Fire events on cooperative gesture triggers

[tomhicks]

User Story

As a developer embedding a MapLibre map with cooperative gestures, I want to receive events when the cooperative gestures are detected and triggered, so that I can put up my own UI instead of hacking the internal one.

Rationale

We want a custom cooperative gesture UI, which I have managed to get pretty much working by overriding the styles globally, but it's starting to get a bit fiddly. For instance, I need to separate the scrim from the message to be able to layer the scrim behind some UI but have the message above it. I can work around this by putting mutation observers on the DOM elements that are modified as the gestures are triggered, but that's a bit flaky.

If we were able to register for an event like "cooperativegestureblocked" or something we would be able to more easily add our own custom UI.

Impact

Makes it harder to implement custom UIs for cooperative gestures.

PR

I'm very happy to make a PR for this, if someone could just point me to the right way to get the event emitter instance into the handler (or to import a singleton).

[HarelM]

I'm not sure how am event will solve your problem, but the code is mainly here:
https://github.com/maplibre/maplibre-gl-js/blob/c0f1936e39528d4b9e3a91463c20cba7c3649a6b/src/ui/handler/cooperative_gestures.ts
I would consider copying the code, modify it and replacing the existing instance:

maplibre-gl-js/src/ui/map.ts

Line 543 in c0f1936

cooperativeGestures: CooperativeGesturesHandler;

[tomhicks]

Ah I think what I need is this._map.fire and then some additional types or whatever to allow that to compile.

Then we should be able to do map.on("cooperativegestureprevented", handler)