Windows Phone Silverlight - setting button IsDisabled if bound data length is 0 - silverlight

Question

Here's what I've got - I'm writing an App that, among other things, reads an RSS feed to get episodes of a certain podcast, then displays each episode's title and description, with a "listen" and "watch" button. But not all the episodes have both options - the RSS will return an empty string instead of a URL for either option if it's not available. So I'm trying to use IValueConverter that I can bind IsDisabled to, which returns true if the bound data length is 0, and false otherwise. For now, I'm just testing it on the "watch" buttons, since the binding will be nearly identical for the "listen" buttons.
A snippet of MainPage.xaml.cs:
using System.Xml.Linq;
using System.Windows.Data;
namespace appname
{
public partial class MainPage : PhoneApplicationPage
{
public MainPage()
{
InitializeComponent();
WebClient PodcastListDownloader = new WebClient();
PodcastListDownloader.DownloadStringCompleted += new DownloadStringCompletedEventHandler(PodcastListDownloadCompleted);
PodcastListDownloader.DownloadStringAsync(new Uri("http://domain.tld/mobile_app/podcastfeed"));
}
void PodcastListDownloadCompleted(object sender, DownloadStringCompletedEventArgs e)
{
if (e.Error != null)
return;
XElement xmlPodcastList = XElement.Parse(e.Result);
PodcastListBox.ItemsSource = from PodcastEpisode in xmlPodcastList.Descendants("item")
select new PodcastItem
{
title = PodcastEpisode.Element("date").Value + " " + PodcastEpisode.Element("title").Value,
subtitle = PodcastEpisode.Element("subtitle").Value,
description = PodcastEpisode.Element("summary").Value,
audio = PodcastEpisode.Element("audio").Value,
video = PodcastEpisode.Element("video").Value,
};
}
private void PlayPodcast(object sender, RoutedEventArgs e)
{
Button btn = (Button)sender;
Microsoft.Phone.Tasks.MediaPlayerLauncher PodcastPlay = new Microsoft.Phone.Tasks.MediaPlayerLauncher();
PodcastPlay.Media = new Uri(btn.Tag.ToString());
PodcastPlay.Show();
}
}
public class PodcastItem
{
public string title { get; set; }
public string description { get; set; }
public string audio { get; set; }
public string video { get; set; }
public string subtitle { get; set; }
}
public class StringLengthVisibilityConverter: IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if (value == null || value.ToString().Length == 0)
{
return false;
}
else
{
return true;
}
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
A snippet of MainPage.xaml:
<phone:PhoneApplicationPage
x:Class="CCoFnow.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:controls="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone.Controls"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d" d:DesignWidth="480" d:DesignHeight="800"
FontFamily="{StaticResource PhoneFontFamilyNormal}"
FontSize="{StaticResource PhoneFontSizeNormal}"
Foreground="{StaticResource PhoneForegroundBrush}"
SupportedOrientations="Portrait" Orientation="Portrait"
shell:SystemTray.IsVisible="False">
<Grid x:Name="LayoutRoot" Background="Transparent">
<!--Panorama control-->
<controls:Panorama Title="AppName">
<controls:Panorama.Background>
<ImageBrush ImageSource="PanoramaBackground.png"/>
</controls:Panorama.Background>
<controls:PanoramaItem Header="Podcast" Foreground="{StaticResource PhoneAccentBrush}">
<ListBox Margin="0,0,-12,0" ItemsSource="{Binding Items}" Name="PodcastListBox">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Margin="0,0,0,17" Width="432">
<TextBlock Text="{Binding title}" TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
<TextBlock Text="{Binding description}" TextWrapping="Wrap" Margin="12,-6,12,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
<StackPanel Orientation="Horizontal">
<Button Content="Listen" Width="215" Tag="{Binding audio}" Click="PlayPodcast"/>
<Button Content="Watch" Width="215" Tag="{Binding video}" Click="PlayPodcast" IsEnabled="{Binding video, Converter={StringLengthVisibilityConverter}}"/>
</StackPanel>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</controls:PanoramaItem>
</controls:Panorama>
</Grid>
</phone:PhoneApplicationPage>
But the debugger is throwing two errors:
1) The tag
'StringLengthVisibilityConverter' does
not exist in XML namespace
'http://schemas.microsoft.com/winfx/2006/xaml/presentation
2) The type
'StringLengthVisibilityConverter' was
not found. Verify that you are not
missing an assembly and that all
referenced assemblies have been built
I set the converter to {StaticResource StringLengthVisibilityConverter} instead (per http://msdn.microsoft.com/en-us/library/system.windows.data.ivalueconverter(v=VS.95).aspx), and now there's just one error: The resource "StringLengthVisibilityConverter" could not be resolved. With this error, I can debug (run) the code, but all the "watch" buttons remain enabled.
So I'm guessing I'm calling it in the wrong namespace, but I can't seem to figure out the correct one. Can someone please point me in the right direction?
Thanks!
Edit: In the process of putting this together, I realized that I need to do this differently - the feed now has additional values I can databind to. However, I'm quite sure I'm going to need this functionality at some point in the future, so I'm going to post anyway. If there's an easy solution for the question, please let me know so I can learn and do it sucessfully next time!

Answer 1

The way you reference the converter isn't quite right. You need an instance of the converter available somwhere, e.g. in the page's Resources section:
<phone:PhoneApplicationPage xmlns:conv="namespace reference for your converter goes here"
...>
<phone:PhoneApplicationPage.Resources>
<conv:StringLengthVisibilityConverter x:Key="Length" />
</phone:PhoneApplicationPage.Resources>
Then you reference that converter by using a StaticResource reference with the x:Key that you gave the converter.
<Button Content="Watch"
Width="215"
Tag="{Binding video}"
Click="PlayPodcast"
IsEnabled="{Binding video, Converter={StaticResource Length}}"/>
I'll leave the discussion of your approach versus using commands and MVVM for another day :)