滚动时出现MvxTableViewSource DequeueReusableCell问题

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?

Answer 1

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.

Answer 2

I spent 10 minutes creating this sample app - github.com/slodge/ListApp

It uses DequeueReusableCell code exactly like the code shown in your question and it seems to work fine

Given this result, I suspect the problem needs investigating within your app.

I would suggest:

1. making a cup of tea,
2. looking at the display carefully - can you identify which cells and which cell values are "incorrect"? can you add a debug field to those cells which tells you whether the cell has the correct list item as it's datacontext
3. once you have identified what is going wrong, look at how those "incorrect" cells and cell values are created and set - this may lead you to identify what the problem is
4. if not, then try to reproduce the effects in a separate test app like https://github.com/slodge/ListApp - this will be something you can share with your colleagues and with others on here in order that they can assist

As far as I can see, this is just normal debugging and development (especially step 1)