Programming iOS 8

Chapter 22. Sensors

A device may contain hardware for sensing the world around itself — where it is located, how it is oriented, how it is moving.

Information about the device’s current location and how that location is changing over time, using its Wi-Fi, cellular networking, and GPS capabilities, along with information about the device’s orientation relative to north, using its magnetometer, is provided through the Core Location framework. You’ll need to import CoreLocation.

Information about the device’s change in speed and attitude using its accelerometer is provided through the UIEvent class (for device shake) and the Core Motion framework, which provides increased accuracy by incorporating the device’s gyroscope, if it has one, as well as the magnetometer; you’ll need to import CoreMotion.

One of the major challenges associated with writing code that takes advantage of the sensors is that different devices have different hardware. If you don’t want to impose stringent restrictions on what devices your app will run on in the first place (UIRequiredDeviceCapabilities in the Info.plist), your code must be prepared to fail gracefully and possibly provide a subset of its full capabilities when it discovers that the current device lacks certain features.

Moreover, certain sensors may experience momentary inadequacy; for example, Core Location might not be able to get a fix on the device’s position because it can’t see cell towers, GPS satellites, or both. And some sensors take time ...