如何引发附加的事件处理程序？

Question

我想创建一个可Panel.OnVisualChildrenChanged Panel事件，该事件在Panel.OnVisualChildrenChanged时Panel.OnVisualChildrenChanged ：

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 );
    }
}

用例将非常简单

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

理想情况下，每次调用Panel.OnVisualChildrenChanged时都会引发该事件。 如果可能的话，我需要添加什么到AddVisualChildrenChanged方法（及其AddVisualChildrenChanged ）上？ 如果那不是完成任务的方法，那么正确的方法是什么？

Answer 1

我想不出一种通过可Panel.OnVisualChildrenChanged的Panel事件触发Panel.OnVisualChildrenChanged的方法。 相反，您可以从任何Panel类派生并重写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;
}

您无法通过附加的Panel事件执行此操作的原因：
1. Panel.Children属性是UIElementCollection ，如果“更改”，则不会公开任何要捕获的事件。
2. Panel.Children属性是只读的，因此您不能将其设置为UIElementCollection的自定义派生实现，它将暴露“已更改”事件。
3. Panel.OnVisualChildrenChanged protected因此您只能在Panel派生类上对其进行访问。