Improving our map annotations

Paul Hudson    @twostraws    May 12th 2022

Right now we’re using MapMarker to place locations in our Map view, but SwiftUI lets us place any kind of view on top of our map so we can have complete customizability. So, we’re going to use that to show a custom SwiftUI view containing an icon and some text to show the location’s name, then take a look at the underlying data type to see what improvements can be made there.

Thanks to the brilliance of SwiftUI, this takes hardly any code at all – replace your existing MapMarker code with this:

MapAnnotation(coordinate: CLLocationCoordinate2D(latitude: location.latitude, longitude: location.longitude)) {
    VStack {
        Image(systemName: "star.circle")
            .resizable()
            .foregroundColor(.red)
            .frame(width: 44, height: 44)
            .background(.white)
            .clipShape(Circle())

        Text(location.name)
    }
}

That’s already an immediate improvement, because now it’s clear what each marker represents – the location name appears directly underneath. However, I want to look beyond just the SwiftUI view: I want to look at the Location struct itself, and apply a few improvements that make it better.

First, I don’t particularly like having to make a CLLocationCoordinate2D inside our SwiftUI view, and I’d much prefer to move that kind of logic inside our Location struct. So, we can move that into a computed property to clean up our code. First, add an import for MapKit into Location.swift, then add this to Location:

var coordinate: CLLocationCoordinate2D {
    CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
}

Now our ContentView code is simpler:

MapAnnotation(coordinate: location.coordinate) {

The second change I want to make is one I encourage everyone to make when building custom data types for use with SwiftUI: add an example! This makes previewing significantly easier, so where possible I would encourage you to add a static example property to your types containing some sample data that can be previewed well.

So, add this second property to Location now:

static let example = Location(id: UUID(), name: "Buckingham Palace", description: "Where Queen Elizabeth lives with her dorgis.", latitude: 51.501, longitude: -0.141)

The last change I’d like to make here is to add a custom == function to the struct. We already made Location conform to Equatable, which means we can already compare one location to another using ==. Behind the scenes, Swift will write this function for us by comparing every property against every other property, which is rather wasteful – all our locations already have a unique identifier, so if two locations have the same identifier then we can be sure they are the same without also checking the other properties.

So, we can save a bunch of work by writing our own == function to Location, which compares two identifiers and nothing else:

static func ==(lhs: Location, rhs: Location) -> Bool {
    lhs.id == rhs.id
}

I’m a huge fan of making structs conform to Equatable as standard, even if you can’t use an optimized comparison function like above – structs are simple values like strings and integers, and I think we should extend that same status to our own custom structs too.

With that in place the next step of our project is complete, so please run it now – you should be able to drop a marker and see our custom annotation, but now behind the scenes know that our code is a little bit neater too!

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