Weak event listener

Erik Arvidsson erik.arvidsson at gmail.com
Mon Mar 25 19:29:49 PDT 2013

WeakMap would not work in this specific case since a WeakMap cannot be

WeakRefs are needed to do data binding and pub sub systems like these.
WeakRefs are very likely to be part of ES7. We haven't really talked much
about them formally but informally TC39 members seems to agree.


On Mon, Mar 25, 2013 at 9:28 AM, Kevin Gadd <kevin.gadd at gmail.com> wrote:

> Most use cases I've personally seen for weak events would be satisfied by
> a WeakMap of weak event target -> event handler. Having the event handler
> itself be weak instead of the event target seems strange to me; it
> introduces a bunch of nondeterministic behavior that isn't desirable -
> sometimes you'll free all your references to a handler and it'll fire
> anyway, because it hasn't been collected. It also introduces the potential
> for accidentally freeing all your references to a handler and having
> important behavior not run. If you get proper WRs this should be pretty
> easy to build if you want it.
> I think the fact that this would make GC behavior observable through the
> execution/non-execution of event handlers probably makes it some sort of
> security hole in the same way that WRs are, but I'm not sure - I've never
> fully understood the threat posed by that information leak.
> -kg
> On Mon, Mar 25, 2013 at 2:55 AM, Marius Gundersen <gundersen at gmail.com>wrote:
>> I tried posting this before, but it seems to have gotten lost on the
>> internet.
>> One thing which is impossible to make in JavaScript today is a weakly
>> referenced event listener system. In such a system an event listener is not
>> strongly referenced by the event system, so events are only dispatched to
>> it as long as another object holds a reference to it. For example:
>> //some weak event system, could be global or created with constructor
>> var events = new WeakEventSystem();
>> (function(){
>>     //This object only exists inside this IIFE, not outside it
>>     var myObject = {
>>         handler: function(event){
>>             console.log("handle event: " + event);
>>         }
>>     };
>>     //add a listener to events, but events only has a weak reference to it
>>     events.addEventListener("someEvent", myObject.handler);
>>     //the following causes the handler above to be called, resulting in
>> output on the console
>>     events.dispatchEvent("someEvent", {params:"someParams"});
>> })();
>> //myObject does not exist anymore
>> //the following does not output anything on the console
>> events.dispatchEvent("someEvent", {params:"someParams"});
>> In this example the event system does not hold a strong reference to the
>> listener/handler function, so if nothing else has a reference to the
>> handler function, it is garbage collected.
>> Any thoughts on this, as an alternative to the more general (and low
>> level) weak references?
>> Marius Gundersen
>> _______________________________________________
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>> https://mail.mozilla.org/listinfo/es-discuss
> --
> -kg
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