controller.h

- (void)viewWillTransitionToSize:(CGSize)size
       withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
    NSUInteger currentAccountIndex = self.currentAccountIndex;
    [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];

    [coordinator animateAlongsideTransition:nil
                                 completion:^ (id<UIViewControllerTransitionCoordinatorContext> context) {
                                     self.currentAccountIndex = currentAccountIndex;
                                     self.accountsCollectionView.contentOffset = CGPointMake(self.accountsCollectionView.width * currentAccountIndex, 0.f);
                                 }];
}