Detect shake for iPhone

// 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