ScrollWatch

jQuery plugin for determining active sections on the page based on scrolling.

Online demo (latest version)

Features

• determining active section(s)
• mapping a class to a list of elements (e.g. menu items)
• many options, including multiple modes of resolution
• works with zoom

Requirements

jQuery 1.9.0 or newer

Browser support

Tested in Mozilla Firefox, Google Chrome, Safari, Opera and MSIE 7+.

Usage

The plugin provides two jQuery methods:

Note

If the sections are inside a custom scrollable element, that element must have position: relative, absolute or fixed.

$(sections).scrollWatch(callback[, options])

Attaches a watcher to the given sections. The callback is then invoked when the focus changes, according to options.

• sections - selector or an array of elements that represent all the possible sections
• callback - function to invoke when the focus changes
• options - object with various settings (see list far below)

Callback arguments

$('div.section').scrollWatch(function (focus, view, watcher) {
    // do something with the current focus
});

1. Object|Object[] focus - the current focus, an object with the following properties:
   (if resolutionMode is none, it will be an array of those objects
   • index - section number (0-based)
   • intersection - null or [y1, y2]
   • section - DOM element of the section
2. Object view - the current view, an object with the following properties:
   • top - top coordinate of the view
   • bottom - bottom coordinate of the view
3. Shira.ScrollWatch.Watcher watcher - instance of the current watcher

$(sections).scrollWatchMapTo(items[, activeClass, options])

Attaches a watcher to the given sections and maps the current focus to the respective item using the specified active class.

• sections - selector or an array of elements that represent all the possible sections
• items - selector or an array of elements to map the "active class" to
• activeClass - class name to add to the active item (defaults to "active")
• options - object with various settings (see list far below)

Example

$('div.section').scrollWatchMapTo('#menu > li');

List of supported options

Option	Default	Description
scroller	null	DOM element to watch for scrolling events. It must be an HTMLElement or Window. If no element is given the first found scrollable offset parent or Window will be used instead.
----------------------	------------------	--------------------------------------------------------
resolutionMode	"height"	Determines how the active section is chosen, one of: height - section that is occupying the largest portion of the view is chosen focus-line - section that is directly intersecting or is closest to the focus line is chosen custom - use a custom resolver none - no resolution is performed (all candidates will be passed to the callback)
----------------------	------------------	--------------------------------------------------------
viewMarginTop	0	Height of an area at the top of the view to be excluded.
----------------------	------------------	--------------------------------------------------------
viewMarginBottom	0	Height of an area at the bottom of the view to be excluded.
----------------------	------------------	--------------------------------------------------------
stickyOffsetTop	5	Height of an area at the top of the scroller that, if intersected by the top of the view, forces the first section to be active regardless of other conditions.
----------------------	------------------	--------------------------------------------------------
stickyOffsetBottom	5	Height of an area at the bottom of the scroller that, if intersected by the bottom of the view, forces the last section to be active regardless of other conditions.
----------------------	------------------	--------------------------------------------------------
clamp	false	When enabled, the space between two sections is considered a part of the first section. Useful when there are large gaps between sections.
----------------------	------------------	--------------------------------------------------------
throttle	true	When enabled, the callback is invoked only when the active section changes When disabled, the callback is invoked on every pulse (e.g. on scroll and resize). This option has no effect when resolutionMode is none.

Options specific to resolutionMode = height

Option	Default	Description
topDownWeight	0	Extra focus height added to the section if it precedes the other. This can be used to prefer earlier sections to later ones.
----------------------	------------------	--------------------------------------------------------

Options for resolutionMode = focus-line

Option	Default	Description
focusRatio	0.3819..	Percentage of the view height that determines position of the focus line.
----------------------	------------------	--------------------------------------------------------
focusOffset	0	Offset added to position of the focus line position after focusRatio is applied. Set focusRatio to 0 if you wish to use the focusOffset as an absolute value.
----------------------	------------------	--------------------------------------------------------
debugFocusLine	false	When enabled, position of the focus line will be displayed when scrolling Intended for debugging purposes.

Options for resolutionMode = custom

Option	Default	Description
resolver	none (required)	Function to invoke when a focus candidate resolution is needed. It must choose and return a single focus object. The following arguments are passed to the function: Object[] candidates - an array of focus objects, each object has the following properties: index - section number (0-based) intersection - null or [y1, y2] section - DOM element of the section