CLLocationManager and its delegate,
MKAnnotation and its Annotation View,
Overlay and its Overlay View

In the JavaScript project, the view controller created a UIWebView that displayed a Google map. In the current project, the view controller creates a MKMapView, which is a subclass of UIView specialized for displaying one Google map. The map fills the MKMapView, so it doesn’t need a backgroundColor.

The view controller also creates a CLLocationManager, which uses a lot of electricity and finds your current location. The view controller acts as the delegate of the CLLocationManager. When the CLLocationManager figures out where you are, the locationManager:didUpdateToLocation:fromLocation: method of the delegate is called. This method receives a CLLocation object containing the latitude, longitude, altitude, accuracy, etc. (Check out the other interesting methods of the delegate.) See Getting the User’s Location.

The locationManager:didUpdateToLocation:fromLocation: method turns off the CLLocationManager to save electricity. It then tells the map to display a 100-meter-wide region containing the current location. With our 2:3 aspect ratio (320 × 480 pixel pairs on an iPhone), the region should be 150 meters from north to south.

locationManager:didUpdateToLocation:fromLocation: also asks the CLGeocoder to look up the street address of the location. See Converting Coordinates into Place Name Information. When the address is found, the CLGeocoder executes the ^{block} of code passed as the second argument of reverseGeocodeLocation:completionHandler:. We saw our first block here.

This block creates an MKAnnotation and puts it into the map. An MKAnnotation can be anything that has a title, subtitle, and latitude and longitude. For example, the Pin object I created is the simplest kind of MKAnnotation. Note that the pin looks like an Apple pin, not a Google Maps pin. See Annotating Maps.

Source code in

  1. main.m
  2. Class MapAppDelegate
  3. Class ViewController creates the MKMapView, the CLLocationManager, and the CLGeocoder.
  4. Class Pin adopts the MKAnnotation protocol.
  5. Property list Map-Info.plist

Create the project

Class ViewController is a subclass of class UIViewController. Class Pin is a subclass of class NSObject. ViewController.h imports the header files CoreLocation.h and MapKit.h. Pin.h imports the header file MapKit.h.

Add the CoreLocation.framework and the MapKit.framework to the project. See Gone for adding a framework to the project.

Allow the app to run only on a device that has the required device capabilities. In the Project Navigator pane of Xcode, open the Supporting Files folder and open the file Map-Info.plist. Select Required device capabilities and add a new line for location-services. See Requiring the Presence of Location Services.

Run the project

If you are running in the Simulator,
System Preferences… → Personal → Security
and enable location services.

“Map” Would Like to Use
Your Current Location.
Don’t Allow/OK

Press OK.

By default, the simulator believes you’re at the Apple Store at the San Francisco State University Downtown Campus, at Latitude 37.785834° North, Longitude 122.406417° West. (A negative longitude is West.) Tap the red pin to display its title and subtitle. Double tap or spread to zoom in, pinch to zoom out. To pinch the simulator, hold down the option key while dragging.

2012-08-03 09:23:22.380 Map[57896:207] locationManager:didUpdateToLocation:fromLocation: Lat 37.785834° Long -122.40641°
2012-08-03 09:23:22.509 Map[57896:207] placemark == Apple Store, San Francisco, Apple Store, San Francisco, 1 Stockton St, San Francisco, CA  94108-5805, United States @ <+37.78584540,-122.40651750> +/- 100.00m, region (identifier <+37.78584545,-122.40652161> radius 18.96) <+37.78584545,-122.40652161> radius 18.96m

Things to try

  1. The old simulators believed they were always in Cupertino, California. The current simulator defaults to thinking it’s at the Apple store at 1 Stockton Street (corner of Ellis Street) in San Francisco. Let’s make the current simulator believe we’re at 7 East 12th Street, NY, NY 10003. First, find the latitude and longitude of this address by pointing your browser at the following URL. See Geocoding Requests.,+New+York,+NY&sensor=false
    Positive latititude is north of the equator; negative longitude is west of the prime meridian.
    "location" : {
                   "lat" : 40.73448420,
                   "lng" : -73.99365349999999
    In the Scheme dropdown menu in the upper left of Xcode, select Map > iPhone 6.0 Simulator (or whatever your version is.) Then click on the word Map > and select Edit Scheme…. In the left panel, select Run In the top bar, select Options.
    Set the Default Location to None and press OK. Then pull down the Debug menu of the simulator and select
    Debug → Location → Custom Location…
    Enter a latitude and longitude for the location
    you would like to simulate.
    Latitude: 40.7343890
    Longitude: -73.9937460

    You may have to Quit iOS Simulator get make the new location take effect.

    2012-08-03 09:45:04.197 Map[76866:207] locationManager:didUpdateToLocation:fromLocation: Lat 40.734389° Long -73.99374°
    2012-08-03 09:45:04.394 Map[76866:207] placemark == 7 E 12th St, 7 E 12th St, New York, NY  10003-4475, United States @ <+40.73453300,-73.99366490> +/- 100.00m

    See Location Simulation. You can specify a GPX file as the Default Location.

  2. Display the red pin whether the geocoder succeeds or fails. If it fails, the pin should display only the latitude and longitude, not the street address.

  3. The MKMapView becomes visible and displays a world map before it is given the location to display. Keep the MKMapView hidden until you give a value to the region property of the MKMapView. Even if the MKMapView is hidden, it will still display the “Map” Would Like to Use Your Current Location action sheet.

  4. Even when the region property has been given a value, it may still take some time until the map has finished downloading from Google into the MKMapView. I wish I could ask you to keep the MKMapView hidden until the mapViewDidFinishLoadingMap: method of the MKMapView’s delegate is called. (Let the view controller be the MKMapView’s delegate. The view controller is now the delegate for two different objects.) But when I run the app on the simulator, mapViewDidFinishLoadingMap: is called only the first time the map is downloaded loaded from Google. Even if I erase the app from the simulator, and quit the simulator, and quit Xcode itself, and then re-launch Xcode, mapViewDidFinishLoadingMap: is not called when I run the app again. The tiles of the map appear anyway, which means they are being cached somewhere.

    In other words, the purpose (or at least the usefulness) of mapViewWillStartLoadingMap: and mapViewDidFinishLoadingMap: is not to tell us when all the tiles have been downloaded. (The tiles have probably already been downloaded.) Their purpose is to delimit the interval of time during which we have to stay connected to the Internet.

    To verify this theory, let the view controller be the delegate of the MKMapView object. The view controller will have to adopt the MKMapViewDelegate protocol, and the delegate property of the MKMapView will have to point to the view controller. Then run the app again, but with a location that is one degree farther north. In locationManager:didUpdateToLocation:fromLocation:, change the second argument of MKCoordinateRegionMakeWithDistance from newLocation.coordinate to

    		newLocation.coordinate.latitude + 1,
    Run the app again and you’ll see that mapViewDidFinishLoadingMap: is called. Run the app yet again and mapViewDidFinishLoadingMap: is not called.

  5. Now that the MKMapView has a delegate, we no longer have to use the default picture (a red pushpin) for the annotation. Get an image (Apple prefers png, but jpg will do) with a transparent background and add it to the project. A ragged example is nyu.png. I made the white background transparent using the form in this page. Add the image file to the project.

    Add the following method to the MKMapViewDelegate (which is the view controller). For simplicity, it always returns an annotation view with exactly the same picture.

    #pragma mark -
    #pragma mark Protocol MKMapViewDelegate
    - (MKAnnotationView *) mapView: (MKMapView *) v
    	viewForAnnotation: (id <MKAnnotation>) annotation {
    	MKAnnotationView *v = [[MKAnnotationView alloc]
    		initWithAnnotation: annotation reuseIdentifier: nil];
    	v.image = [UIImage imageNamed: @"nyu.png"];
    	return v;
    Even better, reuse an existing MKAnnotationView instead of creating a new one with every call to mapView:viewForAnnotation:.

  6. The CLLocation received by the location manager delegate contains more than just a latitude and longitude. It also contains an accuracy in meters. Create a subclass of MKAnnotationView with a drawRect: method that will draw a circle whose radius is the accuracy. See the Overview at the top of this page. Tell the mapView:viewForAnnotation: method to instantiate an object of this new class instead of instantiating a plain vanilla MKAnnotationView.

  7. The above annotation and its annotation view marked a point on the map. An overlay and its overlay view mark a region on the map. Here are examples. A simple example of an overlay and its overlay view is an MKPolygon and its PolygonView. Let’s make one that is a purple rectangle covering the NYU campus.

    Add the following instance variable to the view controller:

    	MKPolygon *polygon;

    Initialize it and add it to the MKMapView at the end of the loadView method of the view controller. (To get the latitudes and longitudes from Google Maps, use the plotter in Manhattan.)

    	//Latitude and longitude of the four corners of NYU campus:
    	CLLocationCoordinate2D coordinates[] = {
    		{40.728804, -73.99500},		//West 4th St. & Mercer St.
    		{40.730978, -73.999625},	//West 4th St. & MacDougal St.
    		{40.730438, -74.000092},	//West 3rd St. & MacDougal St.
    		{40.728202, -73.99555}		//West 3rd St. & Mercer St.
    	const size_t count = sizeof coordinates / sizeof coordinates[0];
    	polygon = [MKPolygon polygonWithCoordinates: coordinates count: count];
    	[(MKMapView *)self.view addOverlay: polygon];

    Add the following method to the view controller.

    #pragma mark -
    #pragma mark Protocol MKMapViewDelegate
    - (MKOverlayView *) mapView: (MKMapView *) v
    	viewForOverlay: (id <MKOverlay>) overlay {
    	MKPolygonView *view = [[MKPolygonView alloc] initWithPolygon: polygon];
    	view.fillColor = [UIColor purpleColor];
    	return view;

    Build and run. Swipe over to the Washington Square campus of NYU and see if it’s covered by a purple rectangle. Then make the rectangle half-transparent (α = ½). Use the official shade of NYU purple:

    	//excerpt from mapView:viewForOverlay:
    	view.fillColor = [UIColor
    		colorWithRed: 82 / 250.0
    		green: 6 / 255.0
    		blue: 145 / 255.0
    		alpha: .5
  8. You can do more than just specify the fillColor of the overlay view. See the other properties inherited from class MKOverlayPathView.

    There are other types of overlays and overlay views. See the Map Kit.

    Class that adopts the
    MKOverlay Protocol
    Class derived from class
    MKPolygon MKPolygonView
    MKCircle MKCircleView
    MKPolyline MKPolylineView