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

It's actually rather very simple.

When you for example call the UICollectionView's reloadData method or it's layout's invalidateLayout method, you do the following:

dispatch_async(dispatch_get_main_queue(), ^{
    [self.collectionView reloadData];
});

dispatch_async(dispatch_get_main_queue(), ^{
    //your stuff happens here
    //after the reloadData/invalidateLayout finishes executing
});

Why this works:

The main thread (which is where we should do all UI updates) houses the main queue, which is serial in nature, i.e. it works in the FIFO fashion. So in the above example, the first block gets called, which has our reloadData method being invoked, followed by anything else in the second block.

Now the main thread is blocking as well. So if you're reloadData takes 3s to execute, the processing of the second block will be deferred by those 3s.

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
})

// In viewDidLoad
[self.collectionView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionOld context:NULL];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary  *)change context:(void *)context
{
    // You will get here when the reloadData finished 
}

- (void)dealloc
{
    [self.collectionView removeObserver:self forKeyPath:@"contentSize" context:NULL];
}