Drawing a FrameworkElement within another FrameworkElement

public class VisualCue : FrameworkElement
{
    public List<Indicator> Indicators { get; set; }

    public VisualCue()
    {
        this.Indicators = new List<Indicator>();
    }

    protected override int VisualChildrenCount
    {
        get { return this.Indicators.Count; }
    }

    protected override Visual GetVisualChild(int index)
    {
        return this.Indicators[index];
    }
}

开发者_高级运维public class Indicator : FrameworkElement
{
    protected override void OnRender(DrawingContext context)
    {
        context.DrawEllipse(Brushes.Red, 
            new Pen(Brushes.Black, 2), new Point(0, 0), 10, 10);

        base.OnRender(context);
    }
}

And in XAML:

<local:VisualCue x:Name="visualCue">
 <local:VisualCue.Indicators>
  <local:Indicator />
 </local:VisualCue.Indicators>
</local:VisualCue>

But the indicator doesn't get drawn. What am I missing?

Have you tried implementing the LogicalChildren method?

protected override IEnumerator LogicalChildren
{
    get { return indicators.GetEnumerator(); }
}

I had all sorts of trouble when I developed my first 'FrameworkElement`.

After extensive research I basically discovered there were a number of methods you have to override in order for it to play nicely with WPF.

At the very least you need to override ArrangeOverride in VisualCue:

protected override Size ArrangeOverride(Size finalSize)
{
    foreach (Indicator indicator in Indicators)
    {
        indicator.Arrange(new Rect(0, 0, 10, 10));
    }

    return finalSize;
}

The values in the Rect passed to Indicator children depends on your layout needs. You should also override MeasureOverride as well.

FrameworkElement.ArrangeOverride method

FrameworkElement.MeasureOverride method