如何创建自定义Adorner ContentControl？

Question

我正在尝试创建一个我可以放置的装饰图层，例如通知。我想尽可能地使它成为一般和XAML编写的，所以我创建了一个名为ContentControl的新AdornerContentControl，使用如下：

<AdornerDecorator Grid.Row="1" Name="RichTextBoxAdornerDecorator">
    <loc:AdornerContentControl x:Name="TestAdorner">
        <loc:AdornerContentControl.Decorator>
            <Button Width="100" Height="100">ZZZ</Button>
        </loc:AdornerContentControl.Decorator>
        <RichTextBox>
            <!-- some more stuff... -->
        </RichTextBox>
    </loc:AdornerContentControl>
</AdornerDecorator>

这是代码：

public class AdornerContentControl : ContentControl {
    private SimpleAdorner _adorner;

        public static readonly DependencyProperty DecoratorProperty = DependencyProperty.Register(
            "Decorator", typeof (Control), typeof (AdornerContentControl), new PropertyMetadata(default(Control),
                OnDecoratorPropertyChanged));

        private static void OnDecoratorPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) { /*...*/ }

        protected override void OnInitialized(EventArgs e)
        {
            base.OnInitialized(e);
            try { _adorner = new SimpleAdorner((UIElement)Content, Decorator); }
            catch (NullReferenceException) { }
        }

        public Control Decorator
        {
            get { return (Control) GetValue(DecoratorProperty); }
            set { SetValue(DecoratorProperty, value); }
        }
}

public class SimpleAdorner : Adorner{
    public SimpleAdorner([NotNull] UIElement adornedElement, Control content) : base(adornedElement)
    {
        _visuals = new VisualCollection(this);
        _adornerLayer = AdornerLayer.GetAdornerLayer(adornedElement);

        if (content == null) return;

        _visuals.Add(_contentPresenter = new ContentPresenter()
        {
            Content = content
        });

        _adornerLayer.Add(this);
    }
}

但AdornerLayer.GetAdornerLayer(adornedElement)始终返回null（SimpleAdorner是在OnInitialized中构建的。）

Should I explicitly create AdornerDecorator？

Am I supposed to call Content.UpdateLayout somewhere？