可以观察到的.FromEvent与DataContextChanged一起使用

Question

我想获得一个控件的DataContext作为一个IObservable。我试过了:

Observable.FromEvent<DependencyPropertyChangedEventHandler,
     DependencyPropertyChangedEventArgs>
     (a => this.DataContextChanged += a, a => this.DataContextChanged -= a)
     .Subscribe(e => MessageBox.Show(e.NewValue.ToString()));

可以很好地编译，但是在调用Subscribe时，它会抛出异常:错误绑定到目标方法。

我的猜测是，这是因为DependencyPropertyChangedEventArgs不从事件参数继承，但我不确定。如有任何正确的建议，我将不胜感激。

Answer 1

FromEvent的另一个重载之一允许您从方法中自己创建委托。允许你这样做:

Observable.FromEvent
    <DependencyPropertyChangedEventHandler, DependencyPropertyChangedEventArgs>
    (a => new DependencyPropertyChangedEventHandler(
        new Action<object, DependencyPropertyChangedEventArgs>((s, e) => a(e))),
        h => this.DataContextChanged += h, h => this.DataContextChanged -= h)
        .Subscribe(e => MessageBox.Show(e.NewValue.ToString()));

似乎可以

Answer 2

您已经'击中了钉子'，由于某种原因DependencyPropertyChangedEventArgs是一个结构体，因此不继承EventArgs。请看这个相关的博客文章:

http://weblogs.asp.net/okloeten/archive/2007/12/10/5430331.aspx

这使得使用通用事件处理机制无法处理此事件。

我猜你将不得不在你自己的代码中处理这个事件，并手动创建一个Observable。

Answer 3

自定义实现将非常简单，如:

Observable.CreateWithDisposable<DependencyPropertyChangedEventHandler>(obs =>
{
   var subject = new Subject<DependencyPropertyChangedEventHandler>();
   //Attach event handler and on event call OnNext of subject with the value
   return subject.Subscribe(obs);
});

UPDATED:处理事件处理程序删除

public class GenericDisposable : IDisposable
        {
            Action _act = null;
            IDisposable _disp = null;
            public GenericDisposable(Action act,IDisposable disp)
            {
                _act = act;
                _disp = disp;
            }
            public void Dispose()
            {
                _act();
                _disp.Dispose();
            }
        }
        public static IObservable<DependencyPropertyChangedEventHandler> CreateForEvent()
        {
            return Observable.Defer(() =>
            {
                return Observable.CreateWithDisposable<DependencyPropertyChangedEventHandler>(obs =>
                {
                    var subject = new Subject<DependencyPropertyChangedEventHandler>();
                    //Attach event handler and on event call OnNext of subject with the value
                    return new GenericDisposable(() => {/*remove event handler*/}, subject.Subscribe(obs));
                });
            });
        }