Added Thumb not rendered

Question

I want to implement my own control derived from FrameworkElement, but the added child elements are not rendered.

I have no idea why.

public class RangeSelection : FrameworkElement
{
    private Thumb thumb = null;

    #region Construction / Destruction

    public RangeSelection()
    {
        this.thumb = new Thumb();
        this.thumb.Width    = 32.0;
        this.thumb.Height   = 32.0;
        this.AddVisualChild(this.thumb);

    }

    #endregion

    protected override Size MeasureOverride(Size availableSize)
    {
        this.thumb.Measure(availableSize);
        return new Size(64.0, 64.0);
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        this.thumb.Arrange(new Rect(0, 0, 64.0, 64.0));
        return base.ArrangeOverride(finalSize);
    }
}

Answer 1

You need to override VisualChildrenCount property and GetVisualChild method. Something like this:

protected override int VisualChildrenCount
{
    get { return thumb == null ? 0 : 1; }
}

protected override Visual GetVisualChild(int index)
{
    if (_child == null)
    {
        throw new ArgumentOutOfRangeException();
    }

    return _child;
}

In case you want more child elements, you should use some kind of collection to store child elements and then you will return collection's count or appropriate element of collection.