Clicking a SegmentedControl to UITableViewCell in Monotouch causes SIGABRT - uitableview

Question

In my Monotouch application for the iPhone, I'm trying to add a segmented controller to a UITableViewCell to act as a toggle for a value in each row. Everything displays correctly, and the first rows will initially work correctly when clicked. However, after the row has left the screen and is reloaded, if I clicked the segmented controller again I receive an exception:
Terminating runtime due to unhandled exception
[ERROR] FATAL UNHANDLED EXCEPTION: System.Exception: Selector invoked from objective-c on a managed object of type MonoTouch.UIKit.UIControlEventProxy (0x16D793C0) that has been GC'ed ---> System.MissingMethodException: No constructor found for MonoTouch.UIKit.UIControlEventProxy::.ctor(System.IntPtr)
I have heard of similar issues with people using Monotouch try to put buttons in table cells. The general recommendation has been to add each table cell to a List to prevent it from being garbage collected, but this has not worked for me.
Is there a solution to get around this problem?
Here is a somewhat simplified version of my GetCell code:
public override UITableViewCell GetCell (UITableView tableView, NSIndexPath indexPath)
{
Hashtable rowData = dataList [indexPath.Row];
FilterListToggleCell cell;
cell = (FilterListToggleCell)tableView.DequeueReusableCell ("FilterTogglePlusCell");
cacheList.Add(cell.Toggle);//cache only the segcontrol
//cacheList.Add(cell);//also previously tried caching the whole cell
cell.Title.Text = (string)rowData["title"];
if(additionalFields.Contains((string)rowData["key"])){
cell.Toggle.SelectedSegment = 0;
} else {
cell.Toggle.SelectedSegment = 1;
}
cell.Toggle.ValueChanged += (sender, e) => {
UISegmentedControl toggle = (UISegmentedControl)sender;
if(toggle.SelectedSegment == 0){
Console.WriteLine("Toggle YES");
} else {
Console.WriteLine("Toggle NO");
}
};
cell.Toggle.Tag = indexPath.Row;
return cell;
}

Answer 1

You're right about the basic condition. This condition happens because there's no managed reference to the UITableViewCell that you return from the GetCell method. As such the GC can collect and free it (and that will cause issues for things like events that wants to call back to the managed instance). See this answer for more details.
Now I'm not sure why this approach did not work for you. However your code above seems incomplete, e.g.
cell = (FilterListToggleCell)tableView.DequeueReusableCell ("FilterTogglePlusCell");
can return null which is when you should be creating more cells (in fact there's no condition where you actually create any new cell, so there's nothing that can be re-used).
Your next line will throw a NullReferenceException when cell is null (because of cell.Toggle).
Note that this does not explain your crash - I suspect there's some missing code that make it works (create cells) and that might not cache them correctly.

Answer 2

I was able to get this working without needing the extra cache based on information from this question: How do I remove/unregister event handler from an event?
The ValueChanged line was replaced with:
cell.Toggle.AddTarget(this, new Selector("ToggleChange"),UIControlEvent.ValueChanged);
and another method was added to my UITableViewDataSource
[Export ("ToggleChange")]
void OnChange(UISementedControl toggle){
if(toggle.SelectedSegment == 0){
Console.WriteLine("Toggle YES");
} else {
Console.WriteLine("Toggle NO");
}
}