Inspect a View's NIB to determine its preferred/natural size

PreferredViewSizeLookup.m

// This code assumes that ARC is enabled.

// Returns the size of this View in its NIB.
+ (CGSize)preferredSize
{
    static NSValue *sizeBox = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // Assumption: The XIB file name matches this UIView subclass name.
        UINib *nib = [UINib nibWithNibName:NSStringFromClass(self) bundle:nil];
        
        // Assumption: The XIB file only contains a single root UIView.
        UIView *rootView = [[nib instantiateWithOwner:nil options:nil] lastObject];
        
        // Cache the size of the UIView in its natural state.
        sizeBox = [NSValue valueWithCGSize:rootView.frame.size];
    });
    return [sizeBox CGSizeValue];
}

/* Example usage
- (CGSize)collectionView:(UICollectionView *)collectionView
                  layout:(UICollectionViewLayout*)collectionViewLayout
  sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return [JASCollectionViewCell preferredSize];
}
*/