How to detect when an iframe has already been loaded

Question

It seems that $('#someIframe').load(function(){...}) won't fire if it is attached after the iframe has finished loading. Is that correct?

What I'd really like is to have a function that is always called once when or after an iframe has loaded. To make this clearer, here are two cases:

• Iframe hasn't loaded yet: run a callback function once it loads.
• Iframe has already loaded: run the callback immediately.

How can I do this?

Answer 1

I've banged my head against a wall until I found out what's happening here.

Background information

• Using .load() isn't possible if the iframe has already been loaded (event will never fire)
• Using .ready() on an iframe element isn't supported (reference) and will call the callback immediately even if the iframe isn't loaded yet
• Using postMessage or a calling a container function on load inside the iframe is only possible when having control over it
• Using $(window).load() on the container would also wait for other assets to load, like images and other iframes. This is not a solution if you want to wait only for a specific iframe
• Checking readyState in Chrome for an alredy fired onload event is meaningless, as Chrome initializes every iframe with an "about:blank" empty page. The readyState of this page may be complete, but it's not the readyState of the page you expect (src attribute).

Solution

The following is necessary:

1. If the iframe is not loaded yet we can observe the .load() event
2. If the iframe has been loaded already we need to check the readyState
3. If the readyState is complete, we can normally assume that the iframe has already been loaded. However, because of the above-named behavior of Chrome we furthermore need to check if it's the readyState of an empty page
4. If so, we need to observe the readyState in an interval to check if the actual document (related to the src attribute) is complete

I've solved this with the following function. It has been (transpiled to ES5) successfully tested in

• Chrome 49
• Safari 5
• Firefox 45
• IE 8, 9, 10, 11
• Edge 24
• iOS 8.0 ("Safari Mobile")
• Android 4.0 ("Browser")

Function taken from jquery.mark

/**  * Will wait for an iframe to be ready  * for DOM manipulation. Just listening for  * the load event will only work if the iframe  * is not already loaded. If so, it is necessary  * to observe the readyState. The issue here is  * that Chrome will initialize iframes with  * "about:blank" and set its readyState to complete.  * So it is furthermore necessary to check if it's  * the readyState of the target document property.  * Errors that may occur when trying to access the iframe  * (Same-Origin-Policy) will be catched and the error  * function will be called.  * @param {jquery} $i - The jQuery iframe element  * @param {function} successFn - The callback on success. Will   * receive the jQuery contents of the iframe as a parameter  * @param {function} errorFn - The callback on error  */ var onIframeReady = function($i, successFn, errorFn) {     try {         const iCon = $i.first()[0].contentWindow,             bl = "about:blank",             compl = "complete";         const callCallback = () => {             try {                 const $con = $i.contents();                 if($con.length === 0) { // https://git.io/vV8yU                     throw new Error("iframe inaccessible");                 }                 successFn($con);             } catch(e) { // accessing contents failed                 errorFn();             }         };         const observeOnload = () => {             $i.on("load.jqueryMark", () => {                 try {                     const src = $i.attr("src").trim(),                         href = iCon.location.href;                     if(href !== bl || src === bl || src === "") {                         $i.off("load.jqueryMark");                         callCallback();                     }                 } catch(e) {                     errorFn();                 }             });         };         if(iCon.document.readyState === compl) {             const src = $i.attr("src").trim(),                 href = iCon.location.href;             if(href === bl && src !== bl && src !== "") {                 observeOnload();             } else {                 callCallback();             }         } else {             observeOnload();         }     } catch(e) { // accessing contentWindow failed         errorFn();     } }; 

Working example

Consisting of two files (index.html and iframe.html): index.html:

<!doctype html> <html> <head>     <meta charset="utf-8">     <title>Parent</title> </head> <body>     <script src="https://code.jquery.com/jquery-1.12.2.min.js"></script>     <script>         $(function() {              /**              * Will wait for an iframe to be ready              * for DOM manipulation. Just listening for              * the load event will only work if the iframe              * is not already loaded. If so, it is necessary              * to observe the readyState. The issue here is              * that Chrome will initialize iframes with              * "about:blank" and set its readyState to complete.              * So it is furthermore necessary to check if it's              * the readyState of the target document property.              * Errors that may occur when trying to access the iframe              * (Same-Origin-Policy) will be catched and the error              * function will be called.              * @param {jquery} $i - The jQuery iframe element              * @param {function} successFn - The callback on success. Will               * receive the jQuery contents of the iframe as a parameter              * @param {function} errorFn - The callback on error              */             var onIframeReady = function($i, successFn, errorFn) {                 try {                     const iCon = $i.first()[0].contentWindow,                         bl = "about:blank",                         compl = "complete";                     const callCallback = () => {                         try {                             const $con = $i.contents();                             if($con.length === 0) { // https://git.io/vV8yU                                 throw new Error("iframe inaccessible");                             }                             successFn($con);                         } catch(e) { // accessing contents failed                             errorFn();                         }                     };                     const observeOnload = () => {                         $i.on("load.jqueryMark", () => {                             try {                                 const src = $i.attr("src").trim(),                                     href = iCon.location.href;                                 if(href !== bl || src === bl || src === "") {                                     $i.off("load.jqueryMark");                                     callCallback();                                 }                             } catch(e) {                                 errorFn();                             }                         });                     };                     if(iCon.document.readyState === compl) {                         const src = $i.attr("src").trim(),                             href = iCon.location.href;                         if(href === bl && src !== bl && src !== "") {                             observeOnload();                         } else {                             callCallback();                         }                     } else {                         observeOnload();                     }                 } catch(e) { // accessing contentWindow failed                     errorFn();                 }             };              var $iframe = $("iframe");             onIframeReady($iframe, function($contents) {                 console.log("Ready to got");                 console.log($contents.find("*"));             }, function() {                 console.log("Can not access iframe");             });         });     </script>     <iframe src="iframe.html"></iframe> </body> </html> 

iframe.html:

<!doctype html> <html> <head>     <meta charset="utf-8">     <title>Child</title> </head> <body>     <p>Lorem ipsum</p> </body> </html> 

You can also change the src attribute inside index.html to e.g. "http://example.com/". Just play around with it.