How to update bindings inside DataTemplate?

Question

Let's say I have the following ListView in my view

<ListView x:Name="ListView1" ItemsSource="{Binding SomeCollection}">
    <ListView.ContextMenu>
        <ContextMenu>
            <MenuItem Header="Do something" Command="{Binding SomeCommand, Mode=OneWay}" />
        </ContextMenu>
    </ListView.ContextMenu>
    <ListView.ItemTemplate>
        <DataTemplate DataType="model:Person">
            <StackLayout>
                <TextBlock Text="{Binding Name}">
                <Image Source="{Binding State, Converter={StaticResource StateToIconConverter}, Mode=OneWay}" />
            </StackLayout>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

Now, this Person is a model and its properties do not have any way to inform the view of them being updated (according to MVVC). But, I need to update the view after SomeCommand gets executed, because items inside SomeCollection get edited.

I tried doing this

public void ExecuteSomeCommand() {
    // change the state of some person inside SomeCollection
    (ListView1.SelectedItems[0] as Person).State = "Some different state";

    // now inform the view of change, so it can reflect in the DataTemplate
    ListView1.GetBindingExpression(ListBox.ItemsSourceProperty).UpdateTarget();
}

And I thought this would propagate into the DataTemplate, but it doesn't. Is there any other way to do this? How should I change my approach?

The ItemsSource Binding is not the Image.Source Binding. Updating the first when the second's source property has changed is pointless. Besides that, consider making Person fire property change notifications. — Clemens, Aug 30 '21 at 15:24
@Clemens I know, but how do I get the reference to the image control if it's inside a DataTemplate? — Sorashi, Aug 30 '21 at 15:28
As another note, do not use the `as` operator without checking its result for `null`. Use an explicit cast instead: `((Person)ListView1.SelectedItem).State = ...`; — Clemens, Aug 30 '21 at 15:28
Let your model implement [INotifyPropertyChanged](https://learn.microsoft.com/en-us/dotnet/api/system.componentmodel.inotifypropertychanged?view=net-5.0). — Olivier Jacot-Descombes, Aug 30 '21 at 15:28
@Clemens I do not use it like this, this is just a sample code which reflects what I want to have done — Sorashi, Aug 30 '21 at 15:29
@OlivierJacot-Descombes that is not good practice, see https://stackoverflow.com/questions/6922130/in-mvvm-model-should-the-model-implement-inotifypropertychanged-interface — Sorashi, Aug 30 '21 at 15:29
@Sorashi Quite the opposite. When you bind to a model property that affects the UI, that property should always fire a change notification. Otherwise wrap the model class in a view model class. — Clemens, Aug 30 '21 at 15:30
@Clemens okay, so in this case, it's ok to use INotifyPropertyChanged? — Sorashi, Aug 30 '21 at 15:31

score 1 · Accepted Answer · answered Aug 30 '21 at 16:07

When the model used in data binding implements the INotifyPropertyChanged Interface the UI is automatically updated when you modify a property of the model.

public class Person : INotifyPropertyChanged
{
    private Image _state;
    public Image State
    {
        get => _state;
        set {
            if (value != _state) {
                _state = value;
                OnPropertyChanged(nameof(State));
            }
        }
    }

    // ... other properties here ...

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

State should not return an Image, but an ImageSource - or anything else that is not a UIElement and can be converted by OP's Binding Converter. E.g. a string, as the code in the questions suggests. — Clemens, Aug 30 '21 at 16:11
Okay, my example must be adapted to the type the OP is using in its model class. — Olivier Jacot-Descombes, Aug 30 '21 at 16:15

How to update bindings inside DataTemplate?

1 Answers1