How to add a spinner icon to button when it's in the Loading state?

Simple solution for Bootstrap 3 using CSS3 animations.

Put the following in your CSS:

.glyphicon.spinning {
    animation: spin 1s infinite linear;
    -webkit-animation: spin2 1s infinite linear;
}

@keyframes spin {
    from { transform: scale(1) rotate(0deg); }
    to { transform: scale(1) rotate(360deg); }
}

@-webkit-keyframes spin2 {
    from { -webkit-transform: rotate(0deg); }
    to { -webkit-transform: rotate(360deg); }
}

Then just add the spinning class to a glyphicon while loading to get your spinning icon:

<button class="btn btn-lg btn-warning">
    <span class="glyphicon glyphicon-refresh spinning"></span> Loading...    
</button>

Based on http://www.bootply.com/128062#

• Note: IE9 and below do not support CSS3 animations.

If you look at the bootstrap-button.js source, you'll see that the bootstrap plugin replaces the buttons inner html with whatever is in data-loading-text when calling $(myElem).button('loading').

For your case, I think you should just be able to do this:

<button type="button"
        class="btn btn-primary start"
        id="btnStartUploads"
        data-loading-text="<i class='icon-spinner icon-spin icon-large'></i> @Localization.Uploading">
    <i class="icon-upload icon-large"></i>
    <span>@Localization.StartUpload</span>
</button>

There's now a full-fledged plugin for that:

http://msurguy.github.io/ladda-bootstrap/