scrollspy.js

(function($) {

	var jWindow = $(window);
	var elements = [];
	var elementsInView = [];
	var isSpying = false;
	var ticks = 0;

	/**
	 * Find elements that are within the boundary
	 * @param {number} top
	 * @param {number} right
	 * @param {number} bottom
	 * @param {number} left
	 * @return {jQuery}		A collection of elements
	 */
	function findElements(top, right, bottom, left) {
		var hits = $();
		$.each(elements, function(i, element) {
			var elTop = element.offset().top,
				elLeft = element.offset().left,
				elRight = elLeft + element.width(),
				elBottom = elTop + element.height();

			var isIntersect = !(elLeft > right ||
				elRight < left ||
				elTop > bottom ||
				elBottom < top);

			if (isIntersect) {
				hits.push(element);
			}
		});

		return hits;
	}

	/**
	 * Called when the user scrolls the window
	 */
	function onScroll() {
		// unique tick id
		++ticks;

		// viewport rectangle
		var top = jWindow.scrollTop(),
			left = jWindow.scrollLeft(),
			right = left + jWindow.width(),
			bottom = top + jWindow.height();

		// determine which elements are in view
		var intersections = findElements(top, right, bottom, left);
		$.each(intersections, function(i, element) {
			var lastTick = element.data('scrollSpy:ticks');
			if (typeof lastTick != 'number') {
				// entered into view
				element.triggerHandler('scrollSpy:enter');
			}

			// update tick id
			element.data('scrollSpy:ticks', ticks);
		});

		// determine which elements are no longer in view
		$.each(elementsInView, function(i, element) {
			var lastTick = element.data('scrollSpy:ticks');
			if (typeof lastTick == 'number' && lastTick !== ticks) {
				// exited from view
				element.triggerHandler('scrollSpy:exit');
				element.data('scrollSpy:ticks', null);
			}
		});

		// remember elements in view for next tick
		elementsInView = intersections;
	}

	/**
	 * Called when window is resized
	*/
	function onWinSize() {
		jWindow.trigger('scrollSpy:winSize');
	}

	/**
	 * Get time in ms
   * @license https://raw.github.com/jashkenas/underscore/master/LICENSE
	 * @type {function}
	 * @return {number}
	 */
	var getTime = (Date.now || function () {
		return new Date().getTime();
	});

	/**
	 * Returns a function, that, when invoked, will only be triggered at most once
	 * during a given window of time. Normally, the throttled function will run
	 * as much as it can, without ever going more than once per `wait` duration;
	 * but if you'd like to disable the execution on the leading edge, pass
	 * `{leading: false}`. To disable execution on the trailing edge, ditto.
	 * @license https://raw.github.com/jashkenas/underscore/master/LICENSE
	 * @param {function} func
	 * @param {number} wait
	 * @param {Object=} options
	 * @returns {Function}
	 */
	function throttle(func, wait, options) {
		var context, args, result;
		var timeout = null;
		var previous = 0;
		options || (options = {});
		var later = function () {
			previous = options.leading === false ? 0 : getTime();
			timeout = null;
			result = func.apply(context, args);
			context = args = null;
		};
		return function () {
			var now = getTime();
			if (!previous && options.leading === false) previous = now;
			var remaining = wait - (now - previous);
			context = this;
			args = arguments;
			if (remaining <= 0) {
				clearTimeout(timeout);
				timeout = null;
				previous = now;
				result = func.apply(context, args);
				context = args = null;
			} else if (!timeout && options.trailing !== false) {
				timeout = setTimeout(later, remaining);
			}
			return result;
		};
	};

	/**
	 * Enables ScrollSpy using a selector
	 * @param {jQuery|string} selector		The elements collection, or a selector
	 * @param {Object=} options						Optional. Set { throttle: number } to change scrollspy throttling. Default: 100 ms
	 * @returns {jQuery}
	 */
	$.scrollSpy = function(selector, options) {
		selector = $(selector);
		selector.each(function(i, element) {
			elements.push($(element));
		});
		options = options || {
			throttle: 100
		};

		if (!isSpying) {
			jWindow.on('scroll', throttle(onScroll, options.throttle || 100));
			jWindow.on('resize', throttle(onScroll, options.throttle || 100));
			isSpying = true;

			// perform a scan once, after current execution context, and after dom is ready
			setTimeout(function() {
				$(document).ready(onScroll);
			}, 0);
		}

		return selector;
	};

	/**
	 * Listen for window resize events
	 * @param {Object=} options						Optional. Set { throttle: number } to change throttling. Default: 100 ms
	 * @returns {jQuery}		$(window)
	 */
	$.winSizeSpy = function(options) {
		$.winSizeSpy = function() { return jWindow; }; // lock from multiple calls
		options = options || {
			throttle: 100
		};
		return jWindow.on('resize', throttle(onWinSize, options.throttle || 100));
	};

	/**
	 * Enables ScrollSpy on a collection of elements
	 * e.g. $('.scrollSpy').scrollSpy()
	 * @param {Object=} options						Optional. Set { throttle: number } to change scrollspy throttling
	 * @returns {jQuery}
	 */
	$.fn.scrollSpy = function(options) {
		return $.scrollSpy($(this), options);
	};

})(jQuery);