problem with pageinit

problem with pageinit

  1. <!DOCTYPE html>
    <html>
    <head>
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <title>a</title>
       
        <link rel="stylesheet" href="http://code.jquery.com/mobile/1.2.0/jquery.mobile-1.2.0.min.css" />
        <script src="http://code.jquery.com/jquery-1.8.2.min.js"></script>
       
        <script>
        $(document).bind('mobileinit',function(){
            $('#a').live('pageinit',function(){
                alert($('#a-header').height());
            });
        });
        </script>
       
        <script src="http://code.jquery.com/mobile/1.2.0/jquery.mobile-1.2.0.min.js"></script>
    </head>
    <body>
    <div data-role="page" id="a">
        <div data-role="header" id="a-header">
            <h1>a</h1>
        </div><!-- /header -->
        <div data-role="content">
            a
        </div><!-- /content -->

        <div data-role="footer">
            a
        </div><!-- /jqm-footer -->
    </div><!-- /page -->
    </body>
    </html>


































why the result is 0???
please help me ,thanks.