Custom dependency property not set on initialization - wpf

Question

I have UserControl ImageView that I want to add a cutom property called UseOverlay to.
<XAML>
<UserControl x:Class="ImageView" .../>
<XAML.cs>
public partial class ImageView : UserControl
{
public static DependencyProperty UseOverlayProperty;
public ImageView()
{
InitializeComponent();
if (UseOverlay)
{
AddOverlay();
}
}
static ImageView()
{
UseOverlayProperty = DependencyProperty.Register("UseOverlay", typeof(bool), typeof(ImageView), new PropertyMetadata(false));
}
public bool UseOverlay
{
get { return (bool)GetValue(UseOverlayProperty); }
set { SetValue(UseOverlayProperty, value); }
}
}
However, when used from another userControl, the property is not set. The ImageView is displayed, but without the overlay, and debugging shows UseOverlay as false.
<ImageView MaxWidth="450" UseOverlay="True"/>
What am I missing?

Answer 1

at the moment UseOverlay used only once in constructor (where it is false according to default value). When UseOverlay="True" is applied nothing happens. You need to add DP ChangedCallback:
DependencyProperty.Register("UseOverlay", typeof(bool), typeof(ImageView),
new PropertyMetadata(false, UseOverlayChangedCallback));
private static void UseOverlayChangedCallback(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
if ((bool) e.NewValue)
((ImageView)obj).AddOverlay();
else
((ImageView)obj).HideOverlay();
}

Answer 2

first you must not use it in the constructor (whithout default value) and use the callback to update the layout
#region --------------------Is playing--------------------
/// <summary>
/// Playing status
/// </summary>
public static readonly DependencyProperty IsPlayingProperty = DependencyProperty.Register("IsPlaying", typeof(bool), typeof(WaitSpin),
new FrameworkPropertyMetadata(new PropertyChangedCallback(OnIsPlayingChanged)));
/// <summary>
/// OnIsPlayingChanged callback
/// </summary>
/// <param name="d"></param>
/// <param name="e"></param>
private static void OnIsPlayingChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
if (System.ComponentModel.DesignerProperties.GetIsInDesignMode(d))
return;
WaitSpin element = d as WaitSpin;
element.ChangePlayMode((bool)e.NewValue);
}
/// <summary>
/// IsPlaying
/// </summary>
[System.ComponentModel.Category("Loading Animation Properties"), System.ComponentModel.Description("Incates wheter is playing or not.")]
public bool IsPlaying
{
get { return (bool)GetValue(IsPlayingProperty); }
set { SetValue(IsPlayingProperty, value); }
}
#endregion
you can add default with this, replace last part of register
new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
RatingValueChanged)