Position是prototype中定义的一个对象，提供了操作DOM中与位置相关的方法，要很好的理解元素在页面中的位置，可以参考这篇文章：Relatively Absolute

具体代码如下，按照代码说说，其中英文是作者的注释，中文红色的才是偶的说明或翻译英文的注释，采用顶式注释法(注释在要说明的代码的上面)说明

// set to true if needed, warning: firefox performance problems
// NOT neeeded for page scrolling, only if draggable contained in
// scrollable elements
//只有在使用拖动的时候元素包含在有滚动条的元素中才需要设置为true
includeScrollOffsets: false,

// must be called before calling withinIncludingScrolloffset, every time the
// page is scrolled
//当页面被scrolled后，使用withinIncludingScrolloffset的时候需要先调用这个方法
prepare: function() {
//横向滚动条滚动的距离
this.deltaX = window.pageXOffset
|| document.documentElement.scrollLeft
|| document.body.scrollLeft
|| 0;
//纵向滚动条滚动的距离
this.deltaY = window.pageYOffset
|| document.documentElement.scrollTop
|| document.body.scrollTop
|| 0;
},

//元素由于滚动条偏移的总距离
realOffset: function(element) {
var valueT = 0, valueL = 0;
do {
valueT += element.scrollTop || 0;
valueL += element.scrollLeft || 0;
element = element.parentNode;
} while (element);
return [valueL, valueT];
},

//元素在页面中由offsetParent累积的offset，当offsetParent都没有滚动条时，就是元素在页面中的位置
cumulativeOffset: function(element) {
var valueT = 0, valueL = 0;
do {
valueT += element.offsetTop || 0;
valueL += element.offsetLeft || 0;
element = element.offsetParent;
} while (element);
return [valueL, valueT];
},

//元素相对于containing block("nearest positioned ancestor")的位置，也就是相对于最近的一个position设置为relative或者absolute的祖先节点的位置，如果没有就是相对于body的位置，跟style.top，style.left一样？
positionedOffset: function(element) {
var valueT = 0, valueL = 0;
do {
valueT += element.offsetTop || 0;
valueL += element.offsetLeft || 0;
element = element.offsetParent;
if (element) {
if(element.tagName=='BODY') break;
var p = Element.getStyle(element, 'position');
if (p == 'relative' || p == 'absolute') break;
}
} while (element);
return [valueL, valueT];
},

//offsetParent
offsetParent: function(element) {
if (element.offsetParent) return element.offsetParent;
if (element == document.body) return element;

while ((element = element.parentNode) && element != document.body)
if (Element.getStyle(element, 'position') != 'static')
return element;

return document.body;
},

// caches x/y coordinate pair to use with overlap
//判断指定的位置是否在元素内
within: function(element, x, y) {
if (this.includeScrollOffsets)
return this.withinIncludingScrolloffsets(element, x, y);
this.xcomp = x;
this.ycomp = y;
this.offset = this.cumulativeOffset(element);

return (y >= this.offset[1] &&
y < this.offset[1] + element.offsetHeight &&
x >= this.offset[0] &&
x < this.offset[0] + element.offsetWidth);
},

//跟within差不多，不过考虑到滚动条，也许是在元素上面，但不是直接在上面，因为滚动条也许已经使元素不可见了
withinIncludingScrolloffsets: function(element, x, y) {
var offsetcache = this.realOffset(element);

this.xcomp = x + offsetcache[0] - this.deltaX;
this.ycomp = y + offsetcache[1] - this.deltaY;
this.offset = this.cumulativeOffset(element);

return (this.ycomp >= this.offset[1] &&
this.ycomp < this.offset[1] + element.offsetHeight &&
this.xcomp >= this.offset[0] &&
this.xcomp < this.offset[0] + element.offsetWidth);
},

// within must be called directly before
//在调用这个方法前，必须先调用within，返回在with指定的位置在水平或者垂直方向上占用的百分比
overlap: function(mode, element) {
if (!mode) return 0;
if (mode == 'vertical')
return ((this.offset[1] + element.offsetHeight) - this.ycomp) /
element.offsetHeight;
if (mode == 'horizontal')
return ((this.offset[0] + element.offsetWidth) - this.xcomp) /
element.offsetWidth;
},

//返回元素相对页面的真实位置
page: function(forElement) {
var valueT = 0, valueL = 0;

var element = forElement;
do {
valueT += element.offsetTop || 0;
valueL += element.offsetLeft || 0;

// Safari fix
if (element.offsetParent==document.body)
if (Element.getStyle(element,'position')=='absolute') break;

} while (element = element.offsetParent);

element = forElement;
do {
if (!window.opera || element.tagName=='BODY') {
valueT -= element.scrollTop || 0;
valueL -= element.scrollLeft || 0;
}
} while (element = element.parentNode);

return [valueL, valueT];
},

//设置target为source的位置，大小
clone: function(source, target) {
var options = Object.extend({
setLeft: true,
setTop: true,
setWidth: true,
setHeight: true,
offsetTop: 0,
offsetLeft: 0
}, arguments[2] || {})

// find page position of source
source = $(source);
var p = Position.page(source);

// find coordinate system to use
target = $(target);
var delta = [0, 0];
var parent = null;
// delta [0,0] will do fine with position: fixed elements,
// position:absolute needs offsetParent deltas
if (Element.getStyle(target,'position') == 'absolute') {
parent = Position.offsetParent(target);
delta = Position.page(parent);
}

// correct by body offsets (fixes Safari)
if (parent == document.body) {
delta[0] -= document.body.offsetLeft;
delta[1] -= document.body.offsetTop;
}

// set position
if(options.setLeft) target.style.left = (p[0] - delta[0] + options.offsetLeft) + 'px';
if(options.setTop) target.style.top = (p[1] - delta[1] + options.offsetTop) + 'px';
if(options.setWidth) target.style.width = source.offsetWidth + 'px';
if(options.setHeight) target.style.height = source.offsetHeight + 'px';
},

//将element的position设置为absolute的模式
absolutize: function(element) {
element = $(element);
if (element.style.position == 'absolute') return;
Position.prepare();

var offsets = Position.positionedOffset(element);
var top = offsets[1];
var left = offsets[0];
var width = element.clientWidth;
var height = element.clientHeight;

element._originalLeft = left - parseFloat(element.style.left || 0);
element._originalTop = top - parseFloat(element.style.top || 0);
element._originalWidth = element.style.width;
element._originalHeight = element.style.height;

element.style.position = 'absolute';
element.style.top = top + 'px';;
element.style.left = left + 'px';;
element.style.width = width + 'px';;
element.style.height = height + 'px';;
},

//将element的position设置为absolute的模式
relativize: function(element) {
element = $(element);
if (element.style.position == 'relative') return;
Position.prepare();

element.style.position = 'relative';
var top = parseFloat(element.style.top || 0) - (element._originalTop || 0);
var left = parseFloat(element.style.left || 0) - (element._originalLeft || 0);

element.style.top = top + 'px';
element.style.left = left + 'px';
element.style.height = element._originalHeight;
element.style.width = element._originalWidth;
}
}

// Safari returns margins on body which is incorrect if the child is absolutely
// positioned. For performance reasons, redefine Position.cumulativeOffset for
// KHTML/WebKit only.
if (/Konqueror|Safari|KHTML/.test(navigator.userAgent)) {
Position.cumulativeOffset = function(element) {
var valueT = 0, valueL = 0;
do {
valueT += element.offsetTop || 0;
valueL += element.offsetLeft || 0;
if (element.offsetParent == document.body)
if (Element.getStyle(element, 'position') == 'absolute') break;

element = element.offsetParent;
} while (element);

return [valueL, valueT];
}
}

终于把Prototype的所有部分都写完了，哈哈，越来越佩服自己的耐力了

下一步决定写写Scriptaculous这个超级流行的效果库