Getting the Heading and Course of a Device
`
`Core Location supports two different ways to get direction-related information:
`• Devices with a magnetometer can report the direction in which a device is pointing, also
`known as its heading.
`• Devices with GPS hardware can report the direction in which a device is moving, also known
`as its course.
`
`Heading and course information don’t represent the same information. The heading of a device
`reflects the actual orientation of the device relative to true north or magnetic north. The course
`of the device represents the direction of travel and doesn’t take into account the device
`orientation. Depending on your app, you might prefer one type of information over the other or
`use a combination of the two. For example, a navigation app might toggle between course and
`heading information depending on the user’s current speed. At walking speeds, heading
`information would be more useful for orienting the user to the current environment, whereas in
`a car, course information provides the general direction of the car’s movement.
`
`Adding a Requirement for Direction-Related
`Events
`
`If your iOS app requires direction-related information in order to function properly, 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, you can associate two relevant strings with the
`UIRequiredDeviceCapabilities key:
`• magnetometer—Include this string if your app requires heading information.
`• gps—Include this string if your app requires course-related information.
`
`Important: If your iOS app uses heading or course events but is able to operate successfully
`without them, don’t include the corresponding string value with the
`UIRequiredDeviceCapabilities key.
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`In both cases, 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.
`To get heading events:
`1. Create a CLLocationManager object.
`2. Determine whether heading events are available by calling the headingAvailable class
`method.
`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 3-1 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.
`
`Listing 3-1  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;
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` }
`
` // 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, check the headingAccuracy property to
`ensure that the data you just received is valid, as shown in Listing 3-2. In addition, if you are
`using the true heading value, also check whether it contains a valid value before using it.
`
`Listing 3-2  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];
`}
`
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`Getting Course Information While the User Is
`Moving
`
`Devices that include GPS hardware can generate information that represents the device’s
`current course and speed. Course information indicates the direction in which the device is
`moving and doesn’t necessarily reflect the orientation of the device itself. As a result, course
`information 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’s 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.
`
`Copyright © 2016 Apple Inc. All Rights Reserved. Terms of Use | Privacy Policy | Updated: 2016-03-21
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