When is DataTemplate used in ContentControl and ItemsControl? - wpf

Question

I have WPF controls that I created that inherits the FrameworkElement class. One of them is done like this:
[ContentProperty("Children")]
public class ItemsElement : FrameworkElement
{
public ItemsElement()
{
Children = new UIElementCollection(this, this);
}
public UIElementCollection Children { get; private set; }
protected override int VisualChildrenCount
{
get
{
if (Children != null)
{
return Children.Count;
}
return 0;
}
}
protected override Visual GetVisualChild(int index)
{
return Children[index];
}
protected override IEnumerator LogicalChildren
{
get
{
if (Children != null)
{
return Children.GetEnumerator();
}
return EmptyEnumerator.Instance;
}
}
protected override Size MeasureOverride(Size availableSize)
{
return availableSize;
}
protected override Size ArrangeOverride(Size finalSize)
{
return finalSize;
}
}
What I want with this code is to have elements that are not 2D-UI elements. I overrided the Measure and Arrange methods so that it does not compute Sizes of the Children. This is a behavior that I want because these computations are heavy and useless.
The problem is if I put ContentControls and ItemsControls in the Children of my ItemsElement class (through XAML), these controls do not seem to generate their Visual children from Content/ItemsSource with the DataTemplate I defined for them in the Resources.
It does work if I add this code at the beginning of the MeasureOverride method:
foreach (UIElement child in Children)
{
child.Measure(availableSize);
}
I can't understand why...
Do you have any idea?
Thanks!

Answer 1

The question really is why are you extending a class who's behaviour you don't want? The answer is too long for here, but you can read it in full in the Control Authoring Overview page on MSDN. But to answer in short, I can only assume that you are extending the wrong class. Maybe you don't even need to extend any class.
In WPF, there are many alternatives to writing a new control. Often the only real need to do that is to perform some special custom drawing for a unique control that cannot be composed by other means. As the linked page says, WPF enables you to customize existing controls by using its rich content model, styles, templates, and triggers. Further on it continues:
Controls that derive from UserControl or Control rely upon composing existing elements. For many scenarios, this is an acceptable solution, because any object that inherits from FrameworkElement can be in a ControlTemplate. However, there are times when a control's appearance requires more than the functionality of simple element composition. For these scenarios, basing a component on FrameworkElement is the right choice.
Finally, the MeasureOverride and ArrangeOverride methods are required to be implemented correctly in order for the FrameworkElement to render its items. You just need to look at MSDN to find this basic information out. From the FrameworkElement.MeasureOverride Method page:
Your implementation should do the following:
1.Iterate your element's particular collection of children that are part of layout, call Measure on each child element.
And from the FrameworkElement.ArrangeOverride Method page:
Parent elements should call Arrange on each child, otherwise the child elements will not be rendered.
Please read the linked pages for further information and in future, please look in MSDN for information regarding your problem control, method, event, etc. before asking questions here.
UPDATE >>>
In response to your somewhat offensive comment, I can see you do have a valid reason to extend the FrameworkElement class, but that doesn't change anything. You say that you've read all the pages that I showed you, but for some reason, you haven't followed the clear instructions on what you have to do to render the items correctly.
There's no need to answer your question regarding the DataTemplate and when exactly it is rendered because it is irrelevant to your problem. Your problem is caused simply because you have not followed the instructions found on MSDN in your class. That's it. One thing that you can check to see if this is correct or not is this.
If you see the full name of the type of objects in the collection when you run your application, then you have a problem with your DataTemplate not being applied. However, if you see no items, then you have a problem with your rendering and that's simply because you haven't implemented those two essential methods correctly.