How does a Panel's DefaultButton property work?

Question

What is happening when you declare a Button to be used as the DefaultButton in an ASP.NET Panel? I understand that ASP.NET will render the contents of the Panel to a div and pass a bunch of stuff to the ViewState. Is there JavaScript inside the ViewState that handles the rendered Button's click event? I thought ViewState was just that - info about state. How does it work?

Answer 1

You're right about the ViewState. It's designed to keep Page and Controls values. That is, their state. You can confirm it here.

About the default button, there is no magic. A javascript is added to the div in order to bind the ENTER key event.

Let's check it out! This code:

<asp:Panel ID="panel" runat="server" DefaultButton="button">
    <asp:Button ID="button" runat="server" Text="this is the button" />
</asp:Panel>

Is rendered to this:

<div id="panel" onkeypress="javascript:return WebForm_FireDefaultButton(event, 'button')">
  <input type="submit" name="button" value="this is the button" id="button">          
</div>

This javascript is generated by the WebForms engine, but we can look for it, if you're curious:

function WebForm_FireDefaultButton(event, target) {
    if (event.keyCode == 13) {
        var src = event.srcElement || event.target;
        if (!src || (src.tagName.toLowerCase() != "textarea")) {
            var defaultButton;
            if (__nonMSDOMBrowser) {
               defaultButton = document.getElementById(target);
            }
            else {
                defaultButton = document.all[target];
            }
            if (defaultButton && typeof(defaultButton.click) != "undefined") {
                defaultButton.click();
                event.cancelBubble = true;
                if (event.stopPropagation) event.stopPropagation();
                return false;
            }
        }
    }
    return true;
}

Notice how it tests if the currently focused control is a textarea. This is because an ENTER inside a textarea is mostly a new line, not a submit.