OnPropertyChanged does not update the View

Question

I have a ListView which has its itemsource to Elements which is a List of Schranken-Objects. The SchrankePresenter is the DataContext of the page(set in the constructor).

The page is displayed correctly, until a button is clicked. If a button is clicked, the value Name of the button should change and the ListView should have one item more, but nothing happens/updates.

Here is the SchrankenPresenter:

 public class SchrankePresenter : INotifyPropertyChanged
    {
        private List<Schranke> _elements;
        public List<Schranke> Elements
        {
            get { return _elements; }
            set
            {
                _elements = value;
                OnPropertyChanged("Elements");
            }
        }
        public ICommand ClickCommand { get; set; }
    private void OnPropertyChanged(string propName)
    {
        Debug.WriteLine($"on Property changed with {propName}");
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public SchrankePresenter()
    {
        var elements = new List<Schranke>();
        for (var i = 1; i < 15; i++)
            elements.Add(new Schranke() { Name = $"{i}: Schranke" });

        Elements = elements;
        Debug.WriteLine("ctor");
        ClickCommand = new DelegateCommand<Schranke>(ClickAction);
    }

    public void ClickAction(Schranke item)
    {
        VibrationDevice.GetDefault().Vibrate(TimeSpan.FromMilliseconds(150));

        Debug.WriteLine($"{item?.Name} was clicked");
        item.Name = "was clicked";
        OnPropertyChanged("Name");
        Elements.Add(new Schranke() { Name = "addednew element" });
        OnPropertyChanged("Elements");

    }

}

The class Schranke has only one member: public string Name { get; set; }

The view looks like:

 <ListView ItemsSource="{Binding Elements, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" x:Name="lv_schranken" Grid.Row="1" SelectedItem="{Binding  SelectedItem}">

            <ListView.ItemTemplate>
                <DataTemplate>
                    <Button Height="80"
                            HorizontalAlignment="Stretch"
                            Command="{Binding ElementName=lv_schranken, Path=DataContext.ClickCommand}"
                            CommandParameter="{Binding}"
                            Style="{StaticResource TransparentStyle}">
                        <Grid>
                                .
                                .  (some grid stuff)
                                .

                            <TextBlock Name="schranken_name"
                                       Grid.Row="1"
                                       Grid.Column="1"
                                       HorizontalAlignment="Center"
                                       VerticalAlignment="Center"
                                       FontWeight="ExtraLight"
                                       FontSize="25"
                                       Foreground="Black"
                                       Text="{Binding Name, Mode=TwoWay}" />
                        </Grid>
                    </Button>
                </DataTemplate>

            </ListView.ItemTemplate>

        </ListView>

PS: if some needs the DelegateCommand - Implementation:

 public class DelegateCommand<T> : ICommand
    {
    private Action<T> _clickAction;

    public DelegateCommand(Action<T> clickAction)
    {
        _clickAction = clickAction;
    }

    public event EventHandler CanExecuteChanged;

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        _clickAction((T)parameter);
    }
}

`OnPropertyChanged("Name");` should be called in `Schranke` class, in `Name` setter. — ASh, Jul 26 '16 at 10:53
If you had done any research at all you would know that one does not bind to `List`. — H.B., Jul 26 '16 at 10:53
It is, but you should should not do it; it's inefficient and essentially a hack. — H.B., Jul 26 '16 at 10:58

score 1 · Accepted Answer · answered Jul 26 '16 at 11:02

If your want to bind to a List of items, you should bind to an ObservableCollection:

Represents a dynamic data collection that provides notifications when items get added, removed, or when the whole list is refreshed.

https://msdn.microsoft.com/de-de/library/ms668604(v=vs.110).aspx

score 0 · Answer 2 · edited May 23 '17 at 12:14

0

For updating the UI from Viewmodel we're required to raised inotifypropertychanged; observablecollection provides the inotifypropertychanged implementation by default; we can refer observablecollection for MVVm pattern.

If you want to use list for the same then you need to implement the inotifypropertychanged on that particular list

edited May 23 '17 at 12:14

Community

1
1

answered Jul 26 '16 at 11:31

Arjun Balip

21
7

OnPropertyChanged does not update the View

2 Answers2