Twitter bootstrap change affix offset

Question

I have site where I want 'sub navigation'. So when you scroll to section, its tool bar will affix below the main toolbar. I have this working, but I can't change the top offset after initailization. The docs say:

affix('refresh')

When using affix in conjunction with adding or removing of elements from the DOM, you'll want to call the refresh method:

but when I try this I get:

Uncaught TypeError: Object # has no method 'refresh'

Here is my code:

$(function () {
    $('.sec-nav').addClass("affix-top");
    var adjustNavs = function (first) {
        $('.sec-nav').each(function (){
            var $p = $(this).closest('.sec');
            var $$ = $p.prevAll('.sec');
            console.log($$);
            var h = 0;
            $$.each(function () { h+=$(this).outerHeight();});
            console.log(h);

            if (first) {
                $(this).affix({offset: {top: h}});
            } else {
                $(this).data('affix', {b: {options: {offset:{top: h}}}});
                console.log(this);
                $(this).affix('refresh')
            }

        });
    }
    adjustNavs(true);
    $('.sec').bind('DOMSubtreeModified', function () {
        adjustNavs();
    });
});

I've tried different variation of

                $(this).data('affix', {b: {options: {offset:{top: h}}}});

including:

                $(this).affix({offset: {top: h}});

I cannot get the offset to change. How do you change the offset of the affix plugin? Thanks.

Answer 1

I figured it out. Instead of dynamically updating the offset, I changed the offset value to a function:

$(function () {
    $('.sec-nav').addClass("affix-top").each(function (){
        var $self = $(this);
        var offsetFn = function () {
            var $p = $self.closest('.sec');
            var $$ = $p.prevAll('.sec');
            var h = 0;
            $$.each(function () { h+=$(this).outerHeight();});
            return h;
        }
        $self.affix({offset: {top: offsetFn}});
    });
});

Now when a section changes due to dom manipulation or window re-size, the offset is automaticly changed due to the functions return value changing.

Answer 2

I simplified Fatmuemoo's answer slightly for a more basic case: My home page's nav bar (the default bootstrap #masthead) sits below a big background slider with id #bg. The height of the slider varies with 3 css breakpoints. We just set the offset of the #masthead to equal the height of the slider. Easy!

    $('header#masthead').affix({ //sticky header at bottom of bg
        offset: {
          top: function(){return $("#bg").outerHeight();}
        }
    });