Also relevant in Firefox for Android ----- Original Message -----
From: "Gavin Sharp" <[email protected]> To: "Firefox Dev" <[email protected]> Cc: "Justin Lebar" <[email protected]> Sent: Tuesday, July 16, 2013 2:52:47 PM Subject: Fwd: Be careful not to leak with event listeners This is good advice for front-end developers in general. Gavin ---------- Forwarded message ---------- From: Justin Lebar <[email protected]> Date: Tue, Jul 16, 2013 at 9:48 AM Subject: Be careful not to leak with event listeners To: "<[email protected]>" <[email protected]> Cc: Kyle Huey <[email protected]>, Andrew McCreight <[email protected]> Dear all, We've been working on some B2G memory leaks lately, and so many of the ones we found have had to do with event listeners, I thought it warranted an e-mail to the list. The essential problem is that an event listener registered on DOM node N lives for as long as N lives. If N is long-lived (e.g. it's the window of the system app), then the event listener will also live for a long time. A closure keeps alive everything it can possibly touch. So if you touch e.g. a DOM node from within an event listener, that node and all of its ancestors and descendents will stay alive at least until the event listener dies. For example, this would cause |iframe| to stay alive for as long as |window| lives, even if you remove all other pointers to |iframe| var iframe = ...; window.addEventListener('mozbrowserfoo', function(e) { iframe.setVisible(false); }); because the mozbrowserfoo event listener lives as long as the window lives, and keeps the iframe alive. It's particularly bad to leak an <iframe mozbrowser> because we then leak all the mozbrowser machinery for that iframe. A solution in this case might be to register the event listener on |iframe| itself, since then the event listener lives only as long as the iframe lives, and therefore doesn't extend the iframe's lifetime. Alternatively, you could pull |iframe| off e.target. One of the bugs we fixed did this: var homescreenIframe = ...; window.addEventListener('something', function(e) { document.doSomething(); }); This leaked homescreenIframe, even though the closure did not touch homescreenIframe. The work-around in this case was to pull |document off e.target.ownerDocument. We believe this may be a bug in the JS engine (bug 894149), and we're working on a fix. For now, please be cautious of touching |document| or |window| from within an event listener: You may end up keeping many things alive that you don't mean to. If you have any questions or doubts, feel free to ask me, khuey, or mccr8. This stuff very tricky to do right, and unfortunately these leaks are difficult to notice and fix, so a bit of prevention may go a long way. Regards, -Justin https://bugzilla.mozilla.org/show_bug.cgi?id=894135 https://bugzilla.mozilla.org/show_bug.cgi?id=894147 https://bugzilla.mozilla.org/show_bug.cgi?id=894081 _______________________________________________ dev-gaia mailing list [email protected] https://lists.mozilla.org/listinfo/dev-gaia _______________________________________________ firefox-dev mailing list [email protected] https://mail.mozilla.org/listinfo/firefox-dev
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