Adding a button in UICollectionViewCell with a click handler using Xamarin.iOS - xamarin.ios

Question

I am rewriting a screen that used a UITableView before to UICollectionView. But I am having trouble with the click handler on a button inside the cell.
collectionView.RegisterNibForCell (DocumentViewCell.Nib, docCellId);
...
public override UICollectionViewCell GetCell (UICollectionView collectionView, MonoTouch.Foundation.NSIndexPath indexPath)
{
var docCell = (DocumentViewCell)collectionView.DequeueReusableCell (docCellId, indexPath);
docCell.BtnDelete.Hidden = !EditMode;
docCell.BtnDelete.TouchUpInside += delegate(object sender, EventArgs e) {
Logging.Debug("Crashes before if reaches here");
};
return docCell;
}
I am aware that the cell is begin reused and this most likely won't work when that happens but now it crashes immediately with just one element in the list when pressing the delete button. Everything works great until I hit the button and then I get the stacktrace below. I see no reason for that happening based on my UITableView code that is almost identical.
Have anyone done this using Nib based CollectionView cells? Any help is very appreciated!
Stacktrace:
at (wrapper managed-to-native) MonoTouch.UIKit.UIApplication.UIApplicationMain (int,string[],intptr,intptr) <IL 0x0009f, 0xffffffff>
at MonoTouch.UIKit.UIApplication.Main (string[],string,string) [0x0004c] in /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:38
at SalesApp.Application.Main (string[]) [0x00000] in /MyApp/Main.cs:18
at (wrapper runtime-invoke) <Module>.runtime_invoke_void_object (object,intptr,intptr,intptr) <IL 0x00050, 0xffffffff>
Native stacktrace:
0 SalesApp 0x0009a85c mono_handle_native_sigsegv + 284
1 SalesApp 0x0000e138 mono_sigsegv_signal_handler + 248
2 libsystem_c.dylib 0x990a78cb _sigtramp + 43
3 ??? 0xffffffff 0x0 + 4294967295
4 UIKit 0x01990258 -[UIApplication sendAction:toTarget:fromSender:forEvent:] + 61
5 UIKit 0x01a51021 -[UIControl sendAction:to:forEvent:] + 66
6 UIKit 0x01a5157f -[UIControl(Internal) _sendActionsForEvents:withEvent:] + 578
7 UIKit 0x01a506e8 -[UIControl touchesEnded:withEvent:] + 546
8 UIKit 0x01c541d3 _UIGestureRecognizerUpdate + 7407
9 CoreFoundation 0x03ecbafe __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 30
Update
This code works fine
public override UICollectionViewCell GetCell (UICollectionView collectionView, MonoTouch.Foundation.NSIndexPath indexPath)
{
var docCell = (DocumentViewCell)collectionView.DequeueReusableCell (DocumentViewCell.Key, indexPath);
docCell.BtnDelete.Hidden = !EditMode;
docCell.BtnDelete.TouchUpInside -= HandleTouchUpInside;
docCell.BtnDelete.TouchUpInside += HandleTouchUpInside;
return docCell;
}
void HandleTouchUpInside (object sender, EventArgs e)
{
Logging.Debug("No crash");
}

Answer 1

The issue is that there's no managed reference to the docCell instance you return from GetCell method. That means the GC can collect it whenever it wants.
When it's needed again it will be resurfaced to the managed world (using the IntPtr constructor). It's gonna be the same (re-used) native instance but a new managed instance. Your crash occurs because the event points to the old, managed, instance (that was collected).
The easy solution is to keep a cache of the created cells as along as the view exists. That will ensure the GC won't collect the (managed) cells before they are used. There's several answers showing this for UITableView.
Note: I thought we fixed (hided) this in recent version of Xamarin.iOS. Maybe it was just for UITableView !?! need to check on this...

Answer 2

I guess DequeueReusableCell returns null as it's the first cell being drawn.
Your code should look like this instead:
public override UICollectionViewCell GetCell (UICollectionView collectionView, MonoTouch.Foundation.NSIndexPath indexPath)
{
var docCell = (DocumentViewCell)collectionView.DequeueReusableCell (docCellId, indexPath);
if (docCell == null)
docCell = new UICollectionViewCell (...);
docCell.BtnDelete.TouchUpInside += delegate(object sender, EventArgs e) {
Logging.Debug("Crashes before if reaches here");
};
return docCell;
}
replace the ellipses (...) by whatever creates your cell.