LongListSelector does not refresh after binding to itemsSource

Question

1. To get data from network and display
2. I use a LongListSelector(articleList) to display the data, the web api works,already verified in visual studio by setting break point.
3. [ISSUE] setting ItemSource to viewModel.ArticleCollection, after retrieve data from web api, the LongListSelector does not refresh.
4. but after chenge code in OnNavigatedTo method in MainPage.xaml.cs , FROM "_viewModel.LoadPage(_searchTerm, _pageNumber++);" TO "articleList.ItemsSource = await CollectionHttpClient.GetAllArticlesAsync();" IT DISPLAYS the web api data.

anyone can help me on this? thanks in advance!

MainPage.xaml

<phone:PhoneApplicationPage
x:Class="CollectionApp.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:vm="clr-namespace:CollectionApp.ViewModels"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"

FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="{StaticResource PhoneFontSizeNormal}"
Foreground="{StaticResource PhoneForegroundBrush}"
SupportedOrientations="Portrait"  Orientation="Portrait"
shell:SystemTray.IsVisible="True">

<phone:PhoneApplicationPage.Resources>

    <vm:ArticleListViewModel x:Key="viewModel"/>

    <DataTemplate x:Key="ResultItemTemplate">
        <Grid Margin="0,6,0,0">
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="*"/>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Rectangle Fill="Gray" Height="50" Width="50" Grid.Row="0" Grid.Column="0" 
                     VerticalAlignment="Top" Margin="0,7,7,0"
                   Grid.RowSpan="2">

            </Rectangle>
            <Image Height="50" Width="50" Grid.Row="0" Grid.Column="0" 
                     VerticalAlignment="Top" Margin="0,7,7,0"
                   Grid.RowSpan="2">
                <Image.Source>
                    <BitmapImage UriSource="{Binding ImagePath}"
                                 CreateOptions="BackgroundCreation"/>
                </Image.Source>
            </Image>
            <TextBlock Text="{Binding Subject}" Grid.Row="0" Grid.Column="1"
                             Foreground="{StaticResource PhoneAccentBrush}" VerticalAlignment="Top"/>

            <TextBlock Text="{Binding Words}" TextWrapping="Wrap"
                           Grid.Row="1" Grid.Column="1"
                           VerticalAlignment="Top"
                           />

        </Grid>
    </DataTemplate>

</phone:PhoneApplicationPage.Resources>

<!--LayoutRoot is the root grid where all page content is placed-->
<Grid x:Name="LayoutRoot" Background="Transparent">

    <!-- LOCALIZATION NOTE:
        To localize the displayed strings copy their values to appropriately named
        keys in the app's neutral language resource file (AppResources.resx) then
        replace the hard-coded text value between the attributes' quotation marks
        with the binding clause whose path points to that string name.

        For example:

            Text="{Binding Path=LocalizedResources.ApplicationTitle, Source={StaticResource LocalizedStrings}}"

        This binding points to the template's string resource named "ApplicationTitle".

        Adding supported languages in the Project Properties tab will create a
        new resx file per language that can carry the translated values of your
        UI strings. The binding in these examples will cause the value of the
        attributes to be drawn from the .resx file that matches the
        CurrentUICulture of the app at run time.
     -->

    <!--Pivot Control-->
    <phone:Pivot Title="{Binding Path=LocalizedResources.ApplicationTitle, Source={StaticResource LocalizedStrings}}">
        <!--Pivot item one-->
        <phone:PivotItem Header="Articles">
            <!--Double line list with text wrapping-->
            <phone:LongListSelector 
                x:Name="articleList"
                Grid.Row="1"   
                Margin="0,0,-12,0" 
                DataContext="{StaticResource viewModel}"
                ItemTemplate="{StaticResource ResultItemTemplate}"   
                ItemsSource="{Binding viewModel.ArticleCollection}"
                >

            </phone:LongListSelector>
        </phone:PivotItem>

        <!--Pivot item two-->
        <phone:PivotItem Header="second">
            <!--Double line list no text wrapping-->
            <phone:LongListSelector Margin="0,0,-12,0" ItemsSource="{Binding Items}">
                <phone:LongListSelector.ItemTemplate>
                        <DataTemplate>
                            <StackPanel Margin="0,0,0,17">
                                <TextBlock Text="{Binding LineOne}" TextWrapping="NoWrap" Margin="12,0,0,0" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
                                <TextBlock Text="{Binding LineThree}" TextWrapping="NoWrap" Margin="12,-6,0,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
                            </StackPanel>
                        </DataTemplate>
                </phone:LongListSelector.ItemTemplate>
            </phone:LongListSelector>
        </phone:PivotItem>
    </phone:Pivot>

    <!--Uncomment to see an alignment grid to help ensure your controls are
        aligned on common boundaries.  The image has a top margin of -32px to
        account for the System Tray. Set this to 0 (or remove the margin altogether)
        if the System Tray is hidden.

        Before shipping remove this XAML and the image itself.-->
    <!--<Image Source="/Assets/AlignmentGrid.png" VerticalAlignment="Top" Height="800" Width="480" Margin="0,-32,0,0" Grid.Row="0" IsHitTestVisible="False" />-->
</Grid>

</phone:PhoneApplicationPage>

MainPage.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Navigation;
using Microsoft.Phone.Controls;
using Microsoft.Phone.Shell;
using CollectionApp.Resources;
using CollectionApp.Network;
using CollectionApp.Models;
using CollectionApp.ViewModels;
using System.Windows.Data;
using System.Diagnostics;
using Windows.UI.Core;

namespace CollectionApp
{
    public partial class MainPage : PhoneApplicationPage
    {
        int _pageNumber = 1;
        string _searchTerm = "";
        ArticleListViewModel _viewModel = new ArticleListViewModel();
        int _offsetKnob = 1;

        // Constructor
        public MainPage()
        {
            InitializeComponent();

            //_viewModel = new ArticleListViewModel();
            _viewModel = (ArticleListViewModel)Resources["viewModel"];
            DataContext = _viewModel;

            articleList.ItemRealized += articleList_ItemRealized;
            this.Loaded += new RoutedEventHandler(MainPage_Loaded);

            //_viewModel = new ArticleListViewModel();
            articleList.ItemsSource = _viewModel.ArticleCollection;

            // Set the data context of the listbox control to the sample data
            //DataContext = App.ViewModel;

            // Sample code to localize the ApplicationBar
            //BuildLocalizedApplicationBar();
        }

        void MainPage_Loaded(object sender, RoutedEventArgs e)
        {
            var progressIndicator = SystemTray.ProgressIndicator;
            if(progressIndicator != null)
            {
                return;
            }

            progressIndicator = new ProgressIndicator();

            SystemTray.SetProgressIndicator(this, progressIndicator);

            Binding binding = new Binding("IsLoading") { Source = _viewModel };
            BindingOperations.SetBinding(progressIndicator, ProgressIndicator.IsVisibleProperty, binding);

            binding = new Binding("IsLoading") { Source = _viewModel};
            BindingOperations.SetBinding(progressIndicator, ProgressIndicator.IsIndeterminateProperty, binding);

            progressIndicator.Text = "Loading new Articles....";            

        }

        private void articleList_ItemRealized(object sender, ItemRealizationEventArgs e)
        {
            if (!_viewModel.IsLoading && articleList.ItemsSource != null && articleList.ItemsSource.Count >= _offsetKnob)
            {
                if (e.ItemKind == LongListSelectorItemKind.Item)
                {
                    if ((e.Container.Content as Article).Equals(articleList.ItemsSource[articleList.ItemsSource.Count - _offsetKnob]))
                    {
                        Debug.WriteLine("Searching for {0}", _pageNumber);
                        //_viewModel.LoadPage(_searchTerm, _pageNumber++);
                    }
                }
            }
        }


        // Load data for the ViewModel Items
        protected async override void OnNavigatedTo(NavigationEventArgs e)
        {
            try
            {
                if (CollectionHttpClient.IsDirty)
                {
                    _viewModel.LoadPage(_searchTerm, _pageNumber++);
                    // articleList.ItemsSource = await CollectionHttpClient.GetAllArticlesAsync(); ;

                }
            }
            catch
            {
                MessageBox.Show("sorry, no data.");
            }
        }

        // Sample code for building a localized ApplicationBar
        //private void BuildLocalizedApplicationBar()
        //{
        //    // Set the page's ApplicationBar to a new instance of ApplicationBar.
        //    ApplicationBar = new ApplicationBar();

        //    // Create a new button and set the text value to the localized string from AppResources.
        //    ApplicationBarIconButton appBarButton = new ApplicationBarIconButton(new Uri("/Assets/AppBar/appbar.add.rest.png", UriKind.Relative));
        //    appBarButton.Text = AppResources.AppBarButtonText;
        //    ApplicationBar.Buttons.Add(appBarButton);

        //    // Create a new menu item with the localized string from AppResources.
        //    ApplicationBarMenuItem appBarMenuItem = new ApplicationBarMenuItem(AppResources.AppBarMenuItemText);
        //    ApplicationBar.MenuItems.Add(appBarMenuItem);
        //}

        private void LongListSelector_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            LongListSelector selector = sender as LongListSelector;

            // verifying sender is actually a LongListSelector
            if (selector == null)
            {
                return;
            }

            NavigationService.Navigate(new Uri("Pages/DetailsPage.xaml?id=" + (selector.SelectedItem as Article).ID, UriKind.RelativeOrAbsolute));

            //
            selector.SelectedItem = null;

        }


    }
}

ArticleListViewModel.cs

using CollectionApp.Models;
using CollectionApp.Network;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;

namespace CollectionApp.ViewModels
{
    public class ArticleListViewModel : INotifyPropertyChanged
    {
        private bool _isLoading = false;

        public bool IsLoading
        {
            get
            {
                return _isLoading;
            }
            set
            {
                _isLoading = value;
                NotifyPropertyChanged("IsLoading");
            }
        }

        public ArticleListViewModel()
        {
            this.ArticleCollection = new ObservableCollection<Article>();
            this.IsLoading = false;
        }


        public ObservableCollection<Article> ArticleCollection
        {
            get;
            private set;
        }

        public void LoadPage(string searchTerm, int pageNumber)
        {
            if (pageNumber == 1)
            {
                this.ArticleCollection.Clear();
            }

            IsLoading = true;
            ReadArticleList();

        }

        public async void ReadArticleList()
        {
            try
            {
                if (CollectionHttpClient.IsDirty)
                {
                    ArticleCollection = await CollectionHttpClient.GetAllArticlesAsync();
                    IsLoading = false;

                }

            }
            catch
            {                
                MessageBox.Show("sorry, no data.");
                IsLoading = false;
            }

        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void NotifyPropertyChanged(String propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (null != handler)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }

    }
}

Answer 1

finally, i came through this. I refresh ArticleCollection every time, this is the cause. ItemRealized event fired every time when item assigned to UI container every time, until there is no new item assign to UI container.

at the last time, the web api return no result since there is no result on the last page. and this time i assign the no result to ArticleCollection. That is why the longlistselector shows nothing

Change the method

public async void ReadArticleList()
        {
            try
            {
                if (CollectionHttpClient.IsDirty)
                {
                    ArticleCollection = await CollectionHttpClient.GetAllArticlesAsync();
                    IsLoading = false;

                }

            }
            catch
            {                
                MessageBox.Show("sorry, no data.");
                IsLoading = false;
            }

        }

into:

private async void ReadArticleList(int pageNumber)
        {
            try
            {

                List<Article> articleList = new List<Article>();
                articleList = await CollectionHttpClient.GetArticlesByPageAsync(pageNumber);

                foreach (var item in articleList)
                {
                    this.ArticleCollection.Add(item);
                }

                IsLoading = false;


            }
            catch
            {
                MessageBox.Show("sorry, no data.");
                IsLoading = false;
            }

        }

Answer 2

I suspect that because you don't raise property changed event when ArticleCollection set. Thats why LongListSelector didn't get refreshed. ObservableCollection only notify UI when item added or removed from collection. But when the ObservableCollection property it self get assigned to new ObservableCollection object, you need to raise property changed event manually. Try to change it as follow, and see if it works now :

private ObservableCollection<Article> _articleCollection;
public ObservableCollection<Article> ArticleCollection
{
    get
    {
        return _articleCollection;
    }
    set
    {
        _articleCollection = value;
        NotifyPropertyChanged("ArticleCollection");
    }
}

Another option is not to reassign ArticleCollection as you currently did in ReadListArticle method :

ArticleCollection = await CollectionHttpClient.GetAllArticlesAsync();

instead, clear the collection and add each new item to collection.