How can I fix the DependencyPropertyDescriptor Add

Question

I know I need to call RemoveValueChanged, but I have not been able to find a reliable place to call this. I'm learning that there probably isn't one.

I looks like I need to find a different way to monitor the change then adding a handler using AddValueChanged. I'm looking for advice on the best way to achieve this. I've seen the recommendation of using a PropertyChangedCallback in the PropertyMetadata, but I'm not sure how to do this when my TextBox and Adorner are not static. Also, the IsFocused property is not a DependencyProperty created in my class.

Thanks.

public sealed class WatermarkTextBoxBehavior
{
    private readonly TextBox m_TextBox;
    private TextBlockAdorner m_TextBlockAdorner;

    private WatermarkTextBoxBehavior(TextBox textBox)
    {
        if (textBox == null)
            throw new ArgumentNullException("textBox");

        m_TextBox = textBox;
    }

    #region Behavior Internals

    private static WatermarkTextBoxBehavior GetWatermarkTextBoxBehavior(DependencyObject obj)
    {
        return (WatermarkTextBoxBehavior)obj.GetValue(WatermarkTextBoxBehaviorProperty);
    }

    private static void SetWatermarkTextBoxBehavior(DependencyObject obj, WatermarkTextBoxBehavior value)
    {
        obj.SetValue(WatermarkTextBoxBehaviorProperty, value);
    }

    private static readonly DependencyProperty WatermarkTextBoxBehaviorProperty =
        DependencyProperty.RegisterAttached("WatermarkTextBoxBehavior",
            typeof(WatermarkTextBoxBehavior), typeof(WatermarkTextBoxBehavior), new UIPropertyMetadata(null));

    public static bool GetEnableWatermark(TextBox obj)
    {
        return (bool)obj.GetValue(EnableWatermarkProperty);
    }

    public static void SetEnableWatermark(TextBox obj, bool value)
    {
        obj.SetValue(EnableWatermarkProperty, value);
    }

    public static readonly DependencyProperty EnableWatermarkProperty =
        DependencyProperty.RegisterAttached("EnableWatermark", typeof(bool),
            typeof(WatermarkTextBoxBehavior), new UIPropertyMetadata(false, OnEnableWatermarkChanged));

    private static void OnEnableWatermarkChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (e.OldValue != null)
        {
            var enabled = (bool)e.OldValue;

            if (enabled)
            {
                var textBox = (TextBox)d;
                var behavior = GetWatermarkTextBoxBehavior(textBox);
                behavior.Detach();

                SetWatermarkTextBoxBehavior(textBox, null);
            }
        }

        if (e.NewValue != null)
        {
            var enabled = (bool)e.NewValue;

            if (enabled)
            {
                var textBox = (TextBox)d;
                var behavior = new WatermarkTextBoxBehavior(textBox);
                behavior.Attach();

                SetWatermarkTextBoxBehavior(textBox, behavior);
            }
        }
    }

    private void Attach()
    {
        m_TextBox.Loaded += TextBoxLoaded;
        m_TextBox.TextChanged += TextBoxTextChanged;
        m_TextBox.DragEnter += TextBoxDragEnter;
        m_TextBox.DragLeave += TextBoxDragLeave;
        m_TextBox.IsVisibleChanged += TextBoxIsVisibleChanged;
    }

    private void Detach()
    {
        m_TextBox.Loaded -= TextBoxLoaded;
        m_TextBox.TextChanged -= TextBoxTextChanged;
        m_TextBox.DragEnter -= TextBoxDragEnter;
        m_TextBox.DragLeave -= TextBoxDragLeave;
        m_TextBox.IsVisibleChanged -= TextBoxIsVisibleChanged;
    }

    private void TextBoxDragLeave(object sender, DragEventArgs e)
    {
        UpdateAdorner();
    }

    private void TextBoxDragEnter(object sender, DragEventArgs e)
    {
        m_TextBox.TryRemoveAdorners<TextBlockAdorner>();
    }

    private void TextBoxIsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        UpdateAdorner();
    }

    private void TextBoxTextChanged(object sender, TextChangedEventArgs e)
    {
        var hasText = !string.IsNullOrEmpty(m_TextBox.Text);
        SetHasText(m_TextBox, hasText);
    }

    private void TextBoxLoaded(object sender, RoutedEventArgs e)
    {
        Init();
    }

    #endregion

    #region Attached Properties

    public static string GetLabel(TextBox obj)
    {
        return (string)obj.GetValue(LabelProperty);
    }

    public static void SetLabel(TextBox obj, string value)
    {
        obj.SetValue(LabelProperty, value);
    }

    public static readonly DependencyProperty LabelProperty =
        DependencyProperty.RegisterAttached("Label", typeof(string), typeof(WatermarkTextBoxBehavior));

    public static Style GetLabelStyle(TextBox obj)
    {
        return (Style)obj.GetValue(LabelStyleProperty);
    }

    public static void SetLabelStyle(TextBox obj, Style value)
    {
        obj.SetValue(LabelStyleProperty, value);
    }

    public static readonly DependencyProperty LabelStyleProperty =
        DependencyProperty.RegisterAttached("LabelStyle", typeof(Style),
            typeof(WatermarkTextBoxBehavior));

    public static bool GetHasText(TextBox obj)
    {
        return (bool)obj.GetValue(HasTextProperty);
    }

    private static void SetHasText(TextBox obj, bool value)
    {
        obj.SetValue(HasTextPropertyKey, value);
    }

    private static readonly DependencyPropertyKey HasTextPropertyKey =
        DependencyProperty.RegisterAttachedReadOnly("HasText", typeof(bool),
            typeof(WatermarkTextBoxBehavior), new UIPropertyMetadata(false));

    public static readonly DependencyProperty HasTextProperty =
        HasTextPropertyKey.DependencyProperty;

    #endregion

    private void Init()
    {
        m_TextBlockAdorner = new TextBlockAdorner(m_TextBox, GetLabel(m_TextBox), GetLabelStyle(m_TextBox));
        UpdateAdorner();

        DependencyPropertyDescriptor focusProp = DependencyPropertyDescriptor.FromProperty(UIElement.IsFocusedProperty, typeof(FrameworkElement));
        if (focusProp != null)
        {
            focusProp.AddValueChanged(m_TextBox, (sender, args) => UpdateAdorner());
        }

        DependencyPropertyDescriptor containsTextProp = DependencyPropertyDescriptor.FromProperty(HasTextProperty, typeof(TextBox));
        if (containsTextProp != null)
        {
            containsTextProp.AddValueChanged(m_TextBox, (sender, args) => UpdateAdorner());
        }
    }

    private void UpdateAdorner()
    {
        if (GetHasText(m_TextBox) ||
            m_TextBox.IsFocused ||
            !m_TextBox.IsVisible)
        {
            // Hide the Watermark Label if the adorner layer is visible
            m_TextBox.ToolTip = GetLabel(m_TextBox);
            m_TextBox.TryRemoveAdorners<TextBlockAdorner>();
        }
        else
        {
            // Show the Watermark Label if the adorner layer is visible
            m_TextBox.ToolTip = null;
            m_TextBox.TryAddAdorner<TextBlockAdorner>(m_TextBlockAdorner);
        }
    }
}

Answer 1

A more lightweight solution for FrameworkElements and FrameworkContentElements is to subscribe to the Unloaded event and remove the handler. This requires a non-anonymous delegate (UpdateAdorner in that case) though:

focusProp.AddValueChanged(m_TextBox, UpdateAdorner);
m_TextBox.Unloaded += (sender, args) => focusProp.RemoveValueChanged(sender, UpdateAdorner);

Answer 2

AddValueChanged of dependency property descriptor results in memory leak as you already know. So, as described here, you can create custom class PropertyChangeNotifier to listen to any dependency property changes.

Complete implementation can be found here - PropertyDescriptor AddValueChanged Alternative.

---

Quote from the link:

This class takes advantage of the fact that bindings use weak references to manage associations so the class will not root the object who property changes it is watching. It also uses a WeakReference to maintain a reference to the object whose property it is watching without rooting that object. In this way, you can maintain a collection of these objects so that you can unhook the property change later without worrying about that collection rooting the object whose values you are watching.

Also for sake of completeness of answer I am posting complete code here to avoid any rot issue in future.

public sealed class PropertyChangeNotifier : DependencyObject, IDisposable
{
    #region Member Variables

    private readonly WeakReference _propertySource;

    #endregion // Member Variables

    #region Constructor
    public PropertyChangeNotifier(DependencyObject propertySource, string path)
        : this(propertySource, new PropertyPath(path))
    {
    }
    public PropertyChangeNotifier(DependencyObject propertySource, DependencyProperty property)
        : this(propertySource, new PropertyPath(property))
    {
    }
    public PropertyChangeNotifier(DependencyObject propertySource, PropertyPath property)
    {
        if (null == propertySource)
            throw new ArgumentNullException("propertySource");
        if (null == property)
            throw new ArgumentNullException("property");
        _propertySource = new WeakReference(propertySource);
        Binding binding = new Binding
        {
            Path = property, 
            Mode = BindingMode.OneWay, 
            Source = propertySource
        };
        BindingOperations.SetBinding(this, ValueProperty, binding);
    }
    #endregion // Constructor

    #region PropertySource
    public DependencyObject PropertySource
    {
        get
        {
            try
            {
                // note, it is possible that accessing the target property
                // will result in an exception so i’ve wrapped this check
                // in a try catch
                return _propertySource.IsAlive
                ? _propertySource.Target as DependencyObject
                : null;
            }
            catch
            {
                return null;
            }
        }
    }
    #endregion // PropertySource

    #region Value
    /// <summary>
    /// Identifies the <see cref="Value"/> dependency property
    /// </summary>
    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value",
    typeof(object), typeof(PropertyChangeNotifier), new FrameworkPropertyMetadata(null, OnPropertyChanged));

    private static void OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        PropertyChangeNotifier notifier = (PropertyChangeNotifier)d;
        if (null != notifier.ValueChanged)
            notifier.ValueChanged(notifier, EventArgs.Empty);
    }

    /// <summary>
    /// Returns/sets the value of the property
    /// </summary>
    /// <seealso cref="ValueProperty"/>
    [Description("Returns/sets the value of the property")]
    [Category("Behavior")]
    [Bindable(true)]
    public object Value
    {
        get
        {
            return GetValue(ValueProperty);
        }
        set
        {
            SetValue(ValueProperty, value);
        }
    }
    #endregion //Value

    #region Events
    public event EventHandler ValueChanged;
    #endregion // Events

    #region IDisposable Members

    public void Dispose()
    {
        BindingOperations.ClearBinding(this, ValueProperty);
    }

    #endregion
}