WPF - How do I use the UserControl with a dependency property and view model?

Question

Hello,

My goal is to have a user select a year and a month. Translate the selection into a date and have the user control send the date back to my view model. That part works for me....However, I cannot get the ViewModel's initial date to set those drop downs.

   public static readonly DependencyProperty Date =
      DependencyProperty.Register("ReturnDate", typeof(DateTime), typeof(DatePicker),
      new FrameworkPropertyMetadata{BindsTwoWayByDefault = true,});

   public DateTime ReturnDate
     {
        get { return Convert.ToDateTime(GetValue(Date)); }
        set
        {
            SetDropDowns(value);
            SetValue(Date, value);
        }
     }

The SetDropDowns(value) just sets the selected items on the combo boxes, however, the program never makes it to that method.

On the view I am using:

  <cc1:DatePicker ReturnDate="{Binding Path=StartDate, Mode=TwoWay}" IsStart="True" />

If this has been answered, then my bad. I looked around and didn't see anything that worked for me. Thus, when the program loads how do I get the value from the view model to a method in order to set the combo boxes?

Thanks,

-Scott

Answer 1

When you use a Dependency Property, the CLR property Setter is never fired (when binding sets the property).

The proper way to do this is to use a PropertyChangedCallback on your dependency property:

public static readonly DependencyProperty Date =
  DependencyProperty.Register("ReturnDate", typeof(DateTime), typeof(DatePicker),
  new FrameworkPropertyMetadata(
        DateTime.Now, 
        FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
        new PropertyChangedCallback(dateChanged)));

public DateTime ReturnDate
 {
    get { return Convert.ToDateTime(GetValue(Date)); }
    set
    {
        SetDropDowns(value);
        SetValue(Date, value);
    }
 }

private static void dateChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
  DatePicker instance = d as DatePicker;
  instance.SetDropDowns((DateTime)e.NewValue);
}