javascript - Detecting when an iframe gets or loses focus

Question

What's the correct way of detecting when an iframe gets or loses focus (i.e. will or will not receive keyboard events)? The following is not working in Fx4:

var iframe = /* my iframe */;
iframe.addEventListener("focus", function() { /* never gets called */ }, false);

Answer 1

You can poll "document.activeElement" to determine if it matches the iframe. Polling isn't ideal, but it works:

function checkFocus() {
  if(document.activeElement == document.getElementsByTagName("iframe")[0]) {
    console.log('iframe has focus');
  } else {
    console.log('iframe not focused');
  }
}

window.setInterval(checkFocus, 1000);