CollectionView dose not show content, when filled in onappearing - xamarin

Question

I have a collectionview that is bound to an ObservableRangeCollectionin my ViewModel.
In my ViewModel there is a Method that runs onAppearing and I want my ColletionViewto be filled from there, but when I do so the collectionveiw dose not display the content only when i reload the content is shown.
View:
<RefreshView Grid.Row="1"
Grid.RowSpan="2"
Command="{Binding RefreshCommand}"
IsRefreshing="{Binding IsBusy, Mode=OneWay}">
<RefreshView.RefreshColor>
<OnPlatform x:TypeArguments="Color">
<On Platform="iOS" Value="White"/>
</OnPlatform>
</RefreshView.RefreshColor>
<CollectionView x:Name="Collection"
ItemsSource="{Binding Locations, Mode=OneWay}"
ItemTemplate="{StaticResource ListDataTemplate}"
RemainingItemsThresholdReachedCommand="{Binding LoadMoreCommand}"
RemainingItemsThreshold="10"
SelectionMode="Single"
BackgroundColor="Transparent"
ItemsLayout="VerticalList"
SelectedItem="{Binding SelectedItem}"
SelectionChangedCommand="{Binding SelectedCommand}">
<CollectionView.EmptyView>
<StackLayout Padding="12">
<Label HorizontalOptions="Center" Text="Keine Daten vorhanden!" TextColor="White"/>
</StackLayout>
</CollectionView.EmptyView>
</CollectionView>
</RefreshView>
ViewModel:
namespace YourPartys.ViewModels
{
public class ListViewModel : ViewModelBase
{
#region Variables
#endregion
#region Propertys
LocationModel selectedItem;
public LocationModel SelectedItem
{
get => selectedItem;
set => SetProperty(ref selectedItem, value);
}
public ObservableRangeCollection<LocationModel> Locations { get;set; } = new ObservableRangeCollection<LocationModel>();
double distance;
public double Distance
{
get => distance;
set => SetProperty(ref distance, value);
}
#endregion
#region Commands
public ICommand FilterButtonCommand { get; }
public ICommand RefreshCommand { get; }
public ICommand SelectedCommand { get; }
public ICommand LoadMoreCommand { get; }
#endregion
//Constructor
public ListViewModel()
{
FilterButtonCommand = new Command(OpenFilter);
RefreshCommand = new AsyncCommand(Refresh);
SelectedCommand = new AsyncCommand(Select);
}
public override async void VModelActive(Page sender, EventArgs eventArgs)
{
base.VModelActive(sender, eventArgs);
var locs = await FirestoreService.GetLocations("Locations");
Locations.AddRange(locs);
}
private void OpenFilter(object obj)
{
PopupNavigation.Instance.PushAsync(new ListFilterPage());
}
private async Task Refresh()
{
IsBusy = true;
var locs = await FirestoreService.GetLocations("Locations");
Locations.AddRange(locs);
IsBusy = false;
}
private async Task Select()
{
if (SelectedItem == null)
return;
var route = $"{nameof(DetailPage)}?Locationid={SelectedItem.Locationid}";
SelectedItem = null;
await AppShell.Current.GoToAsync(route);
}
}
}

Answer 1

There are several problems in your demo.
1.Since you set the BindingContext for your page in xaml as follows:
<ContentPage.BindingContext>
<viewmodels:MainViewModel/>
</ContentPage.BindingContext>
you didn't need to recreate another object MainViewModel in a CS file and reference it. These are two different objects.
MainViewModel viewModel;
viewModel = new MainViewModel();
protected override void OnAppearing()
{
base.OnAppearing();
viewModel.VModelActive(this, EventArgs.Empty);
}
So, you can get the BindingContext in MainPage.xaml.cs in function OnAppearing as follows:
protected override void OnAppearing()
{
base.OnAppearing();
viewModel = (MainViewModel)this.BindingContext;
viewModel.VModelActive(this, EventArgs.Empty);
}
The whole code is
public partial class MainPage : ContentPage
{
MainViewModel viewModel;
public MainPage()
{
InitializeComponent();
// viewModel = new MainViewModel();
}
protected override void OnAppearing()
{
base.OnAppearing();
viewModel = (MainViewModel)this.BindingContext;
viewModel.VModelActive(this, EventArgs.Empty);
}
}
2.when we set the text color of the Label to White,this makes it hard to see the text,so you can reset it to another color,for example Black:
<Label Text="{Binding Name}"
FontSize="30"
TextColor="White"/>