jquery uploadify, if empty

is imposible with jquery uploadify check if empty, when i push upload button? And if empty 开发者_如何学Pythonshow error.

Thanks

EDIT: When user push button "Upload Files" without marking anything file system must show errors (you must select, or etc.)

If you don't understand my problem please look this: http://img696.imageshack.us/img696/8854/erroruploadery.png

Edited (x2):

Something like this:

<script type="text/javascript">
var queueSize = 0;
function startUpload(){
    if (queueSize == 0) {
        alert("Please select a file first.");
    } else {
        $("#fileUpload").fileUploadStart();
    }
}
</script>
<script type="text/javascript">
$("#fileUpload").fileUpload({
    onCancel: function (a, b, c, d) {
        queueSize = d.fileCount;
    },
    onClearQueue: function (a, b) {
        queueSize = b.fileCount;
    },
    onSelectOnce: function (a, b) {
        queueSize = b.fileCount;
    },
    onAllComplete: function () {      
        queueSize = 0;
    },
    onComplete: function (a, b ,c, d, e) {
        queueSize = 0;
    },
});
</script>

Which you would call with:

startUpload();

I did this in much simpler way. Add a div and associate it with uploadify 'queueId'

<asp:FileUpload name="fuFiles" ID="fuFiles" runat="server" />
                        <div id="fuItemsQueue" class="fuItemsQueue">
                        </div>

$("#<%=fuFiles.ClientID %>").uploadify({

                'swf': '../Scripts/uploadify.swf',
                'queueID': 'fuItemsQueue',
.....
            });

Just on the OnCancel Event just count the number of Divs left.

'onCancel': function () {

                    var queueCount = $("#fuItemsQueue > div").size();

                    if (queueCount = 1) {
                        DisableUploadButtons();
                    }
                }

And done!! :)