jQuery中offset()和position()方法

通过源码来学习offset和position方法才能更深入理解其实现
jQuery的offset()方法

获取匹配元素在当前视口的相对偏移。
返回的对象包含两个整形属性：top 和 left。此方法只对可见元素有效。

jQuery的position()方法

获取匹配元素相对父元素的偏移。
返回的对象包含两个整形属性：top 和 left。此方法只对可见元素有效。

jQuery1.4源码：

var rtable = /^t(?:able|d|h)$/i,
	rroot = /^(?:body|html)$/i;

if ( "getBoundingClientRect" in document.documentElement ) {
	jQuery.fn.offset = function( options ) {
		var elem = this[0], box;

		if ( options ) { 
			return this.each(function( i ) {
				jQuery.offset.setOffset( this, options, i );
			});
		}

		if ( !elem || !elem.ownerDocument ) {
			return null;
		}

		if ( elem === elem.ownerDocument.body ) {
			return jQuery.offset.bodyOffset( elem );
		}

		try {
			box = elem.getBoundingClientRect();
		} catch(e) {}

		var doc = elem.ownerDocument,
			docElem = doc.documentElement;

		// Make sure we're not dealing with a disconnected DOM node
		if ( !box || !jQuery.contains( docElem, elem ) ) {
			return box || { top: 0, left: 0 };
		}

		var body = doc.body,
			win = getWindow(doc),
			clientTop  = docElem.clientTop  || body.clientTop  || 0,
			clientLeft = docElem.clientLeft || body.clientLeft || 0,
			scrollTop  = (win.pageYOffset || jQuery.support.boxModel && docElem.scrollTop  || body.scrollTop ),
			scrollLeft = (win.pageXOffset || jQuery.support.boxModel && docElem.scrollLeft || body.scrollLeft),
			top  = box.top  + scrollTop  - clientTop,
			left = box.left + scrollLeft - clientLeft;

		return { top: top, left: left };
	};

} else {
	jQuery.fn.offset = function( options ) {
		var elem = this[0];

		if ( options ) { 
			return this.each(function( i ) {
				jQuery.offset.setOffset( this, options, i );
			});
		}

		if ( !elem || !elem.ownerDocument ) {
			return null;
		}

		if ( elem === elem.ownerDocument.body ) {
			return jQuery.offset.bodyOffset( elem );
		}

		jQuery.offset.initialize();

		var computedStyle,
			offsetParent = elem.offsetParent,
			prevOffsetParent = elem,
			doc = elem.ownerDocument,
			docElem = doc.documentElement,
			body = doc.body,
			defaultView = doc.defaultView,
			prevComputedStyle = defaultView ? defaultView.getComputedStyle( elem, null ) : elem.currentStyle,
			top = elem.offsetTop,
			left = elem.offsetLeft;

		while ( (elem = elem.parentNode) && elem !== body && elem !== docElem ) {
			if ( jQuery.offset.supportsFixedPosition && prevComputedStyle.position === "fixed" ) {
				break;
			}

			computedStyle = defaultView ? defaultView.getComputedStyle(elem, null) : elem.currentStyle;
			top  -= elem.scrollTop;
			left -= elem.scrollLeft;

			if ( elem === offsetParent ) {
				top  += elem.offsetTop;
				left += elem.offsetLeft;

				if ( jQuery.offset.doesNotAddBorder && !(jQuery.offset.doesAddBorderForTableAndCells && rtable.test(elem.nodeName)) ) {
					top  += parseFloat( computedStyle.borderTopWidth  ) || 0;
					left += parseFloat( computedStyle.borderLeftWidth ) || 0;
				}

				prevOffsetParent = offsetParent;
				offsetParent = elem.offsetParent;
			}

			if ( jQuery.offset.subtractsBorderForOverflowNotVisible && computedStyle.overflow !== "visible" ) {
				top  += parseFloat( computedStyle.borderTopWidth  ) || 0;
				left += parseFloat( computedStyle.borderLeftWidth ) || 0;
			}

			prevComputedStyle = computedStyle;
		}

		if ( prevComputedStyle.position === "relative" || prevComputedStyle.position === "static" ) {
			top  += body.offsetTop;
			left += body.offsetLeft;
		}

		if ( jQuery.offset.supportsFixedPosition && prevComputedStyle.position === "fixed" ) {
			top  += Math.max( docElem.scrollTop, body.scrollTop );
			left += Math.max( docElem.scrollLeft, body.scrollLeft );
		}

		return { top: top, left: left };
	};
}

jQuery.offset = {
	initialize: function() {
		var body = document.body, container = document.createElement("div"), innerDiv, checkDiv, table, td, bodyMarginTop = parseFloat( jQuery.css(body, "marginTop") ) || 0,
			html = "<div style='position:absolute;top:0;left:0;margin:0;border:5px solid #000;padding:0;width:1px;height:1px;'><div></div></div><table style='position:absolute;top:0;left:0;margin:0;border:5px solid #000;padding:0;width:1px;height:1px;' cellpadding='0' cellspacing='0'><tr><td></td></tr></table>";

		jQuery.extend( container.style, { position: "absolute", top: 0, left: 0, margin: 0, border: 0, width: "1px", height: "1px", visibility: "hidden" } );

		container.innerHTML = html;
		body.insertBefore( container, body.firstChild );
		innerDiv = container.firstChild;
		checkDiv = innerDiv.firstChild;
		td = innerDiv.nextSibling.firstChild.firstChild;

		this.doesNotAddBorder = (checkDiv.offsetTop !== 5);
		this.doesAddBorderForTableAndCells = (td.offsetTop === 5);

		checkDiv.style.position = "fixed";
		checkDiv.style.top = "20px";

		// safari subtracts parent border width here which is 5px
		this.supportsFixedPosition = (checkDiv.offsetTop === 20 || checkDiv.offsetTop === 15);
		checkDiv.style.position = checkDiv.style.top = "";

		innerDiv.style.overflow = "hidden";
		innerDiv.style.position = "relative";

		this.subtractsBorderForOverflowNotVisible = (checkDiv.offsetTop === -5);

		this.doesNotIncludeMarginInBodyOffset = (body.offsetTop !== bodyMarginTop);

		body.removeChild( container );
		body = container = innerDiv = checkDiv = table = td = null;
		jQuery.offset.initialize = jQuery.noop;
	},

	bodyOffset: function( body ) {
		var top = body.offsetTop,
			left = body.offsetLeft;

		jQuery.offset.initialize();

		if ( jQuery.offset.doesNotIncludeMarginInBodyOffset ) {
			top  += parseFloat( jQuery.css(body, "marginTop") ) || 0;
			left += parseFloat( jQuery.css(body, "marginLeft") ) || 0;
		}

		return { top: top, left: left };
	},
	
	setOffset: function( elem, options, i ) {
		var position = jQuery.css( elem, "position" );

		// set position first, in-case top/left are set even on static elem
		if ( position === "static" ) {
			elem.style.position = "relative";
		}

		var curElem = jQuery( elem ),
			curOffset = curElem.offset(),
			curCSSTop = jQuery.css( elem, "top" ),
			curCSSLeft = jQuery.css( elem, "left" ),
			calculatePosition = (position === "absolute" && jQuery.inArray('auto', [curCSSTop, curCSSLeft]) > -1),
			props = {}, curPosition = {}, curTop, curLeft;

		// need to be able to calculate position if either top or left is auto and position is absolute
		if ( calculatePosition ) {
			curPosition = curElem.position();
		}

		curTop  = calculatePosition ? curPosition.top  : parseInt( curCSSTop,  10 ) || 0;
		curLeft = calculatePosition ? curPosition.left : parseInt( curCSSLeft, 10 ) || 0;

		if ( jQuery.isFunction( options ) ) {
			options = options.call( elem, i, curOffset );
		}

		if (options.top != null) {
			props.top = (options.top - curOffset.top) + curTop;
		}
		if (options.left != null) {
			props.left = (options.left - curOffset.left) + curLeft;
		}
		
		if ( "using" in options ) {
			options.using.call( elem, props );
		} else {
			curElem.css( props );
		}
	}
};


jQuery.fn.extend({
	position: function() {
		if ( !this[0] ) {
			return null;
		}

		var elem = this[0],

		// Get *real* offsetParent
		offsetParent = this.offsetParent(),

		// Get correct offsets
		offset       = this.offset(),
		parentOffset = rroot.test(offsetParent[0].nodeName) ? { top: 0, left: 0 } : offsetParent.offset();

		// Subtract element margins
		// note: when an element has margin: auto the offsetLeft and marginLeft
		// are the same in Safari causing offset.left to incorrectly be 0
		offset.top  -= parseFloat( jQuery.css(elem, "marginTop") ) || 0;
		offset.left -= parseFloat( jQuery.css(elem, "marginLeft") ) || 0;

		// Add offsetParent borders
		parentOffset.top  += parseFloat( jQuery.css(offsetParent[0], "borderTopWidth") ) || 0;
		parentOffset.left += parseFloat( jQuery.css(offsetParent[0], "borderLeftWidth") ) || 0;

		// Subtract the two offsets
		return {
			top:  offset.top  - parentOffset.top,
			left: offset.left - parentOffset.left
		};
	},

	offsetParent: function() {
		return this.map(function() {
			var offsetParent = this.offsetParent || document.body;
			while ( offsetParent && (!rroot.test(offsetParent.nodeName) && jQuery.css(offsetParent, "position") === "static") ) {
				offsetParent = offsetParent.offsetParent;
			}
			return offsetParent;
		});
	}
});

应用场景：
1： 使用position()方法时事实上是把该元素当绝对定位来处理，获取的是该元素相当于最近的一个拥有绝对或者相对定位的父元素的偏移位置
2： 使用position()方法时如果其所有的父元素都为默认定位（static）方式，则其处理方式和offset()一样，是当前窗口的相对偏移
3： 使用offset()方法不管该元素如何定位，也不管其父元素如何定位，都是获取的该元素相对于当前视口的偏移

参考：
http://www.cnblogs.com/believe3301/archive/2008/07/19/1246806.html
http://www.js8.in/602.html
http://article.qmecms.com/frontend/javascript/2010/07/18/jQuery,source,code,analysis,10,jquery,position-297.html