MKPolygon using Swift (Missing argument for parameter 'interiorPolygons' in call)

Question

Fellow Devs, I'm trying to implement a polygon overlay on a mapview as follows:

private func drawOverlayForObject(object: MyStruct) {
    if let coordinates: [CLLocationCoordinate2D] = object.geometry?.coordinates {
        let polygon = MKPolygon(coordinates: coordinates, count: coordinates.count)
        self.mapView.addOverlay(polygon)
    }
}

The following error is presented:

Missing argument for parameter 'interiorPolygons' in call

According to the documentation: Apple Docu:

Mutable Pointers

When a function is declared as taking an UnsafeMutablePointer argument, it can accept any of the following:

nil, which is passed as a null pointer

An UnsafeMutablePointer value

An in-out expression whose operand is a stored lvalue of type Type, which is passed as the address of the lvalue

An in-out [Type] value, which is passed as a pointer to the start of the array, and lifetime-extended for the duration of the call

Now I think that my approach then would be correct, providing a [CLLocationCoordinate2D] array. Did anyone experience the same problem and found a workaround?

thanks Ronny

Mike S · Accepted Answer · 2014-09-24T03:05:52.783

The error you're getting is Swift's cryptic way of saying that it can't find a method which matches your parameters. If you did try passing the interiorPolygons parameter, you'd get an equally confusing:

Extra argument 'interiorPolygons' in call

Your code is pretty close though; you just need a couple of minor changes. In the doc you reference, it says one of the things you can pass is:

An in-out [Type] value, which is passed as a pointer to the start of the array, and lifetime-extended for the duration of the call

So, it's looking for an in-out parameter. Which is done by passing coordinates prefixed with an &, like so:

MKPolygon(coordinates: &coordinates, count: coordinates.count)

But, in-out parameters can't be constants. From the docs:

You can only pass a variable as the argument for an in-out parameter. You cannot pass a constant or a literal value as the argument, because constants and literals cannot be modified.

So, you need to define coordinates with a var first:

if var coordinates: [CLLocationCoordinate2D] = object.geometry?.coordinates

Which makes the entire function look like this:

private func drawOverlayForObject(object: MyStruct) {
    if var coordinates: [CLLocationCoordinate2D] = object.geometry?.coordinates {
        let polygon = MKPolygon(coordinates: &coordinates, count: coordinates.count)
        self.mapView.addOverlay(polygon)
    }
}

Unfortunately this code, at least in Swift 2.1, produces: '&' used with non-inout argument of type 'UnsafeMutablePointer' — Fabrizio Bartolomucci, Dec 19 '15 at 18:32
@FabrizioBartolomucci you'll get that error if `coordinates` is defined with a `let` and not a `var`. Since `let` makes `coordinates` immutable, it can't be used as an `inout` argument (Swift 2.1 still has a semi-cryptic error in this case). — Mike S, Dec 20 '15 at 20:04

score 0 · Answer 2 · answered Dec 20 '15 at 22:15

My final solution by cherry-picking from several tutorials and integrating:

func setPolylineFromPoints(locations:[CLLocation]){
    if locations.count == 0 {
        return;
    }
// while we create the route points, we will also be calculating the bounding box of our route
// so we can easily zoom in on it.
    var pt : UnsafeMutablePointer<MKMapPoint>? // Optional
    pt = UnsafeMutablePointer.alloc(locations.count)
    for idx in 0..<locations.count-1 {
       let location = locations[idx]
       let point = MKMapPointForCoordinate(location.coordinate);
       pt![idx] = point;
    }
    self.polyline = MKPolyline(points:pt!, count:locations.count-1)
// clear the memory allocated earlier for the points
    pt?.destroy()
    pt?.dealloc(locations.count)
}

MKPolygon using Swift (Missing argument for parameter 'interiorPolygons' in call)

2 Answers2