Basic Sensors in iOS

Detecting Shaking

Apple’s shake-detection algorithm analyses eight to ten successive pairs of raw accelerometer triplet values and determines the angle between these readings. If the change in angular velocity between successive data points is large then the algorithm determines that a UIEventSubtypeMotionShake has occurred, and the motionBegan:withEvent: delegate method is called. Conversely, if the change in angular velocity is small and a shake event has been triggered, the motionEnded:withEvent: delegate method is called.

Note

The iPhone is better at detecting side-to-side rather than front-to-back or up-and-down motions. Take this into account in the design of your application.

There are three motion delegate methods, mirroring the methods for gesture handling: motionBegin:withEvent:, motionEnded:withEvent: and motionCancelled:withEvent:. The first indicates the start of a motion event, the second the end of this event. You cannot generate a new motion event for a second (or two) following the first event. The final delegate method is called when a motion is interrupted by a system event, such as an incoming phone call.

Let’s go ahead and add shake detection to our Accelerometer application. You’ll need to add another UILabel to the UI that will change depending on the motion event status. Click on the AccelerometerViewController.h interface file to open it in the Standard Editor and add another UILabel marked as an IBOutlet to the class definition:

@interface AccelerometerViewController ...