WP7 ListBox Binding Doesn't Update

Question

I've been sitting on this problem for hours now, I've got this partial xaml code:

<TextBlock Text="{Binding temprature}" Height="30" HorizontalAlignment="Left" Margin="13,119,0,0" Name="textBlock1" VerticalAlignment="Top" Width="68" />
            <TextBlock Height="30" HorizontalAlignment="Left" Name="commentsTextBlock" Text="Comments:" VerticalAlignment="Bottom" Margin="12,0,0,-31" />
            <ListBox Margin="2,785,-14,-33" ItemsSource="{Binding comments}" DataContext="{Binding}" Name="commentsListBox">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal" Margin="0,0,0,17">
                            <StackPanel Width="311">
                                <TextBlock Text="{Binding poster_username}" TextWrapping="NoWrap" Style="{StaticResource PhoneTextSubtleStyle}" TextTrimming="WordEllipsis" Width="Auto" Foreground="White" FontFamily="Segoe WP Semibold" />
                                <TextBlock Text="{Binding comment_text}" TextWrapping="Wrap" Margin="12,-6,12,0" Style="{StaticResource PhoneTextSubtleStyle}" TextTrimming="WordEllipsis" MaxHeight="100" />
                            </StackPanel>
                        </StackPanel>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>

And I have this class (Thread) which include a List of comments that should be displayed in the ListBox.

    public class Thread : INotifyPropertyChanged
{
    public string title { get; set; }
    public string deal_link { get; set; }
    public string mobile_deal_link { get; set; }
    public string deal_image { get; set; }
    public string deal_image_highres { get; set; }
    public string description { get; set; }
    public string submit_time { get; set; }
    public bool hot_date { get; set; }
    public string poster_name { get; set; }
    public double temperature { get; set; }
    public double price { get; set; }
    public int timestamp { get; set; }
    public string expired { get; set; }
    public Forum forum { get; set; }
    public Category category { get; set; }
    public object merchant { get; set; }
    public object tags { get; set; }
    public int thread_id { get; set; }
    public string visit_link { get; set; }
    public string hot_time { get; set; }
    public int comment_count { get; set; }
    public string availability { get; set; }
    public string can_vote { get; set; }
    public string seen { get; set; }

    public List<Comment> comments { get; set; }
    public event PropertyChangedEventHandler PropertyChanged;

    public void Convert2Unicode()
    {
        UnicodeEncoding unicode = new UnicodeEncoding();
        Byte[] encodedBytes = unicode.GetBytes(title);
        title = String.Format("[{0}]", title);


    }


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

    public void SetComments(string content)
    {
        PaginatedComments commentsList;
        this.comments.Clear();

        DataContractJsonSerializer serializer =
        new DataContractJsonSerializer(typeof(PaginatedComments));
        using (MemoryStream ms = new MemoryStream(Encoding.Unicode.GetBytes(content)))
        {
            commentsList = (PaginatedComments)serializer.ReadObject(ms);
        }


        foreach (var thread in commentsList.data.comments)
        {
            this.comments.Add(thread);
        }

    }
    public Thread()
    {
        comments = new List<Comment>();

    }
    public void addComments()
    {
        List<string> parameters = new List<string>();
        parameters.Add("thread_id=" + thread_id);
        parameters.Add("results_per_page=10");
        parameters.Add("page=1");
        // Set the data context of the listbox control to the sample data
        APICalls.makeRequest(APICalls.ViewingPaginatedComments, parameters, SetComments);

    }
    //
    // Sets the "Seen" variable depending if the item is new (since the last time the application was opened
    //
    public void updateNewItems()
    {
        try
        {
            int last_seen = (int)IsolatedStorageSettings.ApplicationSettings["lastrun"];

            if (last_seen < timestamp)
            {
                seen = "New";
            }
            else
            {
                seen = "";
            }
        }
        catch (System.Exception e)
        {
            IsolatedStorageSettings.ApplicationSettings["lastrun"] = APICalls.GetIntTimestamp();
            IsolatedStorageSettings.ApplicationSettings.Save();
            MessageBox.Show(e.Message);
        }

        IsolatedStorageSettings.ApplicationSettings["lastrun"] = APICalls.GetIntTimestamp();
        IsolatedStorageSettings.ApplicationSettings.Save();

    }
}
public class Comment
{
    public string poster_username { get; set; }
    public string post_date { get; set; }
    public string comment_text { get; set; }
    public int timestamp { get; set; }
    public int like_count { get; set; }
    public string comment_edit_text { get; set; }
}

public class CommentData
{
    public List<Comment> comments { get; set; }
    public int comment_count { get; set; }
    public int total_comments { get; set; }
}

public class PaginatedComments
{
    public string response_status { get; set; }
    public CommentData data { get; set; }
}

If I load the comments into the Thread before changing the DataCotext to this specific thread. the comments are shown, but when I update the comments after changing the DataContext and navigating to the page the comments are not shown (I have other fields in the rest of the xaml page that are binded to the same instance and they work perfectly. only the comments don't work!

I really appreciate your help! Thanks

Answer 1

You should be using

public ObservableCollection<Comment> comments{ get; set;}

instead of

public List<Comment> comments { get; set; } 

The ObservableCollection sends update notices to the view anytime one of the items(in this case a Comment ) is added or removed.

Note: It won't update the Comment . To have the items bound to the Comment update, Comment has to implement INotifyPropertyChanged.

Answer 2

Your property is a simple List<T> . Silverlight needs to be signaled when something changes, using events.

A List<T> does not raise any event when items are being added/removed, so Silverlight cannot detect the new items and thus does not update the UI. The simplest way to make this work is usually to use an ObservableCollection<T> instead of a list. This collection will raise events that Silverlight knows to listen to.

Please note that for this to work you should not call Add/Remove/Clear from any other thread than the U/I (Dispatcher) thread, since Silverlight controls are not thread-safe (and the events are raised on the thread that performs the Add/Remove/Clear call). To do that, simply make sure you call Dispatcher.BeginInvoke from your SetComments method (since it seems to be a callback happening from whatever your fetching mechanism is).

Alternatively, you could also regenerate a brand new List object when fetching comments, and raise the NotifyPropertyCHanged event in the SetComments method, but that would lead to losing the current selection and resetting your list to the top, which is probably not what you want to do.