wpf 如何修复 AttachedBehavior 上的 DependencyPropertyDescriptor AddValueChanged 内存泄漏？

How can I fix the DependencyPropertyDescriptor AddValueChanged Memory Leak on AttachedBehavior?

提问by scuba88

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.

我知道我需要调用 RemoveValueChanged，但我一直无法找到一个可靠的地方来调用它。我正在学习可能没有一个。

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.

我看起来需要找到一种不同的方法来监视更改，然后使用 AddValueChanged 添加处理程序。我正在寻找有关实现这一目标的最佳方法的建议。我已经看到在 PropertyMetadata 中使用 PropertyChangedCallback 的建议，但是当我的 TextBox 和 Adorner 不是静态的时，我不确定如何执行此操作。此外， IsFocused 属性不是在我的类中创建的 DependencyProperty。

Thanks.

谢谢。

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);
        }
    }
}

回答by Rohit Vats

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

AddValueChanged正如您已经知道的那样，依赖属性描述符会导致内存泄漏。所以，作为描述在这里，您可以创建自定义的类PropertyChangeNotifier来听任何依赖属性的变化。

Complete implementation can be found here - PropertyDescriptor AddValueChanged Alternative.

完整的实现可以在这里找到 - PropertyDescriptor AddValueChanged Alternative。

---

---

Quote from the link:

引自链接：

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.

此类利用绑定使用弱引用来管理关联的事实，因此该类不会根植它正在监视的属性更改的对象。它还使用 Wea​​kReference 来维护对其正在监视的属性的对象的引用，而无需将该对象作为根对象。通过这种方式，您可以维护这些对象的集合，以便您以后可以解除属性更改的挂钩，而不必担心该集合是否会根植您正在查看其值的对象。

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

同样为了答案的完整性，我在这里发布完整的代码以避免将来出现任何腐烂问题。

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
}

回答by Lennart

A more lightweight solution for FrameworkElementsand FrameworkContentElementsis to subscribe to the Unloadedevent and remove the handler. This requires a non-anonymous delegate (UpdateAdornerin that case) though:

对于更轻量级的解决方案FrameworkElements，并FrameworkContentElements为订阅Unloaded事件和删除的处理程序。不过，这需要一个非匿名委托（UpdateAdorner在这种情况下）：

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