imageView切换image 动画过度

                    CATransition *transition = [CATransition animation];
                    transition.duration = 2.0f;
                    transition.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];
                    transition.type = kCATransitionFade;
                    [bkView.layer addAnimation:transition forKey:@"transition"];