GetViewForSupplementaryElement not getting called when trying to show heading in UICollectionView

Question

I am trying to setup a UICollectionView within my existing UIViewController. Everything is working except for getting a title to show for each section - I can't figure out what I'm doing wrong.

My code in the UIViewController to initiate the collection view:

public partial class ViewController : UIViewController
{
    //...

    public override void ViewDidLoad ()
    {
        base.ViewDidLoad ();

        CollectionView_Outlet.RegisterClassForCell(typeof(ModifierCell), ModifierCell.CellID);
        CollectionView_Outlet.RegisterClassForSupplementaryView (typeof(Header), UICollectionElementKindSection.Header, Header.HeaderId);
        CollectionView_Outlet.ShowsHorizontalScrollIndicator = false;
        CollectionView_Outlet.Source = new ModifiersSource(this);
        CollectionView_Outlet.BackgroundColor = UIColor.White;
        CollectionView_Outlet.ReloadData();
    }

    //...
}

Then I have created a subclass of UICollectionViewSource:

public class ModifiersSource : UICollectionViewSource
{
    ViewController senderVC;

    public ModifiersSource(ViewController sender)
    {
        senderVC = sender;
    }

    public override nint NumberOfSections(UICollectionView collectionView)
    {
        return 2;
    }

    public override nint GetItemsCount (UICollectionView collectionView, nint section)
    {
        return senderVC.modifiers.Count;
    }

    public override UICollectionViewCell GetCell(UICollectionView collectionView, NSIndexPath indexPath)
    {
        //...
    }


    public override UICollectionReusableView GetViewForSupplementaryElement(UICollectionView collectionView, NSString elementKind, NSIndexPath indexPath)
    {
        var headerView = (Header)collectionView.DequeueReusableSupplementaryView (elementKind, Header.HeaderId, indexPath);
        headerView.Text = "Supplementary View";
        return headerView;  
    }

}

And finally created:

public class Header : UICollectionReusableView
{
    public static NSString HeaderId = new NSString("UserSource1");
    UILabel label;

    public string Text {
        get {
            return label.Text;
        }
        set {
            label.Text = value;
            SetNeedsDisplay ();
        }
    }

    [Export ("initWithFrame:")]
    public Header (RectangleF frame) : base (frame)
    {
        label = new UILabel (){
            Frame = new RectangleF(0,0,300,50), 
            BackgroundColor = UIColor.Red};
        AddSubview (label);
        BackgroundColor = UIColor.White;
    }
}

I've put a breakpoint on the GetViewForSupplementaryElement method but it never gets called. I've also set the following in my StoryBoard:

What am I missing?!

Answer 1

After many attempts of not being able to get the above to work, I manually set UICollectionViewFlowLayout whilst initiating the UIContainerView. Seems to have done the trick, but not sure why it didn't pick up the settings from my StoryBoard. Here is my working code:

public override void ViewDidLoad ()
    {
        base.ViewDidLoad ();

        CollectionView_Outlet.RegisterClassForCell(typeof(ModifierCell), ModifierCell.CellID);
        CollectionView_Outlet.RegisterClassForCell(typeof(ItemOptionCell), ItemOptionCell.CellID);
        CollectionView_Outlet.RegisterClassForSupplementaryView (typeof(Header), UICollectionElementKindSection.Header, Header.HeaderId);
        CollectionView_Outlet.ShowsHorizontalScrollIndicator = false;

        //This is the new bit I added:
        var layout = new UICollectionViewFlowLayout ();
        layout.HeaderReferenceSize = new CGSize (300, 40);
        CollectionView_Outlet.SetCollectionViewLayout (layout, false);

        CollectionView_Outlet.Source = new ModifiersSource(this);
        CollectionView_Outlet.ReloadData();
    }