监听依赖属性的变化

Question

有什么方法可以监听DependencyProperty 的变化吗？我想在值更改但无法使用绑定时收到通知并执行一些操作。它是另一个类的DependencyProperty。

Answer 1

如果它是一个单独类的DependencyProperty，最简单的方法是绑定一个值，然后监听该值的变化。

如果 DP 是您在自己的课程中实现的，那么您可以在创建 DependencyProperty 时使用 register a PropertyChangedCallback。您可以使用它来监听属性的变化。

如果您正在使用子类，您可以使用 OverrideMetadata 将您自己的 PropertyChangedCallback 添加到将被调用的 DP，而不是任何原始的。

Answer 2

如果是这样的话，One hack。您可以使用DependencyProperty 引入静态类。您的源类也绑定到该 dp，而您的目标类也绑定到 DP。

Answer 3

这里肯定少了这个方法：

DependencyPropertyDescriptor
    .FromProperty(RadioButton.IsCheckedProperty, typeof(RadioButton))
    .AddValueChanged(radioButton, (s,e) => { /* ... */ });

Answer 4

我写了这个实用类：

• 它为 DependencyPropertyChangedEventArgs 提供了旧值和新值。
• 源存储在绑定中的弱引用中。
• 不确定公开 Binding 和 BindingExpression 是否是个好主意。
• 没有泄漏。

using System;
using System.Collections.Concurrent;
using System.Windows;
using System.Windows.Data;

public sealed class DependencyPropertyListener : DependencyObject, IDisposable
{
    private static readonly ConcurrentDictionary<DependencyProperty, PropertyPath> Cache = new ConcurrentDictionary<DependencyProperty, PropertyPath>();

    private static readonly DependencyProperty ProxyProperty = DependencyProperty.Register(
        "Proxy",
        typeof(object),
        typeof(DependencyPropertyListener),
        new PropertyMetadata(null, OnSourceChanged));

    private readonly Action<DependencyPropertyChangedEventArgs> onChanged;
    private bool disposed;

    public DependencyPropertyListener(
        DependencyObject source, 
        DependencyProperty property, 
        Action<DependencyPropertyChangedEventArgs> onChanged = null)
        : this(source, Cache.GetOrAdd(property, x => new PropertyPath(x)), onChanged)
    {
    }

    public DependencyPropertyListener(
        DependencyObject source, 
        PropertyPath property,
        Action<DependencyPropertyChangedEventArgs> onChanged)
    {
        this.Binding = new Binding
        {
            Source = source,
            Path = property,
            Mode = BindingMode.OneWay,
        };
        this.BindingExpression = (BindingExpression)BindingOperations.SetBinding(this, ProxyProperty, this.Binding);
        this.onChanged = onChanged;
    }

    public event EventHandler<DependencyPropertyChangedEventArgs> Changed;

    public BindingExpression BindingExpression { get; }

    public Binding Binding { get; }

    public DependencyObject Source => (DependencyObject)this.Binding.Source;

    public void Dispose()
    {
        if (this.disposed)
        {
            return;
        }

        this.disposed = true;
        BindingOperations.ClearBinding(this, ProxyProperty);
    }

    private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var listener = (DependencyPropertyListener)d;
        if (listener.disposed)
        {
            return;
        }

        listener.onChanged?.Invoke(e);
        listener.OnChanged(e);
    }

    private void OnChanged(DependencyPropertyChangedEventArgs e)
    {
        this.Changed?.Invoke(this, e);
    }
}

---

using System;
using System.Windows;

public static class Observe
{
    public static IDisposable PropertyChanged(
        this DependencyObject source,
        DependencyProperty property,
        Action<DependencyPropertyChangedEventArgs> onChanged = null)
    {
        return new DependencyPropertyListener(source, property, onChanged);
    }
}

Answer 5

您可以继承您尝试收听的控件，然后直接访问：

protected void OnPropertyChanged(string name)

没有内存泄漏的风险。

不要害怕标准的 OO 技术。

Answer 6

有多种方法可以实现这一点。这是一种将依赖属性转换为可观察属性的方法，以便可以使用System.Reactive 订阅它：

public static class DependencyObjectExtensions
{
    public static IObservable<EventArgs> Observe<T>(this T component, DependencyProperty dependencyProperty)
        where T:DependencyObject
    {
        return Observable.Create<EventArgs>(observer =>
        {
            EventHandler update = (sender, args) => observer.OnNext(args);
            var property = DependencyPropertyDescriptor.FromProperty(dependencyProperty, typeof(T));
            property.AddValueChanged(component, update);
            return Disposable.Create(() => property.RemoveValueChanged(component, update));
        });
    }
}

用法

记得释放订阅以防止内存泄漏：

public partial sealed class MyControl : UserControl, IDisposable 
{
    public MyControl()
    {
        InitializeComponent();

        // this is the interesting part 
        var subscription = this.Observe(MyProperty)
                               .Subscribe(args => { /* ... */}));

        // the rest of the class is infrastructure for proper disposing
        Subscriptions.Add(subscription);
        Dispatcher.ShutdownStarted += DispatcherOnShutdownStarted; 
    }

    private IList<IDisposable> Subscriptions { get; } = new List<IDisposable>();

    private void DispatcherOnShutdownStarted(object sender, EventArgs eventArgs)
    {
        Dispose();
    }

    Dispose(){
        Dispose(true);
    }

    ~MyClass(){
        Dispose(false);
    }

    bool _isDisposed;
    void Dispose(bool isDisposing)
    {
        if(_disposed) return;

        foreach(var subscription in Subscriptions)
        {
            subscription?.Dispose();
        }

        _isDisposed = true;
        if(isDisposing) GC.SupressFinalize(this);
    }
}