Building a logical tree for a custom control

Vote count:

0

I'm working on a control that has an ObservableCollection as one of its DependencyProperties. This property is set as the DefaultProperty for my control, so I can implicitly add items to the collection in XAML by constructing lines similar to the following:

<MyControl>
    <MyItem/>
    <MyItem/>
    <MyItem/>
</MyControl>

As far as I understand, the WPF engine builds the logical tree when it parses through the XAML. Therefore, each MyItem should be a logical child of the MyControl. Conversely, the MyControl should be the logical parent of each MyItem.

Well, apparently there is more to it. Below is the code that I use to define the relationship above. The custom control contains a DependencyProperty for the ObservableCollection and it is backed by a CLR-property.

[ContentProperty("Items")]
public class MyControl : Control
{
    public static readonly DependencyProperty ItemsProperty = DependencyProperty.Register(
        "Items", 
        typeof(ObservableCollection<MyItem>), 
        typeof(MyControl), 
        new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsRender, OnItemsChangedProperty));

    [Category("MyControl")]
    public ObservableCollection<MyItem> Items
    {
        get { return (ObservableCollection<MyItem>)GetValue(ItemsProperty); }
        set { SetValue(ItemsProperty, value); }
    }

    public MyControl() : base()
    {   //Set a new collection per control, but don't destroy binding.
        SetCurrentValue(ItemsProperty, new ObservableCollection<MyItem>());
    }
}

From with the MyItem (which inherits FrameworkContentElement), I attempt to access its logical parent like so:

if (Parent is FrameworkElement)
{    //Re-Render the parent control.
    ((FrameworkElement)Parent).InvalidateVisual();
}

To my surprise, the parent control is never re-rendered. This is because the MyItem.Parent property is null. Why is this?

How can I instruct the WPF engine to know that MyControl should be the logical parent to MyItem?

asked 22 secs ago

Nick Miller

341

Building a logical tree for a custom control