MvxTableViewSource DequeueReusableCell issue when scrolling

Question

We have set-up our tableviewSource and a tableview but are having issues with the data appearing in the wrong cells and select states being applied to multiple cell when scrolling a large amount of cells.

The table cells are set up according to n+1 tutorial on table cells/table views and only cause problems when part of the table is hidden due to size.

Any ideas on how to solve this?

var menuItem = item as MenuItemViewModel;

if (menuItem != null && menuItem.ViewModelType == null)
{
    NSString cellId = new NSString("Menu Header Cell");
    var headerCell = tableView.DequeueReusableCell(cellId, indexPath) as MenuItemHeaderCell;

    if (headerCell == null)
    {
        headerCell = new UITableViewCell(UITableViewCellStyle.Default, cellId) as MenuItemHeaderCell;
    }
    return headerCell;
}
else
{
    NSString cellId = new NSString("Menu Cell");
    var cell = tableView.DequeueReusableCell(cellId, indexPath) as MenuItemCell;
    if (cell == null)
    {
        cell = new UITableViewCell(UITableViewCellStyle.Default, cellId) as MenuItemCell;
    }
    return cell;
}

Following your comment I have updated the code as follows:

private static readonly NSString MenuHeaderCellIdentifier = new NSString("Menu Header Cell"); private static readonly NSString MenuItemCellIdentifier = new NSString("Menu Cell");

        public MenuTableViewSource(UITableView tableView): base(tableView)
        {
            tableView.RegisterClassForCellReuse(typeof(MenuItemHeaderCell), MenuHeaderCellIdentifier);
            tableView.RegisterClassForCellReuse(typeof(MenuItemCell), MenuItemCellIdentifier);
        }


        protected override UITableViewCell GetOrCreateCellFor(UITableView tableView, NSIndexPath indexPath, object item)
        {
            NSString cellIdentifier;

            var menuItem = item as MenuItemViewModel;
            if (menuItem.ViewModelType == null)
            {
                cellIdentifier = MenuHeaderCellIdentifier;
            }
            else
            {
                cellIdentifier = MenuItemCellIdentifier;

            }
            var cell = (UITableViewCell)TableView.DequeueReusableCell(cellIdentifier, indexPath);
            return cell;

        }

And the bindings are set up as follows:

var source = new MenuTableViewSource(TableView);
TableView.Source = source;
TableView.RowHeight = 46;

var set = this.CreateBindingSet<LeftMenuView, LeftMenuViewModel>();
set.Bind(source).To(vm => vm.MenuItems);
set.Apply();

TableView.ReloadData();

And the re-use issue is still happening. Basically if cells are off screen they start to pick up incorrect values when scrolling. Is there anything else we should be doing?

Do share your code so people can see where you are making a mistake ;) — Mohib Sheth
@MohibSheth is correct - can you post an example DequeueReusableCell that reproduces the effect? Without this, I doubt anyone can really help. — Stuart
The code for new UITableViewCell(...) as MenuItemHeaderCell; is wrong - that as will always be null - but if you've used the RegisterCell APIs then I doubt its ever called. I just tried with a kitten N+1 sample and could not see any effect. Can you post an example that reproduces the effect? — Stuart
@Stuart I have updated the table cell code using this example : github.com/MvvmCross/MvvmCross/wiki/Tables-and-Cells-in-iOS and the cell re-use issue is still occurring any other ideas would be much appreciated. — user1162743

user1162743 user1162743 · Accepted Answer · 2013-11-12T11:17:54

Thanks Stuart for your sample code. After many hours of investigating the issue I found it was only happening on iOS7 after a few more hours I realised it was down to the fact we adding a locally created variable and then delay binding to it, which seemed to result in the value going out of scope. Changing this to a member variable stopped this happening, what a long day, added to the bank of gotcha's.

MvxTableViewSource DequeueReusableCell issue when scrolling

2 Answers