iOS Maps

Using Map Kit in Xamarin.iOS Applications

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iOS includes the MapKit framework, which makes it easy to add maps to an application. Using Map Kit, iOS applications can add interactive maps that support features such as panning and zooming, showing user location and layering rich graphics on a map. This article delves into several of Map Kit's features, showing how to take advantage of them to build geographic features into an application

Overview

Maps are a common feature in all modern mobile operating systems. iOS offers mapping support natively through the Map Kit framework. With Map Kit, applications can easily add rich, interactive maps. These maps can be customized in a variety of ways, such as adding annotations to mark locations on a map, and overlaying graphics of arbitrary shapes. Map Kit even has built-in support for showing the current location of a device.

iOS Maps

Adding a Map

Adding a map to an application is accomplished by adding an MKMapView instance to the view hierarchy, as shown below:

// map is an MKMapView declared as a class variable
map = new MKMapView (UIScreen.MainScreen.Bounds);
View = map;

MKMapView is a UIView subclass that displays a map. Simply adding the map using the code above produces an interactive map:

Map Style

MKMapView supports 3 different styles of maps. To apply a map style, simply set the MapType property to a value from the MKMapType enumeration:

map.MapType = MKMapType.Standard; //road map
map.MapType = MKMapType.Satellite;
map.MapType = MKMapType.Hybrid;

The following screenshot show the different map styles that are available:

Panning and Zooming

MKMapView includes support for map interactivity features such as:

  • Zooming via a pinch gesture
  • Panning via a pan gesture

These features can be enabled or disabled by simply setting the ZoomEnabled and ScrollEnabled properties of the MKMapView instance, where the default value is true for both. For example, to display a static map, simply set the appropriate properties to false:

map.ZoomEnabled = false;
map.ScrollEnabled = false;

User Location

In addition to user interaction, MKMapView also has built-in support for displaying the location of the device. It does this using the Core Location framework. Before you can access the user's location, you must prompt the user. To do this, create an instance of CLLocationManager and call RequestWhenInUseAuthorization.

CLLocationManager locationManager = new CLLocationManager();
locationManager.RequestWhenInUseAuthorization();
//locationManager.RequestAlwaysAuthorization(); //requests permission for access to location data while running in the background

Note that in versions of iOS prior to 8.0, attempting to call RequestWhenInUseAuthorization will result in an error. Make sure to check the version of iOS before making that call if you intend to support versions prior to 8.

Accessing the user's location also requires modifications to Info.plist. The following keys relating to location data should be set:

  • NSLocationWhenInUseUsageDescription - For when you are accessing the user's location while they are interacting with your app.
  • NSLocationAlwaysUsageDescription - For when your app accesses the user's location in the background.

You can add those keys by opening Info.plist and selecting Source at the bottom of the editor.

Once you've updated Info.plist and prompted the user for permission to access their location, you can show the user's location on the map by setting the ShowsUserLocation property to true:

map.ShowsUserLocation = true;

Annotations

MKMapView also supports displaying images, known as annotations, on a map. These can be either custom images or system-defined pins of various colors. For example, the following screenshot shows a map with a both a pin and a custom image:

Adding an annotation

An annotation itself has two parts:

  • The MKAnnotation object, which includes model data about the annotation, such as the title and location of the annotation.
  • The MKAnnotationView , which contains the image to display and optionally a callout that is shown when the user taps the annotation.

Map Kit uses the iOS delegation pattern to add annotations to a map, where the Delegate property of the MKMapView is set to an instance of an MKMapViewDelegate. It is this delegate's implementation that is responsible for returning the MKAnnotationView for an annotation.

To add an annotation, first the annotation is added by calling AddAnnotations on the MKMapView instance:

// add an annotation
map.AddAnnotations (new MKPointAnnotation (){
    Title="MyAnnotation",
    Coordinate = new CLLocationCoordinate2D (42.364260, -71.120824)
});

When the location of the annotation becomes visible on the map, the MKMapView will call its delegate's GetViewForAnnotation method to get the MKAnnotationView to display.

For example, the following code returns a system-provided MKPinAnnotationView:

string pId = "PinAnnotation";

public override MKAnnotationView GetViewForAnnotation (MKMapView mapView, NSObject annotation)
{
    if (annotation is MKUserLocation)
        return null;

    // create pin annotation view
    MKAnnotationView pinView = (MKPinAnnotationView)mapView.DequeueReusableAnnotation (pId);

    if (pinView == null)
        pinView = new MKPinAnnotationView (annotation, pId);

    ((MKPinAnnotationView)pinView).PinColor = MKPinAnnotationColor.Red;
    pinView.CanShowCallout = true;

    return pinView;
}

Reusing Annotations

To conserve memory, MKMapView allows annotation view's to be pooled for reuse, similar to the way table cells are reused. Obtaining an annotation view from the pool is done with a call to DequeueReusableAnnotation:

MKAnnotationView pinView = (MKPinAnnotationView)mapView.DequeueReusableAnnotation (pId);

Showing Callouts

As mentioned earlier, an annotation can optionally show a callout. To show a callout simply set CanShowCallout to true on the MKAnnotationView. This results in the annotation's title being displayed when the annotation is tapped, as shown:

Customizing the Callout

The callout can also be customized to show left and right accessory views, as shown below:

pinView.RightCalloutAccessoryView = UIButton.FromType (UIButtonType.DetailDisclosure);
pinView.LeftCalloutAccessoryView = new UIImageView(UIImage.FromFile ("monkey.png"));

This code results in the following callout:

To handle the user tapping the right accessory, simply implement the CalloutAccessoryControlTapped method in the MKMapViewDelegate:

public override void CalloutAccessoryControlTapped (MKMapView mapView, MKAnnotationView view, UIControl control)
{
    ...
}

Overlays

Another way to layer graphics on a map is using overlays. Overlays support drawing graphical content that scales with the map as it is zoomed. iOS provides support for several types of overlays, including:

  • Polygons - Commonly used to highlight some region on a map.
  • Polylines - Often seen when showing a route.
  • Circles - Used to highlight a circular area of a map.

Additionally, custom overlays can be created to show arbitrary geometries with granular, customized drawing code. For example, weather radar would be a good candidate for a custom overlay.

Adding an Overlay

Similar to annotations, adding an overlay involves 2 parts:

  • Creating a model object for the overlay and adding it to the MKMapView .
  • Creating a view for the overlay in the MKMapViewDelegate .

The model for the overlay can be any MKShape subclass. Xamarin.iOS includes MKShape subclasses for polygons, polylines and circles, via the MKPolygon, MKPolyline and MKCircle classes respectively.

For example, the following code is used to add an MKCircle:

var circleOverlay = MKCircle.Circle (mapCenter, 1000);
map.AddOverlay (circleOverlay);

The view for an overlay is an MKOverlayView instance that is returned by the GetViewForOverlay in the MKMapViewDelegate. Each MKShape has a corresponding MKOverlayView that knows how to display the given shape. For MKPolygon there is MKPolygonView. Similarly, MKPolyline corresponds to MKPolylineView, and for MKCircle there is MKCircleView.

For example, the following code returns an MKCircleView for an MKCircle:

public override MKOverlayView GetViewForOverlay (MKMapView mapView, NSObject overlay)
{
    var circleOverlay = overlay as MKCircle;
    var circleView = new MKCircleView (circleOverlay);
    circleView.FillColor = UIColor.Blue;
    return circleView;
}

This displays a circle on the map as shown:

Local Search

iOS includes a local search API with Map Kit, which allows asynchronous searches for points of interest in a specified geographic region.

To perform a local search, an application must follow these steps:

  1. Create MKLocalSearchRequest object.
  2. Create an MKLocalSearch object from the MKLocalSearchRequest .
  3. Call the Start method on the MKLocalSearch object.
  4. Retrieve the MKLocalSearchResponse object in a callback.

The local search API itself provides no user interface. It doesn’t even require a map to be used. However, to make practical use of local search, an application needs to provide some way to specify a search query and display results. Additionally, since the results will contain location data, it will often make sense to show them on a map.

Adding a Local Search UI

One way to accept search input is with a UISearchBar, which provided by a UISearchController and will display results in a table.

The following code adds the UISearchController (which has a search bar property) in the ViewDidLoad method of MapViewController:

//Creates an instance of a custom View Controller that holds the results
var searchResultsController = new SearchResultsViewController (map);

//Creates a search controller updater
var searchUpdater = new SearchResultsUpdator ();
searchUpdater.UpdateSearchResults += searchResultsController.Search;

//add the search controller
searchController = new UISearchController (searchResultsController) {
                SearchResultsUpdater = searchUpdater
            };

//format the search bar
searchController.SearchBar.SizeToFit ();
searchController.SearchBar.SearchBarStyle = UISearchBarStyle.Minimal;
searchController.SearchBar.Placeholder = "Enter a search query";

//the search bar is contained in the navigation bar, so it should be visible
searchController.HidesNavigationBarDuringPresentation = false;

//Ensure the searchResultsController is presented in the current View Controller 
DefinesPresentationContext = true;

//Set the search bar in the navigation bar
NavigationItem.TitleView = searchController.SearchBar;

Note that you are responsible for incorporating the search bar object into the user interface. In this example, we assigned it to the TitleView of the navigation bar, but if you do not use a navigation controller in your application you will have to find another place to display it.

In this code snippet, we created another custom view controller – searchResultsController – that displays the search results and then we used this object to create our search controller object. We also created a new search updater, which becomes active when the user interacts with the search bar. It receives notifications about searches with each keystroke and is responsible for updating the UI. We will take a look at how to implement both the searchResultsController and the searchResultsUpdater later in this guide.

This results in a search bar displayed over the map as shown below:

Displaying the Search Results

To display search results, we need to create a custom View Controller; normally a UITableViewController. As shown above, the searchResultsController is passed to the constructor of the searchController when it is being created. The following code is an example of how to create this custom View Controller:

public class SearchResultsViewController : UITableViewController
{
    static readonly string mapItemCellId = "mapItemCellId";
    MKMapView map;

    public List<MKMapItem> MapItems { get; set; }

    public SearchResultsViewController (MKMapView map)
    {
        this.map = map;

        MapItems = new List<MKMapItem> ();
    }

    public override nint RowsInSection (UITableView tableView, nint section)
    {
        return MapItems.Count;
    }

    public override UITableViewCell GetCell (UITableView tableView, NSIndexPath indexPath)
    {
        var cell = tableView.DequeueReusableCell (mapItemCellId);

        if (cell == null)
            cell = new UITableViewCell ();

        cell.TextLabel.Text = MapItems [indexPath.Row].Name;
        return cell;
    }

    public override void RowSelected (UITableView tableView, NSIndexPath indexPath)
    {
        // add item to map
        CLLocationCoordinate2D coord = MapItems [indexPath.Row].Placemark.Location.Coordinate;
        map.AddAnnotations (new MKPointAnnotation () {
            Title = MapItems [indexPath.Row].Name,
            Coordinate = coord
        });

        map.SetCenterCoordinate (coord, true);

        DismissViewController (false, null);
    }

    public void Search (string forSearchString)
    {
        // create search request
        var searchRequest = new MKLocalSearchRequest ();
        searchRequest.NaturalLanguageQuery = forSearchString;
        searchRequest.Region = new MKCoordinateRegion (map.UserLocation.Coordinate, new MKCoordinateSpan (0.25, 0.25));

        // perform search
        var localSearch = new MKLocalSearch (searchRequest);

        localSearch.Start (delegate (MKLocalSearchResponse response, NSError error) {
            if (response != null && error == null) {
                this.MapItems = response.MapItems.ToList ();
                this.TableView.ReloadData ();
            } else {
                Console.WriteLine ("local search error: {0}", error);
            }
        });


    }
}

Updating the Search Results

The SearchResultsUpdater acts as a mediator between the searchController's search bar and search results.

In this example we have to first create the search method in the SearchResultsViewController. To do this we must create an MKLocalSearch object and use it to issue a search for an MKLocalSearchRequest, the results are retrieved in a callback passed to the Start method of the MKLocalSearch object. The results are then returned in an MKLocalSearchResponse object containing an array of MKMapItem objects:

public void Search (string forSearchString)
{
    // create search request
    var searchRequest = new MKLocalSearchRequest ();
    searchRequest.NaturalLanguageQuery = forSearchString;
    searchRequest.Region = new MKCoordinateRegion (map.UserLocation.Coordinate, new MKCoordinateSpan (0.25, 0.25));

    // perform search
    var localSearch = new MKLocalSearch (searchRequest);

    localSearch.Start (delegate (MKLocalSearchResponse response, NSError error) {
        if (response != null && error == null) {
            this.MapItems = response.MapItems.ToList ();
            this.TableView.ReloadData ();
        } else {
            Console.WriteLine ("local search error: {0}", error);
        }
    });


}

Then, in our MapViewController we'll create a custom implementation of UISearchResultsUpdating, which is assigned to the SearchResultsUpdater property of our searchController in the Adding a Local Search UI section:

public class SearchResultsUpdator : UISearchResultsUpdating
{
    public event Action<string> UpdateSearchResults = delegate {};

    public override void UpdateSearchResultsForSearchController (UISearchController searchController)
    {
        this.UpdateSearchResults (searchController.SearchBar.Text);
    }
}

The implementation above adds an annotation to the map when an item is selected from the results, as shown below:

Note

UISearchController was implemented in iOS 8. If you wish to support devices earlier than this, then you will need to use UISearchDisplayController.

Summary

This article examined the Map Kit framework for iOS. First, it looked at how the MKMapView class allows interactive maps to be included in an application. Then it demonstrated how to further customize maps using annotations and overlays. Finally, it examined the local search capabilities that were added to Map Kit with iOS 6.1, showing how to use perform location based queries for points of interest and add them to a map.

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