Size methods don't get called for detail view controller in UISplitViewController iOS8

Question

I have a UIViewController containing a UICollectionView using a UICollectionViewFlowLayout. The view controller is the detail view controller of a UISplitViewController.

The view controller is the flow layout's delegate and the sizes of the UICollectionViewCells are calculated according to the available space in the collection view. (for instance itemSize = width/3,height/2)

In iOS8 Apple introduced the API to collapse the master section of the split view controller.

I want to invalidate my collection view's layout when the collection view expands in order to force recalculation of the item sizes.

I thought of adding the appropriate code in the new size methods introduced in iOS8 - but none of them are called when collapsing the master's side.

Here is the code I've tried. It isn't called:

-(void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {

    [coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {

    } completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {
        [self.collectionView.collectionViewLayout invalidateLayout];
        [self.collectionView reloadData];
    }];
}

-(void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection {
    [super traitCollectionDidChange:previousTraitCollection];
    [self.collectionView.collectionViewLayout invalidateLayout];
    [self.collectionView reloadData];
}