How to implement auto-complete for address using Apple Map Kit

Swift 5 + Combine + (Optionally) SwiftUI solution

There seem to be a number of comments on other solutions wanting a version compatible with more recent versions of Swift. Plus, It seems likely that (as I did), people will need a SwiftUI solution as well.

This builds on previous suggestions, but uses Combine to monitor the input, debounce it, and then provide results through a Publisher.

The MapSearch ObservableObject is easily used in SwiftUI (example provided), but could also be used in non-SwiftUI situations as well.

MapSearch ObservableObject

import SwiftUI
import Combine
import MapKit

class MapSearch : NSObject, ObservableObject {
    @Published var locationResults : [MKLocalSearchCompletion] = []
    @Published var searchTerm = ""
    
    private var cancellables : Set<AnyCancellable> = []
    
    private var searchCompleter = MKLocalSearchCompleter()
    private var currentPromise : ((Result<[MKLocalSearchCompletion], Error>) -> Void)?
    
    override init() {
        super.init()
        searchCompleter.delegate = self
        
        $searchTerm
            .debounce(for: .seconds(0.5), scheduler: RunLoop.main)
            .removeDuplicates()
            .flatMap({ (currentSearchTerm) in
                self.searchTermToResults(searchTerm: currentSearchTerm)
            })
            .sink(receiveCompletion: { (completion) in
                //handle error
            }, receiveValue: { (results) in
                self.locationResults = results
            })
            .store(in: &cancellables)
    }
    
    func searchTermToResults(searchTerm: String) -> Future<[MKLocalSearchCompletion], Error> {
        Future { promise in
            self.searchCompleter.queryFragment = searchTerm
            self.currentPromise = promise
        }
    }
}

extension MapSearch : MKLocalSearchCompleterDelegate {
    func completerDidUpdateResults(_ completer: MKLocalSearchCompleter) {
            currentPromise?(.success(completer.results))
        }
    
    func completer(_ completer: MKLocalSearchCompleter, didFailWithError error: Error) {
        //could deal with the error here, but beware that it will finish the Combine publisher stream
        //currentPromise?(.failure(error))
    }
}

SwiftUI interface, including mapped locations

struct ContentView: View {
    @StateObject private var mapSearch = MapSearch()
    
    var body: some View {
        NavigationView {
            Form {
                Section {
                    TextField("Address", text: $mapSearch.searchTerm)
                }
                Section {
                    ForEach(mapSearch.locationResults, id: \.self) { location in
                        NavigationLink(destination: Detail(locationResult: location)) {
                            VStack(alignment: .leading) {
                                Text(location.title)
                                Text(location.subtitle)
                                    .font(.system(.caption))
                            }
                        }
                    }
                }
            }.navigationTitle(Text("Address search"))
        }
    }
}

class DetailViewModel : ObservableObject {
    @Published var isLoading = true
    @Published private var coordinate : CLLocationCoordinate2D?
    @Published var region: MKCoordinateRegion = MKCoordinateRegion()
    
    var coordinateForMap : CLLocationCoordinate2D {
        coordinate ?? CLLocationCoordinate2D()
    }
    
    func reconcileLocation(location: MKLocalSearchCompletion) {
        let searchRequest = MKLocalSearch.Request(completion: location)
        let search = MKLocalSearch(request: searchRequest)
        search.start { (response, error) in
            if error == nil, let coordinate = response?.mapItems.first?.placemark.coordinate {
                self.coordinate = coordinate
                self.region = MKCoordinateRegion(center: coordinate, span: MKCoordinateSpan(latitudeDelta: 0.03, longitudeDelta: 0.03))
                self.isLoading = false
            }
        }
    }
    
    func clear() {
        isLoading = true
    }
}

struct Detail : View {
    var locationResult : MKLocalSearchCompletion
    @StateObject private var viewModel = DetailViewModel()
    
    struct Marker: Identifiable {
        let id = UUID()
        var location: MapMarker
    }
    
    var body: some View {
        Group {
            if viewModel.isLoading {
                Text("Loading...")
            } else {
                Map(coordinateRegion: $viewModel.region,
                    annotationItems: [Marker(location: MapMarker(coordinate: viewModel.coordinateForMap))]) { (marker) in
                    marker.location
                }
            }
        }.onAppear {
            viewModel.reconcileLocation(location: locationResult)
        }.onDisappear {
            viewModel.clear()
        }
        .navigationTitle(Text(locationResult.title))
    }
}

Update - I've created a simple example project here using Swift 3 as the original answer was written in Swift 2.

In iOS 9.3 a new class called MKLocalSearchCompleter was introduced, this allows the creation of an autocomplete solution, you simply pass in the queryFragment as below:

var searchCompleter = MKLocalSearchCompleter()
searchCompleter.delegate = self
var searchResults = [MKLocalSearchCompletion]()

searchCompleter.queryFragment = searchField.text!

Then handle the results of the query using the MKLocalSearchCompleterDelegate:

extension SearchViewController: MKLocalSearchCompleterDelegate {

    func completerDidUpdateResults(completer: MKLocalSearchCompleter) {
        searchResults = completer.results
        searchResultsTableView.reloadData()
    } 

    func completer(completer: MKLocalSearchCompleter, didFailWithError error: NSError) {
        // handle error
    }
}

And display the address results in an appropriate format:

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let searchResult = searchResults[indexPath.row]
    let cell = UITableViewCell(style: .subtitle, reuseIdentifier: nil)
    cell.textLabel?.text = searchResult.title
    cell.detailTextLabel?.text = searchResult.subtitle
    return cell
}

You can then use a MKLocalCompletion object to instantiate a MKLocalSearch.Request, thus gaining access to the MKPlacemark and all other useful data:

let searchRequest = MKLocalSearch.Request(completion: completion!)
let search = MKLocalSearch(request: searchRequest)
search.startWithCompletionHandler { (response, error) in
    if error == nil {
        let coordinate = response?.mapItems[0].placemark.coordinate
    }
}