allow iOS per-frame callback

[tomtaylor]

I'd like to implement a fixed scale overlay view (a CAShapeLayer) on top of a MGLMapView. One that tracks the location of the underlying coordinates and positions itself accordingly. Specifically, I want to draw a line at a fixed screen distance away from and alongside a polyline annotation.

The region change delegate methods let me do this, but they don't fire frequently enough, so the layer jitters as the map scrolls. Looking through the code I can see glkView:drawInRect:, which is used to update the user location annotation view.

Have you considered exposing an API to let me implement something similar myself? I'd consider a MGLMapView subclass, but there isn't a convenient hook into that that point in the drawInRect method.

[incanus]

Related: #1125

This would basically need to happen right about here in the process, between drawing the GL frame and updating the Transform, which affects pixel/geo conversions thereafter.

[incanus]

Also would solve #2045.

[1ec5]

A combination of -mapViewRegionIsChanging: and -mapView:regionDidChangeAnimated: should actually keep your view in sync with the map; if not, that’s a bug. Otherwise, you certainly could subclass MGLMapView and override -glkView:drawInRect:, but make sure to call super’s implementation in that case. (You’ll probably need to declare that your subclass conforms to GLKViewDelegate, since MGLMapView does so privately.)

[1ec5]

@tomtaylor, -[MGLMapViewDelegate mapViewDidFinishRenderingFrame:fullyRendered:] should fire frequently enough for your use case, far more reliably than -mapViewRegionIsChanging: and -mapView:regionDidChangeAnimated:. The method isn’t documented because the fullyRendered parameter doesn’t work as expected yet: #1804. As of #4801, we’re using this callback to update MGLAnnotationViews’ positions on screen as the viewport changes.