1238999 - Add disableEventListener() and enableEventListener() to EventListenerService.cpp

Status: RESOLVED DUPLICATE of bug 1559813

(DevTools :: Inspector, enhancement, P3)

Description

[Mike Ratcliffe [:miker] [:mratcliffe] [:mikeratcliffe]]

It would be awesome if developer tools had a way to toggle events on and off and adding these two methods would make this possible.

Comment 1

[Olli Pettay [:smaug][[e-mail]]]

It is not clear to me what this is about.
Is this about disabling some event listener temporarily or disabling handling certain event type temporarily or what?

Comment 2

[Olli Pettay [:smaug][[e-mail]]]

Also, whatever this is about, would this be global or just some particular event target?

Comment 3

[Mike Ratcliffe [:miker] [:mratcliffe] [:mikeratcliffe]]

Attachment: Screen Shot 2016-01-13 at 10.39.31.PNG

See the attached image to get some idea of the UX:

In the Inspector we show each node and if there are any events attached to that node we show a small button labelled "ev".

Clicking on "ev" shows a popup containing an accordian style list of events attached to that node. The pause button opens that event's handler in the debugger.

We would like to add a button to the popup to toggle the specific event disabled / enabled.

I should have been clearer about the API:
disableEventListener(node, "click", onButtonClicked, true);
enableEventListener(node, "click", onButtonClicked, true);

When the event is disabled it should still be obtainable using getListenerInfoFor() etc. but it should not be triggered.

We considered just removing the event and adding it again but that would result in events being triggered out of sequence.

Comment 4

[Olli Pettay [:smaug][[e-mail]]]

(In reply to Michael Ratcliffe [:miker] [:mratcliffe] from comment #3)
> In the Inspector we show each node and if there are any events attached to
> that node we show a small button labelled "ev".
"events attached" == "event listeners added", right?

> We would like to add a button to the popup to toggle the specific event
> disabled / enabled.
So "event disabled" means the event shouldn't fire at all?
But I think you're talking about event listeners being disabled.

> I should have been clearer about the API:
> disableEventListener(node, "click", onButtonClicked, true);
> enableEventListener(node, "click", onButtonClicked, true);
ok, so some particular event listener for some event target could be disabled.


> We considered just removing the event and adding it again but that would
> result in events being triggered out of sequence.
Right.

So, 
EventListenerManager::Listener struct could have a bool flag to tell whether it is enabled or disabled
and EventListenerManager::Listener::IsListening would just check that and if disabled, return false.

Comment 5

[Mike Ratcliffe [:miker] [:mratcliffe] [:mikeratcliffe]]

We could use the Debugger API to set a hidden (to the user) breakpoint on the first bytecode of the listener, and when the breakpoint hits force an early return rather than executing the body.

This would not require any additional platform work and builds on the debugging primitives we already use.

@jlast Is this something that would be simple to implement? The UI is simple but I am not sure how large a patch this would be on the debugger side.

Comment 6

[Jason Laster [:jlast]]

I think that would be possible. I think Logan's new event system might be a better fit as we can subscribe to specific events i.e. "click" and not just byte offsets. This way we can pause when there is a click event that calls the listener we care about.

Comment 7

[Logan Smyth [:loganfsmyth]]

The downside of the breakpoint approach is that a function can be an event handler on multiple nodes, like

function doThing() {

}
node.addEventListener("click", doThing);

otherNode.addEventListener("click", doThing);

and I wouldn't want disabling the node event listener to necessarily also cancel the listener for otherNode. You presumably want to cancel a given triplet of (Node, EventType, Function) and maybe even taking the event-listener options into account? (passive, capture, etc)

The new logic in https://bugzilla.mozilla.org/show_bug.cgi?id=1529345 should work great for this. It notifies you just before and just after Gecko calls an event handler, and it gives you the Event object, so you can check the event's currentTarget and type, and if it is a partial match for the triplet, you can set an onEnterFrame handler to run code when the listener runs. Then on enter, you could look to see what function you entered, and if it is the one you want to cancel, stop execution of the function immediately.