How do I bind an `DependenyProperty` to another attachable `DependencyProperty`? - wpf

Question

I want to have a singleton that holds some values S1 and S2 that I can bind to. The goal is to have some UIElements update when its value changes. The problem is that I want to use the value inside of a reused DataTemplate. That means that I cannot bind directly to a dependency property of the singleton but this has to be set outside.
To correctly pass updates the values have to be DependencyProperty. Because I dont know to which property I have to bind I created another attachable property AttProperty of the same type as the values. Now I tried to bind the S1 to AttProperty but this gives me an error:
Additional information: A 'Binding' cannot be set on the
'SetAttProperty' property of type 'TextBox'. A 'Binding' can only be
set on a DependencyProperty of a DependencyObject.
So how can I bind with an attachable DependencyProperty to another DependencyProperty?
Here is the code for the singleton I have so far (C#):
public class DO : DependencyObject
{
// Singleton pattern (Expose a single shared instance, prevent creating additional instances)
public static readonly DO Instance = new DO();
private DO() { }
public static readonly DependencyProperty S1Property = DependencyProperty.Register(
"S1", typeof(string), typeof(DO),
new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender));
public string S1
{
get { return (string)GetValue(S1Property); }
set { SetValue(S1Property, value); }
}
public static readonly DependencyProperty AttProperty = DependencyProperty.RegisterAttached(
"Att", typeof(string), typeof(DO),
new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender) );
public static void SetAttProperty(DependencyObject depObj, string value)
{
depObj.SetValue(AttProperty, value);
}
public static string GetAttProperty(DependencyObject depObj)
{
return (string)depObj.GetValue(AttProperty);
}
}
Here is the problematic thing (XAML):
<TextBox Name="Input" Text="" TextChanged="Input_TextChanged" local:DO.AttProperty="{Binding Source={x:Static local:DO.Instance}, Path=S1}" />
Update
With the changes of Bojin Li the errors go away. But one issue remains - if I now try to update the singleton with the help of the attached property like this:
<TextBox local:DO.Att="{Binding Source={x:Static local:DO.Instance}, Path=S1, Mode=TwoWay}" Text="{Binding Path=(local:DO.Att), RelativeSource={RelativeSource Self}, Mode=TwoWay}"/>
Why is the value not propagated to S1 in the singleton?

Answer 1

You need to implement INotifyPropertyChanged and wire the changes up to the dependency property changing.
public class DO : DependencyObject,INotifyPropertyChanged {
// Singleton pattern (Expose a single shared instance, prevent creating additional instances)
public static readonly DO Instance = new DO();
private DO() { }
public static readonly DependencyProperty S1Property = DependencyProperty.Register(
"S1", typeof(string), typeof(DO),
new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender,onS1Changed));
private static void onS1Changed(DependencyObject d, DependencyPropertyChangedEventArgs e) {
DO item = d as DO;
if (item != null) item.OnPropertyChanged(new PropertyChangedEventArgs("S1"));
}
public string S1 {
get { return (string)GetValue(S1Property); }
set { SetValue(S1Property, value); }
}
public static readonly DependencyProperty AttProperty = DependencyProperty.RegisterAttached(
"Att", typeof(string), typeof(DO),
new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender,onAttChanged));
private static void onAttChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
DO item = d as DO;
if (item != null) item.OnPropertyChanged(new PropertyChangedEventArgs("Att"));
}
public static void SetAttProperty(DependencyObject depObj, string value) {
depObj.SetValue(AttProperty, value);
}
public static string GetAttProperty(DependencyObject depObj) {
return (string)depObj.GetValue(AttProperty);
}
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(PropertyChangedEventArgs e) {
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, e);
}
}

Answer 2

I don't think you named your Get/Set Accessors correctly for your attached Property as documented here. Try this instead:
public static readonly DependencyProperty AttProperty = DependencyProperty.RegisterAttached(
"Att", typeof(string), typeof(DO),
new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender));
public static void SetAtt(DependencyObject depObj, string value)
{
depObj.SetValue(AttProperty, value);
}
public static string GetAtt(DependencyObject depObj)
{
return (string)depObj.GetValue(AttProperty);
}
Example Binding:
<TextBlock local:DO.Att="{Binding Source={x:Static local:DO.Instance}, Path=S1}" Text="{Binding Path=(local:DO.Att),RelativeSource={RelativeSource Self}}">