Exposing MGLGeometry_Private.h C++ globals

[ambientlight]

Hi guys,

It would be great to have tile geometry available to a client for feature reconstitution discussed in #9647, so I have exposed tile geometry with:

/* 
 Returns z/x/y tile id given coordinate belongs to.
 */
MGLTileID MGLTileIDContainingCoordinate2D(CLLocationCoordinate2D coordinate, double zoomLevel){
    
    mbgl::LatLng latLng = { coordinate.latitude, coordinate.longitude };
    const double scale = std::pow(2.0, zoomLevel);
    const mbgl::TileCoordinatePoint point = mbgl::Projection::project(latLng, scale) / double(mbgl::util::tileSize);
    return { uint8_t(zoomLevel), uint32_t(point.x), uint32_t(point.y) };
}

/*
 Returns the coordinate bounds corresponding to z/x/y tile id
 */
MGLCoordinateBounds MGLCoordinateBoundsForTileId(MGLTileID tileID){
    
    const mbgl::TileCoordinatePoint point = { double(tileID.x), double(tileID.y) };
    const mbgl::TileCoordinatePoint nextPoint = { double(tileID.x + 1), double(tileID.y + 1) };
    const double scale = std::pow(2.0, tileID.z);
    
    const mbgl::LatLng nw = mbgl::Projection::unproject(point * double(mbgl::util::tileSize), scale);
    const mbgl::LatLng se = mbgl::Projection::unproject(nextPoint * double(mbgl::util::tileSize), scale);
    
    return MGLCoordinateBoundsMake(CLLocationCoordinate2DMake(se.latitude(), nw.longitude()),
                                   CLLocationCoordinate2DMake(nw.latitude(), se.longitude()));
}

in MGLGeometry_Private.h, while MGLTileID is defined as:

/**
 Identifies z/x/y tile.
 */
typedef struct __attribute__((objc_boxable)) MGLTileID {
    uint8_t z;
    uint32_t x;
    uint32_t y;
} MGLTileID;

It does work for me when I wrap these functions inside Objective-C++ methods, however I am hitting linker failures when I try exposing them directly as

/** Returns z/x/y tile id given coordinate belongs to. */
MGLTileID MGLTileIDContainingCoordinate2D(CLLocationCoordinate2D coordinate, double zoomLevel);

/** Returns the coordinate bounds corresponding to z/x/y tile id. */
MGLCoordinateBounds MGLCoordinateBoundsForTileId(MGLTileID tileID);

in MGLGeometry.h.
(ref: GeoThings@2d6d23b)

It feels like I am missing something obvious that I don't know here, any thoughts guys?

[1ec5]

We have no plans to expose any C++ interfaces as part of the iOS or macOS SDK’s public API, because it would break applications written in pure Objective-C or Swift. That said, if there are opportunities to implement these conversions independently in Objective-C, there might be a viable path to adding such an implementation to the public API.

[ambientlight]

Yes that is the approach(tile covertion done seperately at my swift/js client side) I actually followed in the end.

[ambientlight]

But if it will be available in SDK out of the box that would be certainly helpful.

[ambientlight]

@1ec5: does it make sense for me to go with a pull request to js/ios sdks?

[1ec5]

Yes, we’d be open to a PR for the iOS and macOS SDKs at least. MGLComputedShapeSource (#6940 #9983) introduced x/y/z tile coordinates as a concept. For that feature, we ultimately went with a series of method parameters (e.g., -invalidateTileAtX:y:zoomLevel:) instead of a struct. But now that we have APIs that deal in x/y/z and other APIs that deal in coordinate bounds, it does make sense to expose conversion functions publicly.

We have no plans to expose any C++ interfaces as part of the iOS or macOS SDK’s public API, because it would break applications written in pure Objective-C or Swift.

On second thought, C++ is only a concern because these methods are currently inlined. If we move the implementations of these methods to a .mm file – probably a good idea anyways – then they’d be perfectly compatible with pure Objective-C.