How undo a MapKit annotation drag

If, in mapView didChangeDragState, I detect the annotation has been dragged to an unwanted location, I can cancel the drag in mapView:didChangeDragState.

 case .Dragging:  
            let inColorado = StateOutline.inColorado(view.annotation!.coordinate)
            if !inColorado {
                view.dragState = .Canceling
            }

Unfortunately, that leaves the pin at the position in the unwanted location where the drag was cancelled.

One would like to set the annotation to

a valid coordinate
or restore to the pre-drag coordinate

or set to the last valid location of the drag

case .Canceling:
        view.annotation!.coordinate = StateOutline.coloradoCenter()
        view.dragState = .None
    }

That coordinate setting is not permitted because view.annotation!.coordinate is a get-only property.

How might one undo the annotation drag?

Using MKAnnotation setCoordinate isn’t something to consider — it was removed in iOS 8.3.

The only thing that comes to mind is to replace that annotation with a new one and set the coordinate on that. Ideally the pin coordinate would be set to its last valid location.

Solution

Your mistake here is believing that an annotation's coordinate cannot be set. It can. The MKAnnotation protocol does not dictate a settable coordinate, but all actual adopters have one. Just use an MKPointAnnotation. Its coordinate is settable. You may even be using one already!

public class MKPointAnnotation : MKShape {
    public var coordinate: CLLocationCoordinate2D
}

You can even write your own MKAnnotation adopter:

import UIKit
import MapKit

class MyAnnotation : NSObject, MKAnnotation {
    dynamic var coordinate : CLLocationCoordinate2D
    var title: String?
    var subtitle: String?

    init(location coord:CLLocationCoordinate2D) {
        self.coordinate = coord
        super.init()
    }
}