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In the last few months I have been working more intensively with MapKit, doing more advanced operations like clustering map annotations or animating annotation position changes.

I have encountered a few problem resulting in MKMapView quite unexpectedly crashing the whole application that I had to fix, or maybe better to say, work around.

MKMapView crashing the view controller on dismiss

During the application testing I noticed a very strange bug. Sometimes when I dismissed the view controller with MKMapView the application just crashed.

Debugging I noticed that it happened when the annotations on the map were updated just a short while before dismissing the view controller and the crash log pointed to mapView(_:viewFor:).

I guessed that MKMapView was processing annotation changes when the view controller was already deallocated. The MKMapView was still alive, tried to call its delegate, which was that deallocated view controller, and crashed.

The fix for this problem was setting the MKMapView's delegate to nil in the view controller’s deinit method.

deinit {
    mapView.delegate = nil
}

Crashing when animating annotation position changes

The second crash I encountered was a bit more tricky. The application started crashing when I implemented animating the annotation position changes.

The way this works is you have a collection of your annotation objects, each has a coordinate property that needs to be @objc dynamic because MKMapView uses KVO to observe it. When you update this property the annotation changes its position on the map.

If you want to animate the position change on the map, you need to wrap the coordinate property assignment into UIView.animate. Doing this the application started crashing when the user moved the map, or zoomed it, or sometimes just after a while with the user not doing anything a all

The exception said

Collection was mutated while being enumerated.

but the annotation collection was not really mutated as a whole, some annotation in that collection was mutated by updating its coordinate property.

Theory about the crash

The circumstances of the crash led me to believe that there was some timing issue, my code updating the annotation at the same the MKMapView processes it in some way.

Which would make sense, when the user moves the map or zooms it there might be some processing needed to bring annotations into view or hide them.

The interesting thing was this only happened when using annotation clustering. It never happened with “plain” annotations.

With this observation it looked like MKMapView trying to recompute the clusters causing the crash.

In the iOS app I currently work on there is a Notification Service Extension and a Share Extension. Both extensions have been implemented quite some time age and have been working fine.

Recently I got some bug reports that led to discovering some interesting limits about both of those extension types.

Notification Service Extension

The Notification Service Extension is executed when the iOS app receives a push notification and has a chance to modify the payload before iOS displays the push notification.

I use it to change the push notification sound to the sound the user chose in the app, for better personalization.

Another feature is adding a big red warning image as an attachment to the push notification if the push notification is of an alert type.

I already use the image in the main app so I implemented it quite simply, loading it from the asset catalog, saving it into a file and adding that file as an attachment

let image = #imageLiteral(resourceName: "NotificationAlert")
guard let data = image.jpegData(compressionQuality: 0.8) else {
    return failEarly()
}

try data.write(to: tmp.appendingPathComponent("image.png"), options: [])
let imageAttachment = try UNNotificationAttachment(identifier: "image.png", url: fileURL, options: nil)
content.attachments = [imageAttachment]
contentHandler(content.copy() as! UNNotificationContent)

This worked fine on smaller phones but when users started using bigger phone, like iPhone 11, they started complaining that the image is not shown when they receive an alert push notification.

I was able to reproduce the problem and found out the extension crashed exceeding the 24 MB memory limit. But only on bigger phones.

The problem is that manipulating an UIImage instance does not consume the same amount of memory on every device, it depends on the device screen scaling factor.

On smaller devices with smaller scaling factor the image operations take up less memory, below the extension limit, but on bigger devices the memory limit is exceeded.

I solved this problem by just adding the image to the app bundle as a file and using the file directly, without the additional step of using an UIImage.

Annotations are mainly used to displays static “pins” in MKMapView but sometimes you might need to make them move and animate their position changes so it looks better to the users.

There are a few things you need to do to achieve this.

Coordinate property specifics

The MKAnnotation protocol has a coordinate property that is used by the MKMapView to position the annotation to its corresponding location on the map.

If you just update the property you will quickly see that nothing happens, the annotation does not move on the map.

MKMapView uses KVO to know when the coordinate property changes so in Swift you need to mark the coordinate property in your MKAnnotation as @objc dynamic to make it work

final class LocationViewModel: NSObject, MKAnnotation {
    @objc dynamic coordinate: CLLocationCoordinate2D

    ...
}

With this change you will notice that the annotation now moves on the map, but it is not smooth, it basically jumps from the old position to the new one.

If you need to display many annotations in your MKMapView it is recommended to cluster them for better performance.

Map clustering

This means instead of showing all the visible annotations you group annotations that are close together into one single annotation cluster representing them instead.

This cluster annotation usually shows the number of annotations it represents. As you then zoom in to get finer detail the clusters break up and show the actual annotations.

Clustering is supported in MKMapView on iOS 11 and newer, no need to use any custom library. If you need to support older versions of iOS, there are libraries like Cluster that you can use.

Custom cluster view implementation

Let’s say you use a custom annotation view to show your annotations and you want to add support for clustering. You first create a custom MKAnnotationView in the same way

final class LocationDataMapClusterView: MKAnnotationView {

    // MARK: Initialization
    private let countLabel = UILabel()

    override var annotation: MKAnnotation? {
    	didSet {
			 guard let annotation = annotation as? MKClusterAnnotation else {
            	assertionFailure("Using LocationDataMapClusterView with wrong annotation type")
            	return
        	}

    		countLabel.text = annotation.memberAnnotations.count < 100 ? "\(annotation.memberAnnotations.count)" : "99+"
    	}
    }

    override init(annotation: MKAnnotation?, reuseIdentifier: String?) {
        super.init(annotation: annotation, reuseIdentifier: reuseIdentifier)

		displayPriority = .defaultHigh
        collisionMode = .circle

        frame = CGRect(x: 0, y: 0, width: 40, height: 50)
        centerOffset = CGPoint(x: 0, y: -frame.size.height / 2)

       
        setupUI()
    }

    @available(*, unavailable)
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    // MARK: Setup
    private func setupUI() {
        ...
    }
}

The idea is the same as when custom annotation view, but there is a difference. You need to define displayPriority to tell the map that your cluster annotation has a higher priority that normal annotation.

If you want to display completely custom views as “pins” on the map in your iOS application, you should use annotations. All your data needs to be represented as objects conforming to the MKAnnotation protocol, with title, subtitle and coordinate as the required properties.

Custom view implementation

Visually you represent an MKAnnotation with a MKAnnotationView. You can create a custom class that subclasses MKAnnotationView and implement your custom UI in that class.

Here is an sample MKAnnotationView with fixed size that displays just one custom view

final class LocationAnnotationView: MKAnnotationView, Reusable {

    // MARK: Initialization

    override init(annotation: MKAnnotation?, reuseIdentifier: String?) {
        super.init(annotation: annotation, reuseIdentifier: reuseIdentifier)

        frame = CGRect(x: 0, y: 0, width: 40, height: 50)
        centerOffset = CGPoint(x: 0, y: -frame.size.height / 2)

        canShowCallout = true
        setupUI()
    }

    @available(*, unavailable)
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    // MARK: Setup

    private func setupUI() {
        backgroundColor = .clear

        let view = MapPinView()
        addSubview(view)

        view.frame = bounds
    }
}

The MKAnnotationView is by default aligned to its corresponding position on map with the bottom left corner. If your MKAnnotationView looks like a pin for example, you need to align it to the position on the map with the bottom center point. To do that you use the centerOffset property as shown.

Registering the custom view with MKMapView

The next step is to tell MKMapView to user your custom class.

Using single custom MKAnnotationView

If you want to display only one type of annotation in your MKMapView, just register your custom class with the MKMapViewDefaultAnnotationViewReuseIdentifier reuse identifier

mapView.register(LocationAnnotationView.self, forAnnotationViewWithReuseIdentifier: MKMapViewDefaultAnnotationViewReuseIdentifier)

This is enough to make MKMapView to completely handle creating and recycling instances of your custom MKAnnotationViews for you. No need to implement any MKMapView delegate methods for providing annotation views.