CABasicAnimation旋转view，设置anchorPoint的，view会向上浮动一段距离再旋转。

CABasicAnimation旋转view，设置anchorPoint的，view会向上浮动一段距离再旋转。 请问向上浮动是为什么出现，怎么消除？
CABasicAnimation*rotationAnimation = [CABasicAnimation animationWithKeyPath:@"transform.rotation"];
rotationAnimation.toValue = [NSNumber numberWithFloat: M_PI * -0.3 ];
[CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];
rotationAnimation.duration = 0.3;
rotationAnimation.RepeatCount = 0;
rotationAnimation.cumulative = NO;
rotationAnimation.removedOnCompletion = NO;
rotationAnimation.fillMode = kCAFillModeForwards;

button.layer.anchorPoint=CGPointMake(0.5,1);
[button.layer addAnimation:rotationAnimation forKey:@"Rotation"];