本文主要實(shí)現(xiàn)獲取textarea和input光標(biāo)的像素位置，即光標(biāo)的offsetLeft與offsetTop。可實(shí)現(xiàn)如下效果：

　　首先說(shuō)明一下，在網(wǎng)上搜到的很多代碼是如何獲取輸入光標(biāo)位置的如下代碼是如何獲取光標(biāo)的的字符位置，如對(duì)于串“He|llo World!”返回的是光標(biāo)|前的字符數(shù)2，并不是光標(biāo)在頁(yè)面上的像素位置。當(dāng)然，這段代碼對(duì)于獲取光標(biāo)的像素位置能起到一定的輔助作用。

// 獲取光標(biāo)在文本框的位置
function _getFocus(elem) {
var index = 0;
if (document.selection) {// IE Support
elem.focus();
var Sel = document.selection.createRange();
if (elem.nodeName === 'TEXTAREA') {//textarea
var Sel2 = Sel.duplicate();
Sel2.moveToElementText(elem);
var index = -1;
while (Sel2.inRange(Sel)) {
Sel2.moveStart('character');
index++;
};
}
else if (elem.nodeName === 'INPUT') {// input
Sel.moveStart('character', -elem.value.length);
index = Sel.text.length;
}
}
else if (elem.selectionStart || elem.selectionStart == '0') { // Firefox support
index = elem.selectionStart;
}
return (index);
} 　　對(duì)于IE瀏覽器，通過(guò)下面的代碼1實(shí)現(xiàn)起來(lái)比較方便。 //代碼1
if (document.selection) {
elem.focus();
var Sel = document.selection.createRange();
return {
left: Sel.boundingLeft,
top: Sel.boundingTop,
bottom: Sel.boundingTop + Sel.boundingHeight
};
}

　　而對(duì)于firefox之類的瀏覽器則通過(guò)模擬來(lái)實(shí)現(xiàn)，如下圖所示。首先通過(guò)拷貝輸入?yún)^(qū)域的樣式到一個(gè)div層（clone層），然后在此clone層之中的text子層添加光標(biāo)之前的字符，并在text子層之后添加focus層，focus層中包含字符“|”來(lái)模擬光標(biāo)，進(jìn)而通過(guò)獲取focus層的偏移量即可獲得文本光標(biāo)的像素坐標(biāo)位置。

　　具體實(shí)現(xiàn)代碼如下：

var kingwolfofsky = {
/**
* 獲取輸入光標(biāo)在頁(yè)面中的坐標(biāo)
* @param {HTMLElement} 輸入框元素
* @return {Object} 返回left和top,bottom
*/
getInputPositon: function (elem) {
if (document.selection) { //IE Support
elem.focus();
var Sel = document.selection.createRange();
return {
left: Sel.boundingLeft,
top: Sel.boundingTop,
bottom: Sel.boundingTop + Sel.boundingHeight
};
} else {
var that = this;
var cloneDiv = '{$clone_div}', cloneLeft = '{$cloneLeft}', cloneFocus = '{$cloneFocus}', cloneRight = '{$cloneRight}';
var none = '<span style="white-space:pre-wrap;"> </span>';
var div = elem[cloneDiv] || document.createElement('div'), focus = elem[cloneFocus] || document.createElement('span');
var text = elem[cloneLeft] || document.createElement('span');
var offset = that._offset(elem), index = this._getFocus(elem), focusOffset = { left: 0, top: 0 };

if (!elem[cloneDiv]) {
elem[cloneDiv] = div, elem[cloneFocus] = focus;
elem[cloneLeft] = text;
div.appendChild(text);
div.appendChild(focus);
document.body.appendChild(div);
focus.innerHTML = '|';
focus.style.cssText = 'display:inline-block;width:0px;overflow:hidden;z-index:-100;word-wrap:break-word;word-break:break-all;';
div.className = this._cloneStyle(elem);
div.style.cssText = 'visibility:hidden;display:inline-block;position:absolute;z-index:-100;word-wrap:break-word;word-break:break-all;overflow:hidden;';
};
div.style.left = this._offset(elem).left + "px";
div.style.top = this._offset(elem).top + "px";
var strTmp = elem.value.substring(0, index).replace(/</g, '<').replace(/>/g, '>').replace(/\n/g, '<br/>').replace(/\s/g, none);
text.innerHTML = strTmp;

focus.style.display = 'inline-block';
try { focusOffset = this._offset(focus); } catch (e) { };
focus.style.display = 'none';
return {
left: focusOffset.left,
top: focusOffset.top,
bottom: focusOffset.bottom
};
}
},

// 克隆元素樣式并返回類
_cloneStyle: function (elem, cache) {
if (!cache && elem['${cloneName}']) return elem['${cloneName}'];
var className, name, rstyle = /^(number|string)$/;
var rname = /^(content|outline|outlineWidth)$/; //Opera: content; IE8:outline && outlineWidth
var cssText = [], sStyle = elem.style;

for (name in sStyle) {
if (!rname.test(name)) {
val = this._getStyle(elem, name);
if (val !== '' && rstyle.test(typeof val)) { // Firefox 4
name = name.replace(/([A-Z])/g, "-$1").toLowerCase();
cssText.push(name);
cssText.push(':');
cssText.push(val);
cssText.push(';');
};
};
};
cssText = cssText.join('');
elem['${cloneName}'] = className = 'clone' + (new Date).getTime();
this._addHeadStyle('.' + className + '{' + cssText + '}');
return className;
},

// 向頁(yè)頭插入樣式
_addHeadStyle: function (content) {
var style = this._style[document];
if (!style) {
style = this._style[document] = document.createElement('style');
document.getElementsByTagName('head')[0].appendChild(style);
};
style.styleSheet && (style.styleSheet.cssText += content) || style.appendChild(document.createTextNode(content));
},
_style: {},

// 獲取最終樣式
_getStyle: 'getComputedStyle' in window ? function (elem, name) {
return getComputedStyle(elem, null)[name];
} : function (elem, name) {
return elem.currentStyle[name];
},

// 獲取光標(biāo)在文本框的位置
_getFocus: function (elem) {
var index = 0;
if (document.selection) {// IE Support
elem.focus();
var Sel = document.selection.createRange();
if (elem.nodeName === 'TEXTAREA') {//textarea
var Sel2 = Sel.duplicate();
Sel2.moveToElementText(elem);
var index = -1;
while (Sel2.inRange(Sel)) {
Sel2.moveStart('character');
index++;
};
}
else if (elem.nodeName === 'INPUT') {// input
Sel.moveStart('character', -elem.value.length);
index = Sel.text.length;
}
}
else if (elem.selectionStart || elem.selectionStart == '0') { // Firefox support
index = elem.selectionStart;
}
return (index);
},

// 獲取元素在頁(yè)面中位置
_offset: function (elem) {
var box = elem.getBoundingClientRect(), doc = elem.ownerDocument, body = doc.body, docElem = doc.documentElement;
var clientTop = docElem.clientTop || body.clientTop || 0, clientLeft = docElem.clientLeft || body.clientLeft || 0;
var top = box.top + (self.pageYOffset || docElem.scrollTop) - clientTop, left = box.left + (self.pageXOffset || docElem.scrollLeft) - clientLeft;
return {
left: left,
top: top,
right: left + box.width,
bottom: top + box.height
};
}
};

function getPosition(ctrl) {
var p = kingwolfofsky.getInputPositon(ctrl);
document.getElementById('show').style.left = p.left + "px";
document.getElementById('show').style.top = p.bottom + "px";
} 　　

測(cè)試頁(yè)面如下：

<!DOCTYPE html>
<html>
<head>
<title>InputPostion</title>
<script type="text/javascript" src="kingwolfofsky.js""></script>
<script type="text/javascript">
function show(elem) {
var p = kingwolfofsky.getInputPositon(elem);
var s = document.getElementById('show');
s.style.top = p.bottom+'px';
s.style.left = p.left + 'px';
s.style.display = 'inherit';
}
</script>
</head>
<body>
<textarea id="text" onkeyup="show(this)" style="width: 340px; height: 210px;"></textarea>
<br />
<input type="text" onkeyup="show(this)" style="width: 340px;" />
<div id="show" style="width: 34px; height: 13px; background: #eee; position: absolute;border:1px solid grey;font-size:13px; display:none;">Tips</div>
</body>
</html>