbone.History.prototype, {

    // react to a back/forward button, or an href click.  a "soft" route
   checkUrl: function(e) {
        var current = this.getFragment();
        if (current == this.fragment && this.iframe)
            current = this.getFragment(this.getHash(this.iframe));
        if (current == this.fragment) return false;
        if (this.iframe) this.navigate(current);
        // CHANGE: tell loadUrl this is a soft route
        this.loadUrl(undefined, true) || this.loadUrl(this.getHash(), true);
    },

    // this is called in the whether a soft route or a hard Router.navigate call
    loadUrl: function(fragmentOverride, soft) {
        var fragment = this.fragment = this.getFragment(fragmentOverride);
        var matched = _.any(this.handlers, function(handler) {
            if (handler.route.test(fragment)) {
                // CHANGE: tell Router if this was a soft route
                handler.callback(fragment, soft);
                return true;
            }
        });
        return matched;
    },
});

Originally I was trying to do the scroll saving and restoring entirely during the hashchange handler. More specifically, within Router's callback wrapper, the anonymous function that invokes your actual route handler.

route: function(route, name, callback) {
    Backbone.history || (Backbone.history = new Backbone.History);
    if (!_.isRegExp(route)) route = this._routeToRegExp(route);
    if (!callback) callback = this[name];
    Backbone.history.route(route, _.bind(function(fragment, soft) {

        // CHANGE: save scroll position of old route prior to invoking callback
        // & changing DOM
        displayManager.saveScrollPosition(foo.lastRoute);

        var args = this._extractParameters(route, fragment);
        callback && callback.apply(this, args);
        this.trigger.apply(this, ['route:' + name].concat(args));

        // CHANGE: restore scroll position of current route after DOM was changed
        // in callback
        displayManager.restoreScrollPosition(fragment, soft);
        foo.lastRoute = fragment;

        Backbone.history.trigger('route', this, name, args);
    }, this));
    return this;
},

I wanted to handle things this way because it allows saving in all cases, whether an href click, back button, forward button, or navigate() call.

The browser has a "feature" that tries to remember your scroll on a hashchange, and move to it when going back to a hash. Normally this would have been great, and would save me all the trouble of implementing it myself. The problem is my app, like many, changes the height of the DOM from page to page.

For example, I'm on a tall #list view and have scrolled to the bottom, then click an item and go to a short #detail view that has no scrollbar at all. When I press the Back button, the browser will try to scroll me to the last position I was for the #list view. But the document isn't that tall yet, so it is unable to do so. By the time my route for #list gets called and I re-show the list, the scroll position is lost.

So, couldn't use the browser's built-in scroll memory. Unless I made the document a fixed height or did some DOM trickery, which I didn't want to do.

Moreover that built-in scroll behavior messes up the above attempt, because the call to saveScrollPosition is made too late--the browser has already changed the scroll position by then.

The solution to this, which should have been obvious, was calling saveScrollPosition from Router.navigate() instead of the route callback wrapper. This guarantees that I'm saving the scroll position before the browser does anything on hashchange.

route: function(route, name, callback) {
    Backbone.history || (Backbone.history = new Backbone.History);
    if (!_.isRegExp(route)) route = this._routeToRegExp(route);
    if (!callback) callback = this[name];
    Backbone.history.route(route, _.bind(function(fragment, soft) {

        // CHANGE: don't saveScrollPosition at this point, it's too late.

        var args = this._extractParameters(route, fragment);
        callback && callback.apply(this, args);
        this.trigger.apply(this, ['route:' + name].concat(args));

        // CHANGE: restore scroll position of current route after DOM was changed
        // in callback
        displayManager.restoreScrollPosition(fragment, soft);
        foo.lastRoute = fragment;

        Backbone.history.trigger('route', this, name, args);
    }, this));
    return this;
},

navigate: function(route, options) {
    // CHANGE: save scroll position prior to triggering hash change
    nationalcity.displayManager.saveScrollPosition(foo.lastRoute);
    Backbone.Router.prototype.navigate.call(this, route, options);
},

Unfortunately it also means I always have to explicitly call navigate() if I'm interested in saving scroll position, as opposed to just using href="#myhash" in my templates.

Oh well. It works. :-)