gistfile1.m

// in tableVC

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    JotoEventTableViewCell *sizingCell = [tableView dequeueReusableCellWithIdentifier:@"JotoEvent"];
    JotoEvent *event = [self.fetchedResultsController objectAtIndexPath:indexPath];
    [sizingCell updateFriendsForEvent:[event.friends allObjects]];
    sizingCell.frame = CGRectMake(0, 0, self.tableView.frame.size.width, sizingCell.frame.size.height);
    [sizingCell setNeedsLayout];
    [sizingCell layoutIfNeeded];
 
    CGFloat height = [sizingCell.contentView systemLayoutSizeFittingSize:UILayoutFittingCompressedSize].height;
    // Add an extra point to the height to account for the cell separator, which is added between the bottom
    // of the cell's contentView and the bottom of the table view cell.
    height += 1.0f;
    return height;
}


// in cell

- (void) updateFriendsForEvent:(NSArray *)friends {
    self.friends = friends;
    [self.contentView removeConstraint: self.categoriesToSuperViewBottomConstraint]];
    [self.contentView removeConstraint: self.collectionViewToSuperViewBottomConstraint]];
    self.collectionViewToSuperViewBottomConstraint = self.categoriesToSuperViewBottomConstraint = nil;
    [self updateConstraints];
}

- (void) updateConstraints {
    if(!self.categoriesToSuperViewBottomConstraint) {
        self.categoriesToSuperViewBottomConstraint = [NSLayoutConstraint constraintWithItem:self.categoriesLabel attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.contentView attribute:NSLayoutAttributeBottom multiplier:1 constant:8];
        self.collectionViewToSuperViewBottomConstraint = [NSLayoutConstraint constraintWithItem:self.collectionView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.contentView attribute:NSLayoutAttributeBottom multiplier:1 constant:8];
        if(self.friends.count == 0) {
            self.categoriesToSuperViewBottomConstraint.priority = 1000;
            self.collectionViewToSuperViewBottomConstraint = 999;
        } else {
            self.categoriesToSuperViewBottomConstraint.priority = 999;
            self.collectionViewToSuperViewBottomConstraint = 1000;
        }
        [self.contentView addConstraints:@[self.categoriesToSuperViewBottomConstraint, self.collectionViewToSuperViewBottomConstraint]]
        [super updateConstraints];
    }
}