WPF schema - what defines a ListBox to have a ScrollViewer? - wpf

Question

For years, I have felt I don't have a good understanding of WPF because I haven't found an authoritative reference on the possibilities. For example, I just found out that a ListBox has an attached ScrollViewer property.
<ListBox ScrollViewer.HorizontalScrollBarVisibility="Auto">
Other than reading a book or article that tells that, how would I know that ScrollViewer is a valid attached property? Is there a XAML schema document or something? How does Visual Studio Designer know?

Answer 1

ScrollViewer isn't an attached property; it's a class.
ScrollViewer.HorizontalScrollBarVisibility is an attached property. But it's not an attached property that ListBox "has"; it's an attached property that can be attached to any DependencyObject at all, including ListBox.
Here's what you see if you right click on ScrollViewer.SetHorizontalScrollBarVisibility and ScrollViewer.GetHorizontalScrollBarVisibility. A pair of static methods like this is required for an attached property. The first parameter is the thing you're attaching the property to. It doesn't have to be DependencyObject; it could be FrameworkElement, ListBox, ItemsControl, or anything else that can support dependency properties.
// Summary:
// Sets the value of the System.Windows.Controls.ScrollViewer.HorizontalScrollBarVisibility
// dependency property to a given element.
//
// Parameters:
// element:
// The element on which to set the property value.
//
// horizontalScrollBarVisibility:
// The property value to set.
public static void SetHorizontalScrollBarVisibility(DependencyObject element, ScrollBarVisibility horizontalScrollBarVisibility);
//
// Summary:
// Gets the value of the System.Windows.Controls.ScrollViewer.HorizontalScrollBarVisibility
// dependency property from a given element.
//
// Parameters:
// element:
// The element from which the property value is read.
//
// Returns:
// The value of the System.Windows.Controls.ScrollViewer.HorizontalScrollBarVisibility
// dependency property.
public static ScrollBarVisibility GetHorizontalScrollBarVisibility(DependencyObject element);
The ListBox itself quite likely has no clue what ScrollViewer.HorizontalScrollBarVisibility means, or even that it exists. But in the ListBox's template, there's probably a ScrollViewer, which will probably have a binding a lot like this:
<ScrollViewer
HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}"
Put that attached property on any control that might have a ScrollViewer in its template, and if the template was written correctly, the ScrollViewer will use that value.
This is really, really nice because ListBox doesn't have to think about its own scrolling behavior. That's all delegated. Building controls by composition is extremely powerful.
The downside is that the whole thing is just a vast box of undifferentiated barf and it's real hard to make sense out of stuff. Intellisense can't tell you much about the scrolling behavior of ListBox when nobody outside that particular ListBox's template at the moment can even guess what that scrolling behavior might be.
So, in answer to your question: Yes. Basically you just have to read a lot of stuff. And keep a "Cool XAML Tricks" text file to note down cool stuff you hear about that you don't have a use for just yet.
But that's the way this profession has always been. Even with Intellisense, you can't use a class you don't know about.
Today I learned about ColumnDefinition.SharedSizeGroup and Grid.IsSharedSizeScope, and I learned that you can derive value converters from MarkupExtension and give them strongly typed, named properties instead of having to pass some goofy string into CommandParameter.

Answer 2

I believe the best docs we have right now are the MSDN documentation pages for WPF controls. For example, if you look up ListBox, you can find information about the attached ScrollViewer, and also it's full default template.
https://msdn.microsoft.com/en-us/library/cc278062(v=vs.95).aspx