一个简单实现的遮罩层

/**
 * mask - 遮罩
 *
 *
 * Requires:
 * 依赖:
 *      jquery.js
 *
 * How to use:
 * 使用方法:
 *      $.showMask(msg);
 *      $.hideMask();
 *      
css样式：

#ui-mask {
  background-color: #ccc;
  position: absolute;
  z-index: 9999;
  left: 0;
  top: 0;
  display: none;
   100%;
  height: 100%;
  opacity: 0.2;
  filter: alpha(opacity=20);
  -moz-opacity: 0.2;
}

#ui-mask-div {
    font-size: 14px;;
    font-family: Verdana, 微软雅黑, 新宋体, Tahoma;
    z-index: 99999;
    position: fixed;
    padding: 10px 5px 10px 30px;
    border-color: #D4D4D4;
    top: 50%;
    auto;
    border- 2px;
    border-style: solid;
    background: #ffffff url('../loading.gif') no-repeat scroll 5px center;
}
 *      
 *      
 */
(function($) {

    $.fn.showMask = function(msg) {
        return $.showMask(msg);
    }
    
    $.fn.hideMask = function() {
        this.hideMask();
    }

    $.showMask = function(msg) {
        $("html").append('<div id="ui-mask"></div><div id="ui-mask-div">' + msg + '</div>');
        _this_ = $("#ui-mask");
        _this_.height($(document).height());
        this.adjust();
        _this_.css('display', 'block');
    };

    $.hideMask = function() {
        $("#ui-mask").remove();
        $("#ui-mask-div").remove();
    };

    $.adjust = function() {
        var w = $("#ui-mask").width();
        var div = $("#ui-mask-div").css('width').replace('px', '');
        
        var l = (w - div )/ 2;
        if (l < 0)
            l = 0;
        
        $("#ui-mask-div").css({
            left : l + 'px'
        });
    }

})(jQuery);