/**
* 文本框根据输入内容自适应高度
* @param                {HTMLElement}        输入框元素
* @param                {Number}                设置光标与输入框保持的距离(默认0)
* @param                {Number}                设置最大高度(可选)
*/
var autoTextarea = function (elem, extra, maxHeight) {
        extra = extra || 0;
        var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
        isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
                addEvent = function (type, callback) {
                        elem.addEventListener ?
                                elem.addEventListener(type, callback, false) :
                                elem.attachEvent('on' + type, callback);
                },
                getStyle = elem.currentStyle ? function (name) {
                        var val = elem.currentStyle[name];
 
                        if (name === 'height' && val.search(/px/i) !== 1) {
                                var rect = elem.getBoundingClientRect();
                                return rect.bottom - rect.top -
                                        parseFloat(getStyle('paddingTop')) -
                                        parseFloat(getStyle('paddingBottom')) + 'px';        
                        };
 
                        return val;
                } : function (name) {
                                return getComputedStyle(elem, null)[name];
                },
                minHeight = parseFloat(getStyle('height'));
 
        elem.style.resize = 'none';
 
        var change = function () {
                var scrollTop, height,
                        padding = 0,
                        style = elem.style;
 
                if (elem._length === elem.value.length) return;
                elem._length = elem.value.length;
 
                if (!isFirefox && !isOpera) {
                        padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
                };
                scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
 
                elem.style.height = minHeight + 'px';
                if (elem.scrollHeight > minHeight) {
                        if (maxHeight && elem.scrollHeight > maxHeight) {
                                height = maxHeight - padding;
                                style.overflowY = 'auto';
                        } else {
                                height = elem.scrollHeight - padding;
                                style.overflowY = 'hidden';
                        };
                        style.height = height + extra + 'px';
                        scrollTop += parseInt(style.height) - elem.currHeight;
                        document.body.scrollTop = scrollTop;
                        document.documentElement.scrollTop = scrollTop;
                        elem.currHeight = parseInt(style.height);
                };
        };
 
        addEvent('propertychange', change);
        addEvent('input', change);
        addEvent('focus', change);
        change();
};