How do I know that the UICollectionView has been loaded completely?

This worked for me:

[self.collectionView reloadData];
[self.collectionView performBatchUpdates:^{}
                              completion:^(BOOL finished) {
                                  /// collection-view finished reload
                              }];

Swift 4 syntax:

collectionView.reloadData()
collectionView.performBatchUpdates(nil, completion: {
    (result) in
    // ready
})