-(void)fadeOutIn:(UIView *)view duration:(NSTimeInterval)duration
{
    static CGFloat alpha = 1.0;
    [UIView animateWithDuration:duration
                          delay:0.0
                        options: UIViewAnimationOptionRepeat| UIViewAnimationOptionCurveEaseInOut
                     animations:^{
                         alpha = abs((int)(alpha - 1.0));
                         view.alpha = alpha;
                     }
                     completion:^(BOOL finished){
                     }];
}

-(void)fadeOutIn:(UIView *)view duration:(NSTimeInterval)duration
{
    [UIView animateWithDuration:duration
                          delay:0.0
                        options: UIViewAnimationOptionRepeat| UIViewAnimationOptionCurveEaseInOut
                     animations:^{
                         view.alpha = 0.0;
                     }
                     completion:^(BOOL finished){

                         [UIView animateWithDuration:duration
                                               delay:0.0
                                             options: UIViewAnimationOptionRepeat|UIViewAnimationOptionCurveEaseInOut
                                          animations:^{
                                              view.alpha = 1.0;
                                          }
                                          completion:^(BOOL finished){

                                          }];
                     }];
}

[UIView animateKeyframesWithDuration:1 delay:0 options:UIViewAnimationOptionRepeat|UIViewAnimationOptionAutoreverse|UIViewAnimationCurveEaseInOut animations:^{
    self.customIV.alpha = 0;
} completion:^(BOOL finished) {
}];

-(void)fadeOutIn
{
    [UIView animateWithDuration:1
                          delay:0.0
                        options:  UIViewAnimationOptionCurveEaseInOut
                     animations:^{
                         [[self contentView] emailField].alpha = 0.0;
                     }
                     completion:^(BOOL finished){

                         [UIView animateWithDuration:1
                                               delay:0.0
                                             options: UIViewAnimationOptionCurveEaseInOut
                                          animations:^{
                                              [[self contentView] emailField].alpha = 1.0;
                                          }
                                          completion:^(BOOL finished){
                                              [self fadeOutIn];

                                          }];
                     }];
}

[UIView animateWithDuration:duration
                      delay:0.0
                    options: UIViewAnimationOptionRepeat| UIViewAnimationOptionAutoreverse|UIViewAnimationOptionCurveEaseInOut
                 animations:^{
                               view.alpha = 0.0;
                             }
                 completion:nil];

YourView.alpha = 1;// set the initial value of alpha

[UIView animateWithDuration:1 delay:0
                            options: UIViewAnimationOptionAutoreverse | UIViewAnimationOptionRepeat
                         animations:^{
                                YourView.alpha = 0; // set the max value of alpha
                            } completion:nil];

UIView.animateKeyframes(withDuration: 1, delay: 0, options: [UIView.KeyframeAnimationOptions(rawValue: UIView.AnimationOptions.repeat.rawValue), UIView.KeyframeAnimationOptions(rawValue: UIView.KeyframeAnimationOptions.RawValue(UIView.AnimationCurve.easeInOut.rawValue)), UIView.KeyframeAnimationOptions(rawValue: UIView.AnimationOptions.autoreverse.rawValue)] , animations: {
        self.logoCenterImageView.alpha = 0
    }) { finished in
    }