我该如何提出一个附加的事件处理程序？

Question

我想创建一个触发时Panel.OnVisualChildrenChanged被称为附着面板事件：

public class PanelObserver { 
    public static readonly RoutedEvent VisualChildrenChangedEvent; 

    static PanelObserver() { 
     VisualChildrenChangedEvent = EventManager.RegisterRoutedEvent(
      "VisualChildrenChanged", 
      RoutingStrategy.Bubble, 
      typeof(RoutedEventHandler), 
      typeof(Panel)); 
    } 

    public static void AddVisualChildrenChangedHandler(
     DependencyObject d, RoutedEventHandler handler) { 
     //Something needs to be done to d so that when OnVisualChildrenChanged is called, 
     //it will fire handler. 
     (d as Panel)?.AddHandler(VisualChildrenChangedEvent, handler); 
    } 

    public static void RemoveVisualChildrenChangedHandler(
     DependencyObject d, RoutedEventHandler handler) { 
     //Something will likely need to be done to d here as well... 
     (d as Panel)?.RemoveHandler(VisualChildrenChangedEvent, handler); 
    } 
} 

使用情况是相当直截了当

<Grid myns:PanelObserver.VisualChildrenChanged = "SomeEventHandler" /> 

理想情况下，事件将每次Panel.OnVisualChildrenChanged是提高调用。如果可能的话，我需要添加到AddVisualChildrenChanged方法（以及它的对映）以实现此目的吗？如果这不是如何实现这一点，那么完成这个的适当方法是什么？

Answer 1

我不能想办法通过可附着的面板事件触发Panel.OnVisualChildrenChanged。 相反，您可以从任何Panel类派生并覆盖OnVisualChildrenChanged，然后声明另一个事件。

例如：

public class ObservablePanel : Panel 
{ 
    protected override void OnVisualChildrenChanged(DependencyObject visualAdded, DependencyObject visualRemoved) 
    { 
     base.OnVisualChildrenChanged(visualAdded, visualRemoved); 
     VisualChildrenChanged?.Invoke(this, EventArgs.Empty); 
    } 
    public event EventHandler VisualChildrenChanged; 
} 

为什么你无法通过可连接面板事件做的原因：
1. Panel.Children属性是UIElementCollection不公开任何事件捕捉，如果它“改变”。
2. Panel.Children属性为只读属性，因此您无法将其设置为UIElementCollection的自定义派生实现，该实现将显示“已更改”事件。
3. Panel.OnVisualChildrenChanged是protected因此您只能在Panel派生类上访问它。