取消订阅存储在附加属性中的集合的 CollectionChanged 事件
好的，我有一个附加属性（在静态类中声明），它将 INotifyCollectionChanged 属性附加到对象。
设置属性后，我想开始监视集合的更改，然后对集合所附加的对象执行一些操作。
第一次尝试：
private static void MyProperty_OnChanged( DependencyObject d, DependencyPropertyChangedEventArgs e )
{
// We need both the dependency object, and the collection args to process the notification
NotifyCollectionChangedEventHandler changedFunc = (sender, eventArgs) => MyProperty_OnCollectionChanged( d, sender, eventArgs );
if( e.OldValue != null )
e.OldValue.CollectionChanged -= changedFunc; // Can you see the bug?
if( e.NewValue != null )
e.NewValue.CollectionChanged += changedFunc;
}
为了将集合附加到处理程序中的对象，我将 d 拉入闭包。很容易，对吧？
好吧，我相信你能在这里看到这个错误。当集合被删除或替换为新集合时，它无法取消注册事件处理程序，因为changedFunc是具有不同闭包的新处理程序。
那么，这样做的正确方法是什么？
Ok, so I have an attached property (declared in a static class) which attaches an INotifyCollectionChanged property to an object.
When the property is set, I want to start monitoring the collection for changes, and then perform some action on the object to which the collection is attached.
First attempt:
private static void MyProperty_OnChanged( DependencyObject d, DependencyPropertyChangedEventArgs e )
{
// We need both the dependency object, and the collection args to process the notification
NotifyCollectionChangedEventHandler changedFunc = (sender, eventArgs) => MyProperty_OnCollectionChanged( d, sender, eventArgs );
if( e.OldValue != null )
e.OldValue.CollectionChanged -= changedFunc; // Can you see the bug?
if( e.NewValue != null )
e.NewValue.CollectionChanged += changedFunc;
}
In order to get the object the collection is attached to into the handler, I pull d into the closure. Easy enough, right?
Well, I'm sure you can see the bug here. When the collection is removed or replaced with a new collection, it fails to un-register the event handler, because changedFunc is a new handler with a different closure.
So, what's the right way to do this?
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从您的代码中，不清楚这个静态方法位于何处。
有什么方法可以将事件处理程序存储为其所属类的成员变量吗？这将是一个可以根据需要添加和删除的单一参考。当前，每次 MyProperty_OnChanged 触发时，您都会构建一个新的事件处理程序。
或者，您是否可以更改集合以存储对事件处理程序的引用，以便您可以通过 e.OldValue.OldHandler （或类似的）访问它，以便稍后可以访问它？
From your code, it's not clear where this static method lives.
Is there some way you can store the event handler as a member variable of the class it belongs to? That would be a singular reference that could be added and removed as necessary. You are currently constructing a new event handler each time MyProperty_OnChanged fires.
Alternately, can you change your collection to store a reference to the event handler such that you can access it through e.OldValue.OldHandler (or similar) so you would have access to it later?
我通过使用另一个附加属性来保存包含事件处理程序和对
d的引用的私有对象来解决这个问题。实际上，我显式创建了一个闭包类，并保留对它的引用。但这对我来说似乎是一个相当严厉的方法，我真的在寻找一个更优雅的解决方案。
I solved this by using another attached property to hold a private object containing the event handler and reference to
d. Effectively I'm explicitly creating a closure class, and keeping a reference to it.But this seems a pretty heavy handed approach to me, I was really looking for a more elegant solution.