Changing Property of ObservableCollection does nothing - mvvm

Question

In my Xamarin project I have a ListView, which gets populated by an ObservableCollection, which holds "Item" objects with some properties. If I add items to the collection the UI gets updated, but if I change only a property, it won't it does nothing. Even after an UI update through adding an item does nothing, although the property gets correctly changed. How can I make the UI refresh if a property gets changed?
BindableBase is a class from PRISM that implements INotifyPropertyChanged and DelegateCommand implements ICommand, btw.
Here's my XAML:
<ListView ItemsSource="{Binding ListItems}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell x:Name="viewCell">
<ContentView Padding="0,0,0,5"
HeightRequest="50">
<ContentView.GestureRecognizers>
<TapGestureRecognizer BindingContext="{Binding Source={x:Reference listView}, Path=BindingContext}"
Command="{Binding ItemTappedCommand}"
CommandParameter="{Binding Source={x:Reference viewCell}, Path=BindingContext}" />
</ContentView.GestureRecognizers>
<Frame OutlineColor="{Binding Color}" Padding="8">
<StackLayout Orientation="Horizontal" >
<Image x:Name="checkedImage"
HeightRequest="30"
WidthRequest="30"
BackgroundColor="{Binding Color}"
/>
<Label Text="{Binding Text}"
TextColor="{Binding Color}"
Margin="20,0,0,0"
VerticalTextAlignment="Center"
HorizontalOptions="FillAndExpand"/>
<Image Source="{Binding Image}" />
</StackLayout>
</Frame>
</ContentView>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
Here's my ViewModel:
public class DetailPageViewModel : BindableBase
{
public DetailPageViewModel()
{
_listItems.Add(new ViewModels.Item("#123456", "Test1", "XamarinForms.Assets.Yellow.png"));
_listItems.Add(new ViewModels.Item("#654321", "Test3", "XamarinForms.Assets.close.png"));
}
private ObservableCollection<Item> _listItems = new ObservableCollection<Item>();
public ObservableCollection<Item> ListItems
{
get { return _listItems; }
set { SetProperty(ref _listItems, value); }
}
public DelegateCommand<Item> ItemTappedCommand => new DelegateCommand<Item>(ItemTapped);
private void ItemTapped(Item listItem)
{
// Adding an item refreshes the UI.
_listItems.Add(new ViewModels.Item("#654321", "Test3", "XamarinForms.Assets.close.png"));
// But changing the color of an item does nothing. Not even after an UI refresh.
_listItems.ElementAt(_listItems.IndexOf(listItem)).Color="#987654";
}
}
And my Item class:
public class Item
{
public string Color { set; get; }
public ImageSource Check { set; get; }
public string Text { private set; get; }
public ImageSource Image { private set; get; }
public Item(string color, string text, string imageSource)
{
Check = ImageSource.FromResource("XamarinForms.Assets.checkmark-outlined-verification-sign.png");
Color = color;
Text = text;
Image = ImageSource.FromResource(imageSource);
}
}

Answer 1

This is because also your item class needs to implement INotifyPropertyChanged. In your case as you are using Prism you just need to make your item class extend BindableBase (Prism base class which already implements INotifyPropertyChanged for you).
Link: https://github.com/PrismLibrary/Prism/blob/a60d38013c02b60807e9287db9ba7f7506af0e84/Source/Prism/Mvvm/BindableBase.cs
That should make it work.
Also I see in you are doing this:
public ObservableCollection<Item> ListItems
{
get { return _listItems; }
set { SetProperty(ref _listItems, value); }
}
With ObservableCollections you don't need to raise the event manually as they already do it internally. They can be defined as regular properties.
public ObservableCollection<Item> ListItems {get; set;}