Xamarin - Assigning UICollectionViewDelegate wipes out event handlers (and vice versa)

Question

I have a UICollectionView from which I need to listen to scrolling and selection events independently. I assign the Delegate and the Scrolled event handler as follows:

public override void ViewWillAppear(bool animated)
(
    base.ViewWillAppear(animated);
    this.CollectionView.Delegate = this.CollectionViewDelegate;
    this.CollectionView.Scrolled += HandleCollectionViewScrolled;
}

However, after I assign the event handler, the delegate methods no longer get called. And reversing them:

public override void ViewWillAppear(bool animated)
(
    base.ViewWillAppear(animated);
    this.CollectionView.Scrolled += HandleCollectionViewScrolled;
    this.CollectionView.Delegate = this.CollectionViewDelegate;
}

yields the exact opposite result (the delegate methods working but no scrolled listener).

Thinking that the strongly typed delegate's necessary implementation of all methods might be wiping the event handlers out, I tried instead to assign the WeakDelegate property, which is an NSObject subclass that only implements collectionView:didSelectItemAtIndexPath:.

public class MyCollectionViewDelegate : NSObject
{
    public MyCollectionViewDelegate() : base()
    {
    }

    [Export ("collectionView:didSelectItemAtIndexPath:")]
    public void ItemSelected(UICollectionView collectionView, MonoTouch.Foundation.NSIndexPath indexPath)
    {
        Console.WriteLine("It worked.");
    }
}

But again, I get the same result: only the event handler or the delegate fires. Has anyone else experienced this? Is this an issue with Xamarin? I would expect that setting the weak delegate shouldn't necessarily wipe out the event handlers.

It's also worth noting that as a workaround, I tried using KVO. But KVO crashes the app when I try to observe the contentOffset property of the collection view (perhaps I'm using the wrong key path name).

Answer 1

Short Answer:

This is by design. The .NET events are implemented by using an internal *Delegate implementation (there's simply no other way to provide them).

As such you cannot set your own *Delegate without disabling any existing events.

Long Answer:

Here's my blog post that describe this.