4. Data Bind to Anything
There are situations where
you need to data bind to a value and the value type or format does not
quite line up with what you need. The IValueConverter
interface allows your application to data bind to just about anything
including data that does not match the type expected for a particular
control.
As an example, let's say you have a field that takes values of either Yes or No. You could use a ListPicker but a CheckBox makes more sense since it is just two values. You can implement IValueConverter to convert true to a Yes value and No value to false (and vice versa) while still using standard data binding mechanisms.
In this section, we create a custom IValueConverter named HtmlToImageUriConverter.
This very simple converter parses the AppHub Feed html, previously
displayed as raw html text, into a URL that points to an image related
to the content. The URL is extracted from the HTML. Listing 4 shows the code for the converter.
Example 4. WebBrowserHTMLProp Code File
using System;
using System.Windows.Data;
namespace AdvancedDataBinding.Converters
{
public class HtmlToImageUriConverter : IValueConverter
{
public object Convert(object value, Type targetType,
object parameter, System.Globalization.CultureInfo culture)
{
string html = (string)value;
string imageUrl = "";
if (null != html)
{
string[] strings = html.Split('"');
if (strings.Length > 3)
imageUrl = strings[3].Trim();
}
return imageUrl;
}
public object ConvertBack(object value, Type targetType,
object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
}
|
The converter is somewhat
brittle in that it depends on the URL value to be the fourth string when
split, but one wouldn't expect the feed format to change very often,
and it makes for an interesting sample. The converter is made available
to the DatabindToAnything.xaml View via an xml namespace import:
xmlns:converters="clr-namespace:AdvancedDataBinding.Converters"
The converter is added as a resource in this XAML:
<phone:PhoneApplicationPage.Resources>
<converters:HtmlToImageUriConverter x:Key="HtmlToImageConverter"/>
</phone:PhoneApplicationPage.Resources>
Applying the converter is a
simple modification in XAML or visually in Expression Blend. Here is the
updated ListBoxDataTemplate:
<DataTemplate>
<StackPanel Margin="0,0,0,30">
<TextBlock Text="{Binding Title}" Margin="0,0,12,0" Style="{StaticResource PhoneTextLargeStyle}" />
<Image delay:LowProfileImageLoader.UriSource=
"{Binding Description, Converter={StaticResource HtmlToImageConverter}}" Margin="0,6,0,4" Width="100" HorizontalAlignment="Left" />
<HyperlinkButton NavigateUri="{Binding Link}" FontFamily="Segoe WP SemiLight" TargetName="_blank"
FontSize="18.667" FontStyle="Italic" Content="More..." HorizontalAlignment="Left" Margin="-12,0,0,0"/>
</StackPanel>
</DataTemplate>
The Binding.Converter parameter is configured to the added StaticResource to apply the converter for the Image object. Note that this sample also implements the LowProfileImageLoader for better performance as well as the PerformanceProgressBar to give better visual status to the user. Figure 6 shows the resulting output with a nice image instead of the raw HTML that was displayed previously.
