// 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 { 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