Detect when an iframe starts to load new URL

I found a better solution if iframe and the container page is same origin, don’t have to put extra code into the inner page:

<iframe src="https://stackoverflow.com/questions/17315013/same-origin.com" onload="content_finished_loading(this)"></iframe>
<script>
    var indicator = document.querySelector('.loading-indicator');
    var content_start_loading = function() {
        indicator.style.display = 'block';
    };

    var content_finished_loading = function(iframe) {
        indicator.style.display = 'none';
        // inject the start loading handler when content finished loading
        iframe.contentWindow.onunload = content_start_loading;
    };
</script>