Lags when adding new annotations backed by MLNAnnotationView

[kryzhikh]

MapLibre iOS Version

6.18.1

iOS Version

18.3.1

Device

iPhone 16 pro

What happened?

In my project I use view annotations and have a logic where new annotations are being added as the camera moves along the map. When there are already quite a few annotations on the map, adding new ones produces significant UI lags. The more annotations are added, the heavier the lags are.

Also, while trying to research the issue, I've noticed, that when I remove out of screen annotations, they are actually not being removed from annotationContainerView, but seem to be kept for reusing. Even if I try to remove all the annotations from the map, seems like only the visible annotation are actually being removed from annotationContainerView, while non of them are needed any more.

Steps to reproduce

Preconditions:

1. mapView(_: MLNMapView, viewFor: any MLNAnnotation) -> MLNAnnotationView? that returns MLNAnnotationView for any added annotation. Same reuseIdentifier is user for all annotation views, dequeueReusableAnnotationView is also used prior to new view creation.

Steps to reproduce UI lags

1. Implement mapViewRegionIsChanging(_: MLNMapView) method of MLNMapViewDelegate with adding new annotations as the camera is moving (mapView.addAnnotations())
2. move the camera along the map making new annotations to be added
3. the more annotations are added, the more laggy is the movement.

Steps to reproduce annotation view is not actually deleted

1. add 3000 annotations on the map
2. zoom in so that only some of the annotations remain visible
3. delete all the added annotations from the map (mapView.removeAnnotations(annotations))
4. check annotationContainerView subviews count - there are still a lot of them

Relevant log output

Additional context

That doesn't seem correct, when all the annotations are being removed from the map some of them (in some cases all of them) are actually kept as subviews of annotationContainerView.

Also analising MLNMapView.mm code I've notices some points:

1. there is updateAnnotationContainerViewWithAnnotationViews method that seems to be called to add new annotation views on the map, and it recreates annotationContainerView on each call. I'm not sure why is that needed, but probably this might be a reason of lags when adding new annotations
2. according to removeAnnotations method implementation seems like it should delete annotation view from annotationContainerView and from reuse queue no matter if it is visible or not, but for some reason that does not happen for out of screen annotations. I've noticed the following code in removeAnnotations which seems to be a bug and might be the reason of the issue:

if (![annotationViewReuseQueue containsObject:annotationView])
{
    [annotationViewReuseQueue removeObject:annotationView];
}

Screencast for deleting all annotations at the moment when only some of them are visible

Screencast for laggy map panning while adding new annotations

(The label on the bottom depicts the number of annotationContainerView subviews got as mapView.subviews.first(where: { $0 is MTKView })?.subviews.first)