Mapbox-gl-native: [core] Add callback for collision detection

[julianrex]

It would be useful to have a callback when collision detection has run. This could be a new RendererObserver function or an additional parameter to onDidFinishRenderingFrame (proof of concept by @pozdnyakov here: https://github.com/mapbox/mapbox-gl-native/pull/15440).

This can be useful after camera changes, when the developer then wants to query features (e.g. annotations) in the camera change completion callback/block (for example see @astojilj's comment here).

/cc @tobrun

[asheemmamoowala]

This can be useful after camera changes, when the developer then wants to query features

How does a a developer know when to and when not to depend on this new callback? If a style has no symbol layer, it would never invoke this callback. It makes it hard to write generic code to do this after a camera change is complete because it requires knowing if the style currently loaded has symbol layers

[julianrex]

This is a good point - I was thinking that we would need a separate iOS delegate method anyway (referencing "collision detection" isn't particularly user friendly), so that logic could potentially check for symbol layers. Do you think that would be sufficient @asheemmamoowala @pozdnyakov?

[pozdnyakov]

referencing "collision detection" isn't particularly user friendly

right, we probably should not expose "placement update" status but rather use it as an internal flag in the SDK implementation.

[johnnewman]

This would be a great feature for my use case on iOS. I'm drawing features in a symbol layer that all have iconAllowsOverlap set to false. I also have a UI component on screen that corresponds to each of the visible features in that style layer. To draw this custom component, I need to know which features are going to be hidden by the SDK's collision detection and which will be visible.

The delegate mapViewDidBecomeIdle: is reliable, but its execution can be delayed. Since there isn't a fast callback for this case, my solution was to create a timer that executes visibleFeaturesInRect:inStyleLayersWithIdentifiers: a few times every second after adding the style layer.

This solution isn't super clean. A callback would make things much better.

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