Updating observablecollection with async list item.

I am having a list and i populate the list with a method using for loop. just a number from 1 - 10. Then i created a list view and populate it with Observable collection by converting the list i get from method into observable collection. While executing this program i can able to see the list view populated with numbers.
But when i populate the same list with async method i can't see the list view get populate. I used the breakpoints to check whether the methods are working or not. But the methods are all working fine. even my observable collection get populated. but i can't able see the list view.

My async program:


                 Margin="10,0,0,0" Foreground="Red"/>








                                           Foreground="Red" Name="MyProgress"/>



                              ItemTemplate="{StaticResource ListViewTemplate}">

My class:

   public class ListItem
   {
      public float Number { get; set; }
   }

   public class ListManager
   {
      public async static Task> GetItemsAsync()
   {
      var listItems = new List();
      Rootobject temperatures = await getTemperatureAsync();

      for (int i = 0; i < 3; i++)
   {
      listItems.Add(new ListItem() { Number = temperatures.hourly.data[i].temperature });
   }
      return listItems;
   }

   private async static Task getTemperatureAsync()
   {
      HttpClient client = new HttpClient();
      var jsonData = await client.GetStringAsync("https://api.darksky.net/forecast/apikey               /13.08,80.27?units=si");
      var parsedData = JsonConvert.DeserializeObject(jsonData);
      return parsedData;
   }
   }

Mainpage.xaml.cs

   ObservableCollection ListItems;
   public MainPage()
   {
      this.InitializeComponent();
      ThisMethod().GetAwaiter();
   }

   private async Task ThisMethod()
   {
      MyProgress.Visibility = Visibility.Visible;
      ObservableCollection items = new ObservableCollection(await          ListManager.GetItemsAsync());
      ListItems = items;
      MyProgress.Visibility = Visibility.Collapsed;
   }