UIAccelerometer Handle - Shake

// Ensures the shake is strong enough on at least two axes before declaring it a shake.
// "Strong enough" means "greater than a client-supplied threshold" in G's.
static BOOL L0AccelerationIsShaking(UIAcceleration* last, UIAcceleration* current, double threshold) {
        double
                deltaX = fabs(last.x - current.x),
                deltaY = fabs(last.y - current.y),
                deltaZ = fabs(last.z - current.z);
 
        return
                (deltaX > threshold && deltaY > threshold) ||
                (deltaX > threshold && deltaZ > threshold) ||
                (deltaY > threshold && deltaZ > threshold);
}
 
@interface L0AppDelegate : NSObject <UIApplicationDelegate> {
        BOOL histeresisExcited;
        UIAcceleration* lastAcceleration;
}
 
@property(retain) UIAcceleration* lastAcceleration;
 
@end
 
@implementation L0AppDelegate
 
- (void)applicationDidFinishLaunching:(UIApplication *)application {
        [UIAccelerometer sharedAccelerometer].delegate = self;
}
 
- (void) accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
 
        if (self.lastAcceleration) {
                if (!histeresisExcited && L0AccelerationIsShaking(self.lastAcceleration, acceleration, 0.7)) {
                        histeresisExcited = YES;
 
                        /* SHAKE DETECTED. DO HERE WHAT YOU WANT. */
 
                } else if (histeresisExcited && !L0AccelerationIsShaking(self.lastAcceleration, acceleration, 0.2)) {
                        histeresisExcited = NO;
                }
        }
 
        self.lastAcceleration = acceleration;
}
 
// and proper @synthesize and -dealloc boilerplate code
 
@end