Карта форм Xamarin, не добавляющая булавки на созданный новый штифт

Question

У меня есть карта форм Xamarin, которая показывает коллекцию Пинов. Когда я добавляю новое местоположение, он не показывает вновь созданный контакт, пока я не нажму 'Refre sh'.

У меня изначально была подписка AddLocation в конструкторе ViewModel, и она работала, но добавила закрепите 3 раза в базе данных и карту. После перемещения подписки AddLocation и отмены подписки на их собственные методы в ViewModel и вызова затем OnAppearing () и OnDisappearing () новый вывод не отображается.

Карта находится на странице с именем ItemPage.xaml

<ContentPage.ToolbarItems>
    <ToolbarItem Text="Add" Clicked="ToolbarItem_Clicked" />
    <ToolbarItem Text="Refresh" Clicked="ToolbarItem_Clicked_1" />
</ContentPage.ToolbarItems>

<ContentPage.Content>
    <!--The map-->
    <ContentView Content="{Binding Map}" />
</ContentPage.Content>

и код позади -

[DesignTimeVisible(false)]
public partial class ItemPage : ContentPage
{
    ItemViewModel viewModel;

    public ItemPage()
    {
        InitializeComponent();

        BindingContext = viewModel = new ItemViewModel();
    }

    private void Button_Clicked(object sender, EventArgs e)
    {

    }

    protected override void OnAppearing()
    {
        base.OnAppearing();
    }

    async void ToolbarItem_Clicked(object sender, EventArgs e)
    {
        await Navigation.PushModalAsync(new NavigationPage(new NewLocationPage()));
    }

    private void ToolbarItem_Clicked_1(object sender, EventArgs e)
    {
        viewModel.LoadLocationsCommand.Execute(null);
    }
}

Модель представления -

public class ItemViewModel : BaseViewModel
{

    public ObservableCollection<MapPinModel> Locations { get; set; }

    public ObservableCollection<ItemModel> Items { get; set; }

    public Command LoadLocationsCommand { get; set; }

    public Command LoadItemsCommand { get; set; }

    public Map Map { get; private set; }

    public ItemViewModel()
    {
        Title = "Items";
        Locations = new ObservableCollection<MapPinModel>();
        Items = new ObservableCollection<ItemModel>();

        LoadLocationsCommand = new Command(async () => await ExecuteLoadLocationsCommand());

        LoadItemsCommand = new Command(async () => await ExecuteLoadItemsCommand());



        Map = new Map(MapSpan.FromCenterAndRadius(
            new Xamarin.Forms.Maps.Position(53.70251232638285, -1.8018436431884768),
            Distance.FromMiles(0.5)))
        {
            IsShowingUser = true,
            VerticalOptions = LayoutOptions.FillAndExpand
        };

        this.LoadLocationsCommand.Execute(null);
    }

    public void UnsubscribeMessages()
    {
        MessagingCenter.Unsubscribe<NewLocationPage, ItemLocationModel>(this, "AddLocation");
    }

    public void SubscribeMessages()
    {
        MessagingCenter.Subscribe<NewLocationPage, ItemLocationModel>(this, "AddLocation", async (obj, item) =>
        {
            await ItemDataStore.AddItemLocationAsync(item);
        });
    }

    async Task ExecuteLoadItemsCommand()
    {
        if (IsBusy)
            return;

        try
        {
            Items.Clear();
            var items = await ItemDataStore.GetItemsAsync(true);

            foreach (var item in items)
            {
                Items.Add(item);
            }
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex);
        }
        finally
        {
            IsBusy = false;
        }
    }

    async Task ExecuteLoadLocationsCommand()
    {
        Map.Pins.Clear();

        var locations = await ItemDataStore.GetLocationsAsync(true);

        foreach (Feature feature in locations.Features)
        {
            if (!feature.Geometry.Type.Equals(GeoJSONObjectType.Point))
                continue;

            Point point = feature.Geometry as Point;

            GeoJSON.Net.Geometry.Position s = point.Coordinates as GeoJSON.Net.Geometry.Position;

            MapPinModel pin = new MapPinModel
            {
                PopupContent = feature.Properties["popupContent"].ToString(),
                ScientificName = feature.Properties["scientificname"].ToString(),
                ItemDescription = feature.Properties["description"].ToString(),
                AddedBy = feature.Properties["username"].ToString(),
                Latitude = s.Latitude.ToString(),
                Longitude = s.Longitude.ToString()
            };

            Xamarin.Forms.Maps.Position position = new Xamarin.Forms.Maps.Position(Convert.ToDouble(pin.Latitude), Convert.ToDouble(pin.Longitude));

            Pin newPin = new Pin
            {
                Label = pin.PopupContent,
                Type = PinType.Place,
                Position = position
            };

            Map.Pins.Add(newPin);
        }
    }
}

Наконец, страница нового местоположения -

[DesignTimeVisible(false)]
public partial class NewLocationPage : ContentPage
{
    ItemViewModel viewModel;

    public ItemLocationModel Location { get; set; }

    public NewLocationPage()
    {
        InitializeComponent();

        Location = new ItemLocationModel();

        BindingContext = viewModel = new ItemViewModel();

        BindingContext = this;

        viewModel.SubscribeMessages();
    }

    async void Save_Clicked(object sender, EventArgs e)
    {
        var location = await Geolocation.GetLastKnownLocationAsync();

        Location.Latitude = location.Latitude.ToString();
        Location.Longitude = location.Longitude.ToString();

        Location.DatePosted = DateTime.Now;

        Location.ItemId = ((ItemModel)itemsPicker.SelectedItem).Id;

        Location.SecretLocation = secretLocation.IsToggled;

        MessagingCenter.Send(this, "AddLocation", Location);

        await Navigation.PopModalAsync();

        Location = null;

    }

    async void Cancel_Clicked(object sender, EventArgs e)
    {
        await Navigation.PopModalAsync();
    }

    protected override void OnAppearing()
    {
        viewModel.SubscribeMessages();

        base.OnAppearing();

        if (viewModel.Items.Count == 0)
            viewModel.LoadItemsCommand.Execute(null);

        itemsPicker.ItemsSource = viewModel.Items;
    }

    protected override void OnDisappearing()
    {
        viewModel.LoadLocationsCommand.Execute(null);

        viewModel.UnsubscribeMessages();

        base.OnDisappearing();
    }
}