ios - I need help animating UICollectionView cell size when collection view size changes -

i have uicollectionview takes entire screen, , inside cells same exact size collection view. when user hits button, collection view animates take half screen (and view shown beside it). fine, cells still size of original collection view.

i can call invalidatelayout on collection view's layout, , cause collectionview:layout:sizeforitematindexpath: called , cells recalcualted, , sizes changes, these not animate, snap new size.

is there way animate these cell size changes @ same time, or in same animation block animates size change parent collection view?

 [_collectionview performbatchupdates:^{      (int =0; i<count; i++) {         nsindexpath *indexpath = [nsindexpath indexpathforrow:i insection:0];         uicollectionviewcell *cell = [_collectionview cellforitematindexpath:indexpath];         cell.layer.transform = catransform3dmakescale(0.9, 0.9, 1);     }  } completion:nil];