fix() in event propagation is slow

[olau]

The fix() function called when an event is propagated deep copies the event to be able to set a few attributes. Unfortunately, the event object is quite large so this is really expensive for events like mousemove. On my 950 MHz PIII wiggling the mouse when an empty callback is set with mousemove() uses up 100% of the CPU time in Firefox.

There's a longer discussion on the mailing list here: ​http://groups.google.com/group/jquery-dev/browse_thread/thread/cc768c85d5514098

I've tried shallow copying the event with "for (v in originalEvent) event[v] = originalEvent[v];", but it's still slow.

The only solution I can think of is to copy a couple of documented standard values from the event object, normalizing them as needed, and then let people access the browser event object with an originalEvent field. If you can tell me which values to copy, I'd be happy to supply a patch.

[davidserduke]

possible patch

[davidserduke]

This patch uses Ariel Flesler's idea (posted in the above jquery-dev thread noted above) of prototype inheritance to clone the event node when possible instead of using extend(). It falls back to extend() when/if it fails like it seemed to do in my IE7 tests.

There are all sorts of possible side effects but it definitely sped up the event code during mousemove in my limited tests.

[davidserduke]

used with FireFox and FireBug profiling to test mousemove event speed

[davidserduke]

Unfortunately metaKey will be changed in the event when the ctrl key is held down. So this fix to speed can cause a JS error in that case.

[nostrademons]

If the issue is read-only properties, how about assigning the new custom properties *first* and then setting the new event's prototype afterwards. The new event is just a normal empty JavaScript object, then, so it shouldn't run into any issues with read-only errors on assignment. Then you could set the prototype chain as the last operation in the function, which'll make sure that the returned object returns the original object's values for anything that hasn't been fixed up. It seems like this will work on any browser that conforms to the ECMAScript standard for assignment & prototype access.

[brandon]

This should be addressed in Rev. [5344].