MKOverlayRenderer.setNeedsDisplay Causes MKTileOverlayRenderer To Re-Render Tiles

Bug Report ID

FB9957545

Note: this is still broken as of iOS 15 (Xcode 13.3).

Notes

This demo app exposes a MapKit bug that re-renders MKTileOverlayRenderer tiles when:

  • another MKOverlayRenderer calls any of the setNeedsDisplay methods.
  • an overlay is added to the map (e.g. simple MKPolyline + MKPolylineRenderer).

Here are the main players in this demo:

  • MapViewController:
    • Basic MKMapView host view controller and MKMapDelegate.
    • Contains two toolbar buttons to add and remove a GridOverlay.
  • A GridOverlay renders using a DelayedGridOverlayRenderer.
  • A DelayedGridOverlayRenderer is a simple MKOverlayRenderer that calls the setNeedsDisplay function “later”.
    • Simulates asynchronously generating/ loading tiles (but for simplicity just renders a box around the tile).

Feedback ID: FB9957545

Demo showing the “flickering” bug

Bug

func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
    switch overlay {
    case let tileOverlay as MKTileOverlay:
        // Note: Replace the `MKTileOverlayRenderer` with the `HackTileOverlayRenderer` to stop re-drawing madness.
        return MKTileOverlayRenderer(tileOverlay: tileOverlay)
    case is GridOverlay:
        return DelayedGridOverlayRenderer()
    default:
        return MKOverlayRenderer(overlay: overlay)
    }
}

Demo that includes a hack to eliminate the “flickering” bug

Fixed

func mapView(_ mapView: MKMapView, rendererFor overlay: MKOverlay) -> MKOverlayRenderer {
    switch overlay {
    case let tileOverlay as MKTileOverlay:
        // Note: Replace the `HackTileOverlayRenderer` with the `MKTileOverlayRenderer` to expose re-drawing madness.
        return HackTileOverlayRenderer(tileOverlay: tileOverlay)
    case is GridOverlay:
        return DelayedGridOverlayRenderer()
    default:
        return MKOverlayRenderer(overlay: overlay)
    }
}

Possible workaround (aka super-duper hack)

It turns out that simply subclassing MKTileOverlayRenderer, overriding draw(_:zoomScale:in), and calling super resolves the re-drawing problem. I can’t explain why. Possible ObjC runtime shenanigans?

Here’s the full hack-workaround.

import MapKit

final class HackTileOverlayRenderer: MKTileOverlayRenderer {

    override func draw(_ mapRect: MKMapRect, zoomScale: MKZoomScale, in context: CGContext) {
        super.draw(mapRect, zoomScale: zoomScale, in: context)
    }
}

Other

This sample demo uses OpenStreetMap tiles to help demonstrate the MapKit bug. https://www.openstreetmap.org/copyright

GitHub

View Github