I found a strange behavior of Sivlerlight plugin on browsers besides IE. 

Check the blowing sample.  There are a Silverlight application(XMLViewer.xap) inside a DIV which named dvSL and two buttons, btnShow and btnHide, with onclick event to set getElementById("dvSL").style.display to "" or "none" to change visibility of dvSL.  You can try the live demo.

<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>Load Event Test</title>
    <script type="text/javascript">
        function onSilverlightLoad(s, e) {
            alert("Silverlight Loaded!");
        }
        function show() { document.getElementById("dvSL").style.display = ""; }
        function hide() { document.getElementById("dvSL").style.display = "none"; }
    </script>
</head>
<body>
    <form id="form1" runat="server" style="height:100%">
    <input type="button" id="btnShow" value="Show" onclick="show();" />
    <input type="button" id="btnHide" value="Hide" onclick="hide();" />
    <div id="dvSL">
        <object data="data:application/x-silverlight-2," 
                type="application/x-silverlight-2" width="200" height="200">
          <param name="source" value="SL/XMLViewer.xap"/>
          <param name="onLoad" value="onSilverlightLoad" />
          <param name="background" value="white" />
          <param name="minRuntimeVersion" value="4.0.50401.0" />
          <param name="autoUpgrade" value="true" />
          <a href="http://go.microsoft.com/fwlink/?LinkID=149156&v=4.0.50401.0" 
             style="text-decoration:none">
               <img src="http://go.microsoft.com/fwlink/?LinkId=161376" 
              alt="Get Microsoft Silverlight" style="border-style:none"/>
          </a>
        </object>
        <iframe id="_sl_historyFrame" 
        style="visibility:hidden;height:0px;width:0px;border:0px">
        </iframe>
    </div>
    </form>
</body>
</html>

According to my expection, the visibility change only affect the "visual visibility" of elements, including Siverlight object in the page, but not the "residency in memory" of elements. 

In IE8, it's true! No matter how many times you press btnHide and btnShow, you will get only once "Silverlight Loaded!" popup.  But in browsers besides IE, each time you press btnShow to make Sivlerlight object visible again, you get one "Silverlight Loaded!" popup. I have tested this on Chrom, Firefox, Safari and Opera, the Silverlight seems reloaded each time when its container becomes visible from hidden status.  So, remember to check your logic when you try to toggle the visibility of Silverlight host.

PS: I found a related post in Silverlight forum and found "window.focus()" workaround, but it failed in my test. (if I realized it correctly)

function hide() { window.focus(); document.getElementById("dvSL").style.display = "none"; }

【中文摘要】

發現當網頁上的Silveright由隱藏切換為顯示時,在IE以外的瀏覽器(Chrome, Firefox, Safari, Opera)上會觸發重新載入,連帶地Silverlight的OnLoad事件也會被反覆觸發,應用時請小心。這裡有Demo給大家實驗看看。Silverlight論壇上有篇相關文章 ,提到用window.focus()的做法避開,但在我的測試中並未成功。


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