Getting Direction-Related Events

Adding a Requirement for Direction-Related Events

If your app requires some form of direction-related information in order to function properly, you should include the UIRequiredDeviceCapabilities key in the app’s Info.plist file. This key contains an array of strings indicating the features that your app requires of the underlying iOS-based device. The App Store uses this information to prevent users from installing apps on a device without the minimum required hardware.

For direction-related events, there are two relevant strings you can associate with this key:

• magnetometer—Include this string if your app requires the presence of heading information.

• gps—Include this string if your app requires the presence of course-related information.

In both cases, you should also include the location-services string in the array. For more information about the UIRequiredDeviceCapabilities key, see Information Property List Key Reference.

Getting Heading-Related Events

Heading events are available to apps running on a device that contains a magnetometer. A magnetometer measures nearby magnetic fields emanating from the Earth and uses them to determine the precise orientation of the device. Although a magnetometer can be affected by local magnetic fields, such as those emanating from fixed magnets found in audio speakers, motors, and many other types of electronic devices, Core Location is smart enough to filter out fields that move with the device.

Heading values can be reported relative either to magnetic north or true north on the map. Magnetic north represents the point on the Earth’s surface from which the planet’s magnetic field emanates. This location is not the same as the North Pole, which represents true north. Depending on the location of the device, magnetic north may be good enough for many purposes, but the closer to the poles you get, the less useful this value becomes.

The steps for receiving heading events are as follows:

1. Create a CLLocationManager object.

2. Determine whether heading events are available by calling the headingAvailable class method. (In iOS 3.x and earlier, check the value of the headingAvailable property instead.)

3. Assign a delegate to the location manager object.

4. If you want true north values, start location services.

5. Call the startUpdatingHeading method to begin the delivery of heading events.

Listing 2-1 shows a custom method that configures a location manager and starts the delivery of heading events. In this case, the object is a view controller that displays the current heading to the user. Because the view controller displays the true north heading value, it starts location updates in addition to heading updates. This code runs in iOS 4.0 and later

Listing 2-1  Initiating the delivery of heading events

- (void)startHeadingEvents {

   if (!self.locManager) {

      CLLocationManager* theManager = [[[CLLocationManager alloc] init] autorelease];

      // Retain the object in a property.

      self.locManager = theManager;

      locManager.delegate = self;

   }

   // Start location services to get the true heading.

   locManager.distanceFilter = 1000;

   locManager.desiredAccuracy = kCLLocationAccuracyKilometer;

   [locManager startUpdatingLocation];

   // Start heading updates.

   if ([CLLocationManager headingAvailable]) {

      locManager.headingFilter = 5;

      [locManager startUpdatingHeading];

   }

}

The object you assign to the delegate property must conform to the CLLocationManagerDelegate protocol. When a new heading event arrives, the location manager object calls the locationManager:didUpdateHeading: method to deliver that event to your app. Upon receiving a new event, you should check the headingAccuracy property to ensure that the data you just received is valid, as shown in Listing 2-2. In addition, if you are using the true heading value, you should also check to see if it contains a valid value before using it.

Listing 2-2  Processing heading events

- (void)locationManager:(CLLocationManager *)manager didUpdateHeading:(CLHeading *)newHeading {

   if (newHeading.headingAccuracy < 0)

      return;

   // Use the true heading if it is valid.

   CLLocationDirection  theHeading = ((newHeading.trueHeading > 0) ?

            newHeading.trueHeading : newHeading.magneticHeading);

   self.currentHeading = theHeading;

   [self updateHeadingDisplays];

}

Getting Course Information While the User Is Moving

Devices that include GPS hardware can generate information indicating the device’s current course and speed. Course information is used to indicate the direction in which the device is moving and does not necessarily reflect the orientation of the device itself. As a result, it is primarily intended for apps that provide navigation information while the user is moving.

The actual course and speed information is returned to your app in the same CLLocation objects you use to get the user’s position. When you start location updates, Core Location automatically provides course and speed information when it is available. The framework uses the incoming location events to compute the current direction of motion. For more information on how to start location updates, see “Getting the User’s Location”