// this code will cause a UIView to shake--good for "login error"

CAKeyframeAnimation * anim = [ CAKeyframeAnimation animationWithKeyPath:@"transform" ] ;
anim.values = [ NSArray arrayWithObjects:
                   [ NSValue valueWithCATransform3D:CATransform3DMakeTranslation(-5.0f, 0.0f, 0.0f) ],
                   [ NSValue valueWithCATransform3D:CATransform3DMakeTranslation(5.0f, 0.0f, 0.0f) ], 
                   nil ] ;
anim.autoreverses = YES ;
anim.repeatCount = 2.0f ;
anim.duration = 0.07f ;
    
[ viewToShake.layer addAnimation:anim forKey:nil ] ;