Animate MapKit annotation coordinate change in Swift?

Question

When using MapKit in iOS 8 in Swift, how do I make it possible to animate a change in the map position of a custom annotation? I am saying this:

UIView.animateWithDuration(0.25) {
    var loc = ann.coordinate
    loc.latitude = loc.latitude + 0.0005
    loc.longitude = loc.longitude + 0.001
    ann.coordinate = loc
}

...where ann is a custom annotation, MyAnnotation. The annotation is jumping, not animating, to the new coordinate position.

The annoying thing is that the animation works perfectly if I write MyAnnotation in Objective-C. But if I write it in Swift, I don't get the animation any more!

Just FYI, here is my Swift code for MyAnnotation:

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

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

score 69 · Accepted Answer · answered Apr 21 '15 at 15:24

You almost have it right! Just change this line in your MyAnnotation implementation:

var coordinate : CLLocationCoordinate2D

to this:

dynamic var coordinate : CLLocationCoordinate2D

That will fix it.

Why does this matter? Because annotation animation relies on KVO (key value observing). In Swift, a property is not key value observable unless it is marked as dynamic. (To get a little more technical, that in turn is because KVO observation works by swizzling the implementation of the property setter method - the runtime reaches right into your class and changes that property's setter code. But in Swift, it can't do that unless you explicitly mark the property as dynamic.)

Animate MapKit annotation coordinate change in Swift?

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