Programming iOS 6, 3rd Edition

Acceleration and Attitude

Acceleration results from the application of a force to the device, and is detected through the device’s accelerometer, supplemented by the gyroscope if it has one. Gravity is a force, so the accelerometer always has something to measure, even if the user isn’t consciously applying a force to the device; thus the device can report its attitude relative to the vertical.

Acceleration information can arrive in two ways:

As a prepackaged UIEvent

You can receive a UIEvent notifying you of a predefined gesture performed by accelerating the device. At present, the only such gesture is the user shaking the device.

With the Core Motion framework

You instantiate CMMotionManager and then obtain information of a desired type. You can ask for accelerometer information, gyroscope information, or device motion information (and you can also use Core Motion to get magnetometer information); device motion combines the gyroscope data with data from the other sensors to give you the best possible description of the device’s attitude in space.

Shake Events

A shake event is a UIEvent (Chapter 18). Receiving shake events is rather like receiving remote events (Chapter 27), involving the notion of the first responder. To receive shake events, your app must contain a UIResponder which:

• Returns YES from canBecomeFirstResponder
• Is in fact first responder

This responder, or a UIResponder further up the responder chain, should implement some or all of these methods:

motionBegan:withEvent: ...