ItemsSource doesn't show/bind when ObservableCollection is filled before loading the page - xamarin

Question

I have a carouselview, in that view I have an ObservableCollection binded as an itemssource. I am able to bind the collection and it would show when I execute the viewmodel's command in the OnAppearing event.
Code that works:
Second Page
public partial class SecondPage : ContentPage
{
public Coll(bool hard, string subject)
{
InitializeComponent();
var vm = (DataSelectionViewModel)BindingContext;
vm.Hard = hard;
vm.Subject = subject;
/* had to set "hard" and "subject" here again, otherwise data won't load */
}
protected override async void OnAppearing()
{
var vm = (DataSelectionViewModel)BindingContext;
base.OnAppearing();
await vm.LoadData.ExecuteAsync().ConfigureAwait(false);
}
}
The viewmodel for second page
public class DataSelectionViewModel : BaseViewModel
{
private string subject;
public string Subject { get => subject; set => SetProperty(ref subject, value); }
private bool hard;
public bool Hard { get => hard; set => SetProperty(ref hard, value); }
public ObservableCollection<Items> FilteredData { get; set; }
public UserSelectionViewModel()
{
_dataStore = DependencyService.Get<IDataStore>();
LoadData= new AsyncAwaitBestPractices.MVVM.AsyncCommand(FilterData);
FilteredData = new ObservableCollection<Items>();
}
public async Task FilterData()
{
FilteredData.Clear();
var filtereddata = await _dataStore.SearchData(Hard, Subject).ConfigureAwait(false);
foreach (var data in filtereddata)
{
FilteredData.Add(data);
}
}
}
First Page where second page gets Hard and Subject values
private async void ButtonClick(object sender, EventArgs e)
{
var vm = (BaseViewModel)BindingContext;
vm.Hard = HardButtonSelected == Hard;
vm.Subject = vm.Subject.ToLower();
await Navigation.PushAsync(new SecondPage(vm.Hard, vm.Subject));
}
So I want to change my code so that if I press the button on the first page, data instantly starts to filter and add to the ObservableCollection and when it's finished, then navigate to the second page. However if I try to load it to the BaseViewModel and then get the data from the second viewmodel it won't show the data.
Code that doesn't work:
Second Page
public partial class SecondPage : ContentPage
{
public SecondPage()
{
InitializeComponent();
}
}
The viewmodel for second page
public class DataSelectionViewModel : BaseViewModel
{
public ObservableCollection<Items> FilteredData { get; set; }
public UserSelectionViewModel()
{
FilteredData = new ObservableCollection<Items>();
}
}
BaseViewModel
public class BaseViewModel : INotifyPropertyChanged
{
private string subject;
public string Subject { get => subject; set => SetProperty(ref subject, value); }
private bool hard;
public bool Hard { get => hard; set => SetProperty(ref hard, value); }
public ObservableCollection<Items> FilteredData { get; set; }
/* BaseViewModel has implementation of SetProperty */
}
First Page where second page gets Hard and Subject values
private async void ButtonClick(object sender, EventArgs e)
{
var vm = (BaseViewModel)BindingContext;
vm.Hard = HardButtonSelected == Hard;
vm.Subject = vm.Subject.ToLower();
}
First Page viewmodel
public class FirstPageViewModel : BaseViewModel
{
public IAsyncCommand MehetButtonClickedCommand { get; }
readonly IPageService pageService;
readonly IFeladatokStore _feladatokStore;
public FeladatValasztoViewModel()
{
_dataStore = DependencyService.Get<IDataStore>();
ButtonClickedCommand = new AsyncCommand(ButtonClicked);
pageService = DependencyService.Get<IPageService>();
}
private async Task ButtonClicked()
{
await FilterData();
await pageService.PushAsync(new SecondPage());
}
private async Task FilterData()
{
FilteredData.Clear();
var datas = await _dataStore.SearchData(Subject, Hard).ConfigureAwait(false);
foreach (var data in datas)
{
FilteredData.Add(data);
}
}
So basically this gives a null exception error. I also tried giving the ObservableCollection as an argument for SecondPage(ObservableCollection x) and that did work, but because I had to make another ObservableCollection for it and copy from one to another it stopped being async and froze for a couple of seconds. So my question is how can I make this async?

Answer 1

To avoid delay, build the new collection in a private variable. Then set the property to that variable:
// Constructor with parameter
public SomeClass(IList<Items> data)
{
SetFilteredDataCopy(data);
}
public ObservableCollection<Items> FilteredData { get; set; }
private void SetFilteredDataCopy(IList<Items> src)
{
var copy = new ObservableCollection<Items>();
foreach (var item in src)
copy.Add(item);
FilteredData = copy;
//MAYBE OnPropertyChanged(nameof(FilteredData));
}