Detect when an iframe starts to load new URL

Solution 1:

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="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>

Solution 2:

I came up with following solution - which is only possible because we control the content of the iframe content and the host-window

Inside the iframe we add following script to the page footer (all pages use the same template, so this is a change to a single file)

<script>
window.onunload = function() {
    // Notify top window of the unload event
    window.top.postMessage('iframe_change', '*');
};
</script>

Inside the host-window we add this script to monitor the iframe state

function init_content_monitor() {
    var content = jQuery('.iframe');

    // The user did navigate away from the currently displayed iframe page. Show an animation
    var content_start_loading = function() {
        alert ('NOW: show the animation');
    }

    // the iframe is done loading a new page. Hide the animation again
    var content_finished_loading = function() {
        alert ('DONE: hide the animation');
    }

    // Listen to messages sent from the content iframe
    var receiveMessage = function receiveMessage(e){
        var url = window.location.href,
            url_parts = url.split("/"),
            allowed = url_parts[0] + "//" + url_parts[2];

        // Only react to messages from same domain as current document
        if (e.origin !== allowed) return;
        // Handle the message
        switch (e.data) {
            case 'iframe_change': content_start_loading(); break;
        }
    };
    window.addEventListener("message", receiveMessage, false);

    // This will be triggered when the iframe is completely loaded
    content.on('load', content_finished_loading);
}