Why isn't MvxStandardTableViewSource SelectedItem updating properly?

Question

I'm trying to bind a UITableView to an ObservableCollection<MyTypeViewModel> using MvxStandardTableViewSource, and I'm getting weird behavior and bindings that aren't working

The (partial) code is this:

tblFeatures = new UITableView ();
mSource = new MvxStandardTableViewSource(tblFeatures, "TitleText Name");

var set = this.CreateBindingSet<MyTypeView, MyTypeViewModel> ();

set.Bind(mSource).To (vm => vm.Objects);
set.Bind(mSource).For(s => s.SelectedItem).To (vm => vm.SelectedObject);

set.Apply ();

What I'm seeing is that when I select an item in the table, it does update SelectedObject with the new value. But when I change SelectedObject by some other means, the table does not update the displayed selected item. I have verified that the SelectedObject really is being changed, by setting a breakpoint and by binding another control to it; the other control (a label) does change as the selection changes, but the UITableView does not.

Am I doing something wrong, or could this be an issue in MVVMCross?

Stuart Stuart · Accepted Answer · 2013-07-05T05:37:38

The selecteditem binding was requested as a 'nice to have' and was mainly requested for the user story where a user clicks on a list item - so this is currently a 'one way to source' implementation.

The very basic details and some code can be found via - https://github.com/slodge/MvvmCross/issues/278

In truth, the current binding is abusing the 'selected' paradigm - as it doesn't really reflect the selection state of the cell and also has no handling of mutiselect - instead it just lets the viewmodel know the latest tapped item.

If there are genuine user stories/requirements for full two'way binding, then these should be addable - eg if the user story were for scrolling the item into view then 'scrollToRowAt' could be used - see How to scroll UITableView to specific position - but this again might not be true 'selection' within the uitableview.

if you wanted to implement this yourself it should be fairly straight-forward to override the binding with your own one (see the custom binding n+1 in http://mvvmcross.wordpress.com for an intro on custom bindings).

could this be an issue in MVVMCross?

Overall I think I'd categorise it as a known limitation of that particular binding caused by lack of user demand and by some confusion over whether selection genuinely means selection in the touch screen era.

if it was logged as a bug I'd say it wasn't.. If it was logged ss a feature request I'd try getting as much user input as i could about what devs really want from this, whether they want true cell selection or scroll into view, both or something more.

Why isn't MvxStandardTableViewSource SelectedItem updating properly?

2 Answers