求高手解决！！！
请帮我详细解释一下这段代码？
(function($) {
// @todo Document this.
$.extend($,{ placeholder: {
browser_supported: function() {
return this._supported !== undefined ?
this._supported :
( this._supported = !!('placeholder' in $('<input type="text">')[0]) );
},
shim: function(opts) {
var config = {
color: '#888',
cls: 'placeholder',
selector: 'input[placeholder], textarea[placeholder]'
};
$.extend(config,opts);
return !this.browser_supported() && $(config.selector)._placeholder_shim(config);
}
}});
$.extend($.fn,{
_placeholder_shim: function(config) {
function calcPositionCss(target)
{
var op = $(target).offsetParent().offset();
var ot = $(target).offset();
return {
top: ot.top - op.top,
left: ot.left - op.left,
width: $(target).width()
};
}
return this.each(function() {
var $this = $(this);
if( $this.is(':visible') ) {
if( $this.data('placeholder') ) {
var $ol = $this.data('placeholder');
$ol.css(calcPositionCss($this));
return true;
}
var possible_line_height = {};
if( !$this.is('textarea') && $this.css('height') != 'auto') {
possible_line_height = { lineHeight: $this.css('height'), whiteSpace: 'nowrap' };
}
var ol = $('<label />')
.text($this.attr('placeholder'))
.addClass(config.cls)
.css($.extend({
position:'absolute',
display: 'inline',
float:'none',
overflow:'hidden',
textAlign: 'left',
color: config.color,
cursor: 'text',
paddingTop: $this.css('padding-top'),
paddingRight: $this.css('padding-right'),
paddingBottom: $this.css('padding-bottom'),
paddingLeft: $this.css('padding-left'),
fontSize: $this.css('font-size'),
fontFamily: $this.css('font-family'),
fontStyle: $this.css('font-style'),
fontWeight: $this.css('font-weight'),
textTransform: $this.css('text-transform'),
backgroundColor: 'transparent',
zIndex: 99
}, possible_line_height))
.css(calcPositionCss(this))
.attr('for', this.id)
.data('target',$this)
.click(function(){
$(this).data('target').focus();
})
.insertBefore(this);
$this
.data('placeholder',ol)
.keypress(function(){
ol.hide();
}).blur(function() {
ol[$this.val().length ? 'hide' : 'show']();
}).triggerHandler('blur');
$(window)
.resize(function() {
var $target = ol.data('target');
ol.css(calcPositionCss($target));
});
}
});
}
});
})(jQuery);
jQuery(document).add(window).bind('ready load click', function() {
if (jQuery.placeholder) {
jQuery.placeholder.shim();
}
});
